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Dam Safety Guidance Manual Published by the Oklahoma Water Resources Board Dam Safety Program 2011 For Oklahoma Dam Owners FPO Mark Harrison, Oklahoma Conservation Commission Oklahoma Water Resources Board - Dam Safety Program - March 2011 1 Acknowledgement Dam safety has attracted a great deal of attention in recent years, and in preparing this manual, information from a number of sources was used. The National Dam Safety Program, instituted in response to several major dam failures in the early 1970’s, fo-cused on the problem nationwide. Federal Emergency Management Agency (FEMA) has taken the lead in providing assistance to states in promoting dam safety. The National Dam Safety Program Act of 1996 continues to reinforce the commitment by the Federal Government to dam safety. Special recognition is given to the Federal Emergency Management Agency (FEMA) and the Association of State Dam Safety Officials (ASDSO) for their leadership in developing effective dam safety programs and policies for the furtherance of dam safety. Their diligence in assisting the U.S. dam safety community was an important factor in the issuance of the FEMA grant. The cooperation of the owners of dams within Oklahoma is essential to the success of the State’s dam safety effort. The ultimate purpose of such a program is the protection of the lives and property of citizens of Oklahoma. Cover and other photographs courtesy of Oklahoma Conservation Commission. This publication was funded by a grant from the Federal Emergency Management Agency (FEMA), National Dam Safety Program Grant Agreement 2010-RC-50-0006. Cover Photo: Kadashan No. 2 in Wagoner County Courtesy Mark Harrison, Oklahoma Conservation Commission 2 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Contents DAM SAFETY: AN OWNER’S GUIDANCE MANUAL................................4 Introduction to Dam Safety......................................................4 Hazards, Risk, Failures...............................................................5 Inspection Guidelines................................................................5 Instrumentation and Monitoring Guidelines.........................5 Maintenance Guidelines............................................................6 Emergency Action Plan Guidelines.........................................6 Operation Plan Guidelines........................................................7 Measures to Reduce the Consequences of Dam Failure..................................................7 Water Retention Ability...........................................................15 Seepage Through a Dam.........................................................16 Seepage Around a Dam...........................................................16 Release of Water........................................................................16 Principal or Mechanical Spillway...........................................17 Drawdown Facility...................................................................17 Emergency (Auxiliary) Spillway.............................................17 CHAPTER 3: HAZARDS, RISKS, FAILURES.........................................19 General.......................................................................................19 Hazards as Sources of Risks....................................................19 Natural Hazards That Threaten Dams..................................20 Hazards from Human Activity...............................................21 Site-Specific Structural Risk....................................................22 Sources of Dam Failure............................................................23 Three Categories of Structural Failure..................................23 Failures.......................................................................................23 Age and Its Relation to Failure...............................................25 CHAPTER 4: DEVELOPING A PERSONAL SAFETY PROGRAM..........27 Objectives of a Safety Program...............................................27 Guidelines for Assessing Existing Conditions......................27 Procedural Guidelines – A Source Book...............................29 Documenting the Safety Program..........................................29 CHAPTER 5: INSPECTION GUIDELINES.............................................31 Introduction..............................................................................31 Organizing for Inspection.......................................................32 Embankment Dams and Structures.......................................33 Upstream Slope.........................................................................34 Downstream Slope...................................................................35 Crest...........................................................................................36 Seepage Areas............................................................................37 Concrete Dams and Structures...............................................38 Spillways....................................................................................40 Procedure for Inspection of the Spillway..............................44 Inlets, Outlets, and Drains......................................................45 Inspecting the outlet system...................................................47 General Areas............................................................................48 DAM SAFETY: AN OWNER’S GUIDANCE MANUAL CHAPTER 1: AN APPROACH TO DAM SAFETY....................................9 General.........................................................................................9 Urgency for Safety......................................................................9 Dam Ownership and Safety......................................................9 Role of the Dam Owner in Dam Safety.................................10 The Role of Consultants in Dam Safety................................11 Role of the Oklahoma Water Resources Board....................12 CHAPTER 2: INTRODUCTON TO DAMS.............................................13 General.......................................................................................13 The Watershed System.............................................................13 Types of Dams...........................................................................14 Components..............................................................................14 Construction Materials............................................................14 MARK HARRISON, OKLAHOMA CONSERVATION COMMISSION Oklahoma Water Resources Board - Dam Safety Program - March 2011 3 Contents CHAPTER 6: INSTRUMENTATION AND MONITORING GUIDELINES...........................................71 General.......................................................................................71 Reasons for Instrumentation..................................................71 Instrument Types and Usage..................................................72 Visual Observations.................................................................72 Movements................................................................................72 Pore Pressure and Uplift Pressure..........................................74 Water Level and Flow...............................................................74 Water Quality............................................................................74 Temperature..............................................................................74 Crack and Joint Size.................................................................75 Seismic Activity........................................................................75 Weather......................................................................................75 Stress and Strain.......................................................................75 Automated Data-Acquisition Systems...................................75 Frequency of Monitoring........................................................76 CHAPTER 7: MAINTENANCE INSPECTION GUIDELINES..................79 General.......................................................................................79 Maintenance Priorities.............................................................79 Immediate Maintenance..........................................................79 Required Maintenance at Earliest Possible Date..................79 Continuing Maintenance........................................................80 Specific Maintenance Items....................................................80 Earthwork Maintenance and Repair......................................80 Riprap Maintenance and Repair.............................................82 Controlling Vegetation............................................................84 Controlling Livestock..............................................................85 Controlling Animal Damage..................................................85 Controlling Damage From Traffic.........................................86 Mechanical Maintenance.........................................................86 Electrical Maintenance............................................................88 Cleaning.....................................................................................88 Concrete Maintenance.............................................................89 Metal Component Maintenance.............................................89 CHAPTER 8: EMERGENCY ACTION PLAN GUIDELINES....................91 General.......................................................................................91 Contents of Guidelines............................................................92 Description of the Project.......................................................92 Notification Flowchart.............................................................92 Emergency Detection, Evaluation, and Classifications.......93 Responsibilities.........................................................................93 Preparedness.............................................................................94 Inundation Maps......................................................................94 Implementation........................................................................94 CHAPTER 9: GUIDELINES FOR OPERATIONS....................................97 General.......................................................................................97 Plan Guidelines.........................................................................97 Background Data......................................................................98 Operating Instructions and Records......................................98 Schedule of Routine Tasks.......................................................99 Record Keeping.......................................................................100 CHAPTER 10: REDUCING THE CONSEQUENCES OF A DAM FAILURE.....................................................101 Supplements to a Dam Safety Program...............................101 Liability....................................................................................101 Measures to Reduce the Consequences of Dam Failures............................................103 Insurance.................................................................................104 Government Assistance.........................................................104 DAM SAFETY: AN OWNER’S GUIDANCE MANUAL apendix a.....................................................................106 iNSPECTION EQUIPMENT AND CHECKLIST.............107 APPENDIX B: REPORT FORM.......................................111 APPENDIX C: GLOSSARY...............................................113 REFERENCES.....................................................................121 Oklahoma Water Resources Board - Dam Safety Program - March 2011 4 DAM SAFETY: An Owner’s Guidance Manual Introduction to Dam Safety The need for dam safety is urgent. Across the United States thousands of dams are now in place with many more built each year. Dams—essential elements of the national infrastructure—supply water for households and businesses and cooling water for power plants, offer opportunities for recreation, and help control floods. Should a dam fail, many lives and many dollars’ worth of property are at risk. The legal and moral responsibility for dam safety rests with the dam owner. Existing dams are aging and new ones are being built in hazardous areas. At the same time, devel-opment continues in potential inundation zones downstream. More people are at risk from dam failure than ever, despite better engineering and construction methods, and continued deaths and property losses from dam failures are to be expected. Society and individuals alike may profit from dam operations. Dam ownership, however, is neither justified nor effective if one cannot assure the safety of citizens and property. The costs of dam safety are small in comparison to the consequences following a dam failure, particularly in today’s litigious society. Liability due to dam failure can easily offset years of profitability. You can directly influence the safety of a dam by developing a safety program which includes inspection, monitoring through instrumentation, maintenance of the structure, and proactive emergency planning. A high-quality safety program is attuned to the dam structure and its im-mediate environment and depends on the owner’s knowledge of the dam and how it works. Lakes in Oklahoma and in other parts of the country may either be human-built or exist because of geologic activities such as landslides, erosion, or glaciation. The majority of dams are human structures constructed of earthfill or concrete. It is important that you, as a dam owner, be aware of the different types of dams, their essential components and their function, as well as the important physical conditions likely that influence them. As in the case with buildings, highways, and other works that we construct, dams require an on-going maintenance program to insure their continued useful life. This fact has not always been appreciated. Often there is a tendency to neglect them once construction is completed. As is the case with buildings, highways, and other works that we construct, dams require an on-going maintenance program to ensure their continued useful life. This fact has not always been fully appreciated. Often there is a tendency to neglect them once construction is completed. Oklahoma Water Resources Board - Dam Safety Program - March 2011 5 National statistics show that dam failure is an all too common problem. It is imperative that you, as a dam owner or operator, familiarize yourself with the risks and hazards of dam ownership. Risk has greatly increased for existing dams as developers have been allowed to construct be-low dams within their inundation zone and new dams are being built in areas where the geology may be inappropriate. Other risks include natural phenomena such as floods, earthquakes, and landslides. These hazards threaten dam structures and their surroundings. Floods that exceed the capacity of a dam’s spillway and then erode the dam or abutments are particularly hazardous. Seismic activity which appears to be on the rise in Oklahoma may also cause cracking or seepage. Similarly, debris from landslides may block a dam’s spillway and cause an overflow event that erodes the abutments and ultimately weakens the structure. Hazards, Risk, Failures The three major categories of dam failure are overtopping by flood, foundation defects, and piping. For earthen dams, the major reason for failure has been piping or seepage. For concrete dams, the major reasons for failure have been associated with foundations. Overtopping has been a significant cause of dam failure, primar-ily where a spillway was inadequate. Inspection Guidelines An effective inspection program is essential to identification of problems and for safe main-tenance of a dam. The program should involve three types of inspections: (1) periodic technical inspections; (2) periodic maintenance inspec-tions; and (3) informal observations by project personnel as they operate the dam. Technical inspections involve specialists familiar with the design and construction of dams and include assessments of structure safety. Maintenance inspections are performed more frequently than technical inspections in order to detect, at an early stage, any detrimental developments in the dam. These involve assessment of operational capabil-ity as well as structural stability. The third type of inspection is actually a continuing effort by on-site project personnel (dam tenders, powerhouse operators, maintenance personnel) performed in the course of their normal duties. A fact sheet on Dam Inspection Guidelines is available online at www.owrb.ok.gov/damsafety. Instrumentation and Monitoring Guidelines A dam’s instrumentation furnishes data for determining if the structure is functioning as intended and continuing surveillance to warn of any unsafe developments. Monitoring physi-cal phenomena that can lead to a dam failure may draw on a wide spectrum of instruments and procedures ranging from very simple to very complex. Any program of dam- safety instrumentation must involve proper design consistent with other project components. The program must be based on prevailing geotech-nical conditions at the dam, and must include consideration of the hydrologic and hydraulic factors present before and after the project is in operation. Instrumentation designed for moni-toring potential deficiencies at existing dams must take into account the threat to life and property that the dam presents. Thus, the extent and nature of the instrumentation depends not only on the complexity of the dam and the size of the reservoir, but also on the potential for deaths and property losses downstream. An instrumentation program should involve instruments and evaluation methods that are as simple and straight forward as the project will allow. The involvement of qualified personnel in the design, installation, consistent and regular monitoring, and evaluation of an instrumenta-tion system is of prime importance to the suc-cess of the program. Specific information that instrumentation can provide includes: 6 Oklahoma Water Resources Board - Dam Safety Program - March 2011 • warning of a problem, i.e. settlement , movement, seepage, stability • definition and analysis of a problem, such as locating areas of concern • proof that behavior of the dam is as expected • evaluating any remedial actions Maintenance Guidelines A good maintenance pro-gram will protect a dam against deterioration and prolong its life. A poorly maintained dam will deteriorate, and may fail. Nearly all the components of a dam and the materials used for its construction are susceptible to damaging deterioration if not properly maintained. A good maintenance pro-gram protects both you and the general public. The cost of a proper maintenance program is small compared to the cost of major repairs or the loss of life and property and resultant litiga-tion. You should develop a basic maintenance program based primarily on systematic and frequent inspections. Inspections, as noted in Chapter 5, should be carried out monthly and after major floods or earthquakes. During each inspection, fill out a checklist of items requiring maintenance. An Inspection Checklist is avail-able online at www.owrb.ok.gov/damsafety. Emergency Action Plan Guidelines History has shown that dams sometimes fail and that often these failures cause loss of life, injuries and extensive property damage. You should prepare for this possibility by developing an emergency action plan which provides a systematic means to: • identify potential problems that could threaten a dam • determine who would be at risk should a failure occur • expedite effective response actions to prevent failure • develop a notification plan for evacuating people to reduce loss of life and property damage should failure occur You are responsible for preparing a plan cov-ering these measures and listing actions that you and operating personnel should take. You should be familiar with the local government officials and agencies responsible for warn-ing and evacuating the public. An Emergency Action Guide is available online at www.owrb. ok.gov/damsafety. It is important that you make full use of others who are concerned with dam safety. Emergency plans will be more effective if they integrate the actions of others who can expedite response. People and organizations with whom you should consult in preparing an emergency action plan include numerous local participants, state and federal agencies. An essential part of the emergency action plan is a list of agencies and persons to be notified in the event of a potential failure. Possible inclu-sions for this list should be obtained from and coordinated with local law enforcement agencies and local disaster emergency services. These are key institutions that can activate public warn-ing and evacuation procedures or that might be able to assist you, the dam owner, in delaying or preventing failure. Certain key elements must be included in every notification plan. Information about potential inundation (flooding) areas and travel times for the breach (flood) wave is essential. Inunda-tion maps are especially useful in local warning and evacuation planning, including identifying evacuation routes. Oklahoma Water Resources Board - Dam Safety Program - March 2011 7 Operation Plan Guidelines Establishing an operation procedure or plan calls for detailed: • data on the physical characteristics of dam and reservoir • descriptions of dam components • operating instructions for operable mechanisms • instructions for inspections Measures to Reduce the Consequences of Dam Failure Liabilities that are determined following a dam failure strongly affect organizations and individuals, governments and dam owners alike. Establishing liability is the legal means de-veloped by society to recover damages due to some intentional or negligent wrong (in this case, a lack of dam safety) and represents another perspective on the dam safety problem. A thorough understanding of this legal process can help you decide the steps necessary to reduce liability. You can directly and indirectly influence the use of a variety of measures that will serve to reduce the consequences of dam failure. For example, insurance against the costs that will accrue after a failure will save you money by spreading costs to multiple dam owners. Some land use measures instituted by gov-ernments represent better means of mitigating future disasters. Land use measures that restrict living or de-veloping in inundation zones radically improve safety and are among the most effective ways to save lives and preserve property over the long term; however, such steps are not always acceptable to the local population or government. Thus, increasing public awareness and governmental planning are vital measures that must be considered as ways to reduce the consequences of dam failure. • instrumentation and monitoring guidelines • guidelines for maintenance • guidelines for emergency operations • bibliographic references Establish a schedule for both day-to-day tasks and tasks performed less frequently throughout the year. The schedule should formalize inspec-tion and maintenance procedures so that even an inexperienced person can determine when a task is to be done. Oklahoma Water Resources Board - Dam Safety Program - March 2011 9 CHAPTER 1: An Approach to Dam Safety 1 public and private agencies, and private citizens. Typical reasons for building dams include water storage for human consumption, agricultural production, power generation, flood control, reduction of soil erosion, industrial use, and recreation. Thus, dam owners serve society by meeting important state needs and may also personally profit from dam operations. How-ever, those are not sufficient reasons for build-ing or owning a dam if the owner cannot keep people and property safe in potential inunda-tion zones. Both financially and morally, successful dam ownership and the maintenance of safety standards go hand in hand. Investment in dam safety should be accepted as an integral part of project costs and not viewed as an expendable item that can be eliminated if a budget becomes tight (Jansen, 1980). The potential cost and sta-tistical likelihood of dam failure to both life and property are simply too high to ignore. As national needs for water intensify and its value increases, more dams are being built. At the same time, many existing dams are reach-ing or passing their design life spans and, for various reasons, people continue to settle near dams. As builders use poorer sites for dams or as areas below a dam develop, the job of protect-ing life and property becomes more difficult. Therefore, as dam construction continues and the population grows, exposure of the public to dam failure hazards increases and the overall safety problem becomes more difficult. General This manual is a safety guide for the dam owner. The continuing need for dam safety is critical because of the thousands of dams now in place and the many new ones being built each year. Although these dams are essential elements of the national infrastructure, the risks to the public posed by their possible failure are great; a large and growing number of lives and valuable property are at stake. Though many are concerned about dam safety, the legal and moral responsibility essentially rests with the dam owner. Urgency for Safety The critical need for dam safety is clear. World and national statistics on dam failures show an unaccept-able record of deaths and property losses. The record for U.S. losses from major dam failures in recent years, shown in Table 1 is also discouraging. Actual national losses are much higher than indicated be-cause the statistics shown exclude small dam failures and combinations of dam failure with natural flood-ing events. Two examples are dams that failed near Hearne, Texas in May 2004 and the Johnstown, Pennsylvania, disaster of 1889 which is still regarded as one of the nation’s great catastrophes. The poten-tial for future similar catastrophes due to dam failure remains strong. Only a cooperative effort in dam safety involving owners and communities can lessen this potential. Dam Ownership and Safety This manual can be applied to dams owned and operated by a wide range of organizations and people, including state and local governments, 10 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Name & Location of Dam Date of Failure Number of Lives Lost Damages Mohegan Park, CT 3/1963 6 $3 million. Little Deer Creek, UT 6/1963 1 Summer cabins damaged. Baldwin Hills, CA 12/1963 5 41 houses destroyed, 986 houses damaged, 100 apartment buildings damaged. Swift, MT 6/1964 19 Unknown. Lower Two Medicine, MT 6/1968 9 Unknown. Lee Lake, MA 3/1968 2 6 houses destroyed, 20 houses damaged, 1 manufacturing plant damaged or destroyed. Buffalo Creek, WV 2/1972 125 546 houses destroyed, 538 houses damaged. Lake “O” Hills, AR 4/1972 1 Unknown. Canyon Lake, SD 6/1972 33 Unable to assess damage because dam failure accompanied damage caused by natural flooding. Bear Wallow, NC 2/1976 4 1 house destroyed. Teton, ID 6/1976 11 771 houses destroyed, 3,002 houses damaged, 246 businesses damaged or destroyed. Laurel Run, PA 7/1977 40 6 houses destroyed, 19 houses damaged. Sandy Run & 5 others, PA 7/1977 5 Unknown. Kelly Barnes, GA 11/1977 39 9 houses, 18 house trailers, & 2 college buildings destroyed; 6 houses, 5 college buildings damaged. Lawn Lake, CO 7/1982 3 18 bridges destroyed, 117 businesses & 108 houses damaged. Campgrounds, fisheries, power plant damaged. D.M.A.D., UT 6/1983 1 Unknown. Nix Lake Dam, TX 3/1989 1 Unknown. Silver Lake, MI 5/2003 0 $102,000,000. Big Bay Lake, MS 3/2004 0 98 houses, 2 churches, fire station, bridge, $2.2 million Kaloko Res., HI 3/2006 7 Unknown. Source: Graham, 1983, 2004 TABLE 1.1. Loss of Life and Property Dama ge From Notable U.S. Dam Failures , 1963-2006 Governments across the nation have shown increasing concern for this problem and have enacted laws, statutes, and regulations that in-crease the dam owner’s responsibility. In most states, including Oklahoma, owners are held strictly liable for losses or damages resulting from dam failure. Concurrently, liability insur-ance costs have risen rapidly. Role of the Dam Owner in Dam Safety An owner should be aware of and use both direct and indirect means of achieving dam safety. The owner can monitor and work on factors directly in his control (for example, structural integrity), which are detailed below. However, the owner may also seek to influence governmental policy and work for positive change in statutes and laws that affect dam safety (example, zoning laws). Such indirect influence by an owner could con-tribute significantly to reducing the likelihood and consequences of dam failure and, thus, to overall community safety. Liability, insurance coverage, and the roles of the state and federal governments should all be well-understood by an owner. Liability can apply not only to the individual dam owner, but also to any company or organization that possesses the dam, or any person who operates or maintains it and, potentially, even those who live around a lake. If an unsafe condition existed prior to a new dam owner’s term of ownership, the new owner cannot be relieved of liability should the Oklahoma Water Resources Board - Dam Safety Program - March 2011 11 dam fail during this term. Thus, the potential owner must carefully inspect the structural integrity of any dam prior to purchase and then inspect, maintain, and repair it thereafter. Legally, the dam owner must do what is neces-sary to avoid injuring persons or property which usually applies to circumstances and situations which a reasonable person could anticipate. In order to meet your responsibility to maintain the dam in a reasonable and safe condition, you, the owner, should conduct regular inspections of the dam and maintain or repair deficient items. Regular inspections by qualified professionals are necessary to identify and correct any problems. A dam owner should have a thorough understand-ing of the dam’s physical and social environment. This would include: knowledge of natural and technological hazards that threaten it, an under-standing of the developing human settlement pat-terns around the dam, and an under-standing of any events that can lead to structural failure. It is a good idea for every owner of a dam to pause and consider what lies below their dam. Several questions need to be asked. • What is the nature of the land use downstream: wooded or agricultural land, scattered homes, roads, villages, urban? • How many structures are located within a half mile, a mile or several miles of the dam? • How are downstream structures located with regard to the watercourse or floodplain, with respect to both distances from the watercourse or river and elevation above it? • What is the first-floor elevation of homes located downstream. Are they only a few feet above the level of the water surface, or are they on bluffs high above it and out of danger? • Is the valley below the dam characterized by steep hills, or is there a broad floodplain? This is an important consideration, as it determines whether water released in a dam failure or during flooding would soon spread out and lose its force or whether a destructive wall of water would travel a long distance downstream. Owning a dam brings many different concerns and possible rewards, but in the end success will largely be measured by a continuing record of safety. Owners can also influence the safety of dams in more direct ways. They can and should de-velop their own safety programs, which should include such important elements as inspection, monitoring through instrumentation, mainte-nance, emergency action planning, and proper operation. Such programs are directly related to a specific dam’s structure and its immediate environment and depend on the owner’s knowl-edge of the dam and how it works. The Role of Consultants in Dam Safety A dam is a special kind of structure, simple in concept but with many complicated com-ponents. There is no such thing as a standard dam design; furthermore, each dam site is unique. The existence of a dam necessitates the involvement of many specialists to analyze, design, build, inspect, and repair it. This wide variety of consultants will include civil, geo-technical, mechanical, and electrical engineers, geologists and hydrologists. As owner, you should know more about your dam than anyone else. A consultant can advise you on such important items as: • the design and construction of a new dam • the overall stability of the dam under normal and flood conditions • any repairs or maintenance needed by the dam and appurtenant works • the severity of any problems and indicate in what order to repair them • cost estimates for repair work • adequacy of the spillway to pass the design flood 12 Oklahoma Water Resources Board - Dam Safety Program - March 2011 • an assessment of downstream hazards • the dam owner’s preparation and procedures to deal with emergency conditions Hazardous conditions at the dam should be reported verbally and in writing to the dam owner and the OWRB. A written report from the owner’s consultant is essential for every inspection. It is uncommon that a dam owner has all of the technical skills needed to monitor the condition of the dam. Thus, the role of the consulting engineer is critical in dam safety Role of the Oklahoma Water Resources Board The Oklahoma Water Resources Board is responsible for administrating state dam safety laws. The staff of the OWRB has four primary areas of activity in the dam safety program: (1) review and approval of plans and specifications of new dams, (2) review of plans and specifications for repairs, modification, or rehabilitation work, (3) periodic inspections of construction work on new and existing dams, and (4) review of inspection reports and approval of emergency action plans. Oklahoma Water Resources Board - Dam Safety Program - March 2011 13 CHAPTER 2: Introduction to Dams 2 General The purpose of a dam is to impound (store) water for any of several reasons, e.g., flood control, water supply for hu-mans or livestock, irrigation, energy generation, recreation, or pollution control. This manual primarily concentrates on earthen dams, which constitute the majority of structures in place and under development in Oklahoma. The Watershed System Water from rainfall or snowmelt naturally runs downhill into a stream valley and then into larger streams or other bodies of water. The “watershed system” refers to the drainage process through which rainfall or snowmelt is collected into a particular stream valley during natural runoff (directed by gravity). Dams con-structed across such a valley then impound the runoff water and release it at a controlled rate. During periods of high runoff, water stored in the reservoir typically increases, and overflow through a spillway may occur. During periods of low water flow is normally controlled. Hence, with the insertion of a dam into a watershed very high runoffs (floods) and very low runoffs (drought periods) are generally avoided. Mark Harrison, Oklahoma Conservation Commission 14 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Types of Dams Dams may either be human-built or result from natural phenomena, such as landslides or glacial deposition. The majority of dams are human struc-tures normally constructed of earthfill or concrete. Naturally occurring lakes may also be modified by adding a spillway to allow for safe, efficient release of excess water from the resulting reservoir. Dam owners should be aware of the different types of dam’s essential components of a dam how the components function, and important physical conditions likely to affect a dam. Human-built dams may be classified according to the type of construction materials used, the methods used in construction, their slope or cross-section, the way they resist the forces of the water pressure behind them, the means of controlling seepage, and oc-casionally, their purpose. Components : The components of a typical dam are illustrated in Figure 2.1. Nearly all dams possess the features shown or variations of those features. Definitions of the terms are given in the Glos-sary. The various dam components are discussed in greater detail later on. Construction Materials : The materials used for construction of dams include earth, rock, tailings from mining or milling, concrete, masonry, steel, and any combination of those materials. 1. Embankment Dams: Embankment dams, the most common type in use today, have the general shape shown in Figure 2.1. Their side slopes typically have a grade of two to one (horizontal to vertical) or flatter. Their capacity for water retention is due to the low permeability of the entire mass (in the case of a homogeneous embankment) or of a zone of low-permeability material (in the case of a zoned embankment dam). Material used for embankment dams include natural soil or rock obtained from borrow areas or nearby quarries, or waste materials obtained from mining or milling operations. If the natural material has a high permeability, then a zone of very low permeability material must be included in the dam to retain water. 2. Concrete Dams: Concrete dams may be categorized into gravity and arch dams according to the designs used to resist the stress due to reservoir water pressure. A concrete gravity dam (shown in Figure 2.2) is the most common form of concrete dam. In it, the mass weight of the concrete and friction resist the reservoir water pressure. A buttress dam is a specific type of gravity dam in which the large mass of concrete is reduced, and the forces are diverted to the dam foundation through vertical or sloping buttresses. Gravity Figure 2.1. Embankment dam (U.S. Army Corps of Enginers). Oklahoma Water Resources Board - Dam Safety Program - March 2011 15 dams are constructed of non-reinforced vertical blocks of concrete with flexible seals in the joints between the blocks. Concrete arch dams are typically rather thin in cross-section. The reservoir water forces acting on an arch dam are carried laterally into the abutments. The shape of the arch may resemble a segment of a circle or an ellipse, and the arch may be curved in the vertical plane as well. Such dams are usually built from a series of thin vertical blocks that are keyed together, with water stops between the blocks. Variations of arch dams include multi-arch dams, in which more than one curved section is used, and arch gravity dams, which combine some features of the two types. A recently developed method for constructing concrete gravity dams involves the use of a relatively weak concrete mix which is placed and compacted in a manner similar to that used for earthfill dams. Roller-compacted concrete has the advantages of decreased cost and time. In addition, there are no joints where seepage could occur. 3. Other Types: Various construction techniques could be used in a single dam. For example, a dam could include an earthen or rock fill embankment as well as a portion made of concrete. In such a case, the concrete section would normally contain the spillway or other outlet works. A recent design for low-head dams (with a minimal height of water behind the dam uses inflatable rubber or plastic materials anchored at the bottom by a concrete slab. Some dams are constructed for special purposes, such as diversion of water, or permit construction of other facilities in river valleys. These dams are called diversion dams and cofferdams, respectively. Water Retention Ability Because the purpose of a dam is to retain water effectively and safely, its water-retention ability is of prime importance. Water may pass from the reservoir to the downstream side of a dam by: • seeping through the dam • seeping through the abutments • seeping under the dam • overtopping the dam • passing through the outlet works • passing through or over a primary spillway • passing over an emergency spillway The first three ways water pass from a reser-voir are considered undesirable, particularly if the seepage is not limited in area or volume. Overtopping of an embankment dam is also very undesirable because the embankment mate-rial may be eroded away. Additionally, only a few concrete dams have been designed to be overtopped. Water normally leaves a dam by passing through an outlet works or spillway. Water should pass over an emergency spillway only during periods of very high reservoir levels and high water inflow. Figure 2.2. CONCRETE GRAVITY dam (U.S. Army Corps of Engineers ). 16 Oklahoma Water Resources Board - Dam Safety Program - March 2011 tion, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is undesirable. To minimize this possibility, dams are constructed with internal impermeable barriers and internal drainage facilities such as drainpipes or filter sys-tems, or other drainage systems such as toe, blanket, or chimney drains. Flow through a dam foundation may be diminished by grouting known or suspected highly permeable material, constructing a cutoff wall or trench below a dam, or constructing an upstream impermeable blanket. Figure 2.3 illustrates a cutoff trench. In summary, the overall water retention ability of a dam depends on its permeability, the abutments, the foundation, and the efforts made to reduce that permeability or restrict the flow of water through these components. Should high permeability oc-cur, seepage can lead to piping, which will likely result in failure. Release of Water Intentional release of water, as stated earlier, is confined to water releases through a service spillway or outlet works or over emergency spillways. Principal or Mechanical Spilway : The principal or mechanical spillway maintains the normal water level in the reservoir. Its function is to pass expected flood flows past the dam safely and without erosion. It may consist of a pipe through the dam or a system of gates that discharge water into a concrete spillway. Either method uses the Seepage Through a Dam: All embankment dams and most concrete dams allow some seep-age. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is impor-tant to control the quantity of seepage by using low permeability materials in construction and by channeling and restricting the flow so that embankment materials do not erode. Seepage through a concrete dam is usually minimal and is almost always through joints between blocks, or through cracks or deteriorated concrete which may have developed. Maintenance of these joints and cracks is therefore essential. The seepage water should be collected and channelized, so that its quantity can be measured and erosion minimized. Seepage Around a Dam: Seepage around the ends of a dam through the abutment materials or under a dam, through the dam foundation material, may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it less stable. Seepage through abutments or foundations can dissolve the constituents of certain rocks such as limestone, dolomite, or gypsum so that any cracks or joints in the rock become progressively larger and in turn allow more seepage. Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When the entrance point is reached, water may then flow without restric- Figure 2.3. Embankment dam WITH A CUT-OFF TRENCH (U.S. Army Corps of Enginers ). Oklahoma Water Resources Board - Dam Safety Program - March 2011 17 overflow principle. When the reservoir reaches a certain level, water flows into a standpipe or riser pipe (Figure 2.4) or over a gate. Intake structures for spillways must have systems that prevent clog-ging by trash or debris. Drawdown Facility : All dams should have some type of drawdown facility which can: • quickly lower the water level if failure of the dam is imminent. • serve the operational purposes of the reservoir. • lower the water level for dam repairs. • periodically raise and lower the pool level to kill weeds and mosquitoes. The valve regulating the drawdown facility should be on the upstream end of the conduit to minimize the risk to the dam posed by a pos-sible internal rupture of the pipe. Emergency (Auxiliary ) Spilway : As the name implies an emergency spillway functions during emergency conditions to prevent over-topping of a dam. A typical emergency spillway is an excavated channel in earth (Figure 2.5) or rock near one abutment of a dam. An emergency spillway should always discharge away from the toe of a dam to avoid its erosion. Furthermore, the spillway should be constructed in such a manner that the spillway itself will not seriously erode when it is in use. Obviously erosional fail-ure of the spillway could be as catastrophic as failure of the dam itself. An emergency spillway should be sized to convey the so-called “design flood”, the rare, large-magnitude flood used to establish design criteria. The spillways of many existing dams are now considered undersized because standards for the design flood have increased over the years. Figure 2.4. Principal Spilway Figure 2.5. Emergency Spilway . Mark Harrison, Oklahoma Conservation Commission Oklahoma Water Resources Board - Dam Safety Program - March 2011 19 CHAPTER 3: Hazards, Risks, and Failures 3 General Dam failures are severe threats to life and property and are now being recorded and documented much more thoroughly than in the past. Recorded losses have been high. Statistics on losses of life and property fully justify the need for dam owners to better understand the risks to the public posed by dams, the kinds of haz-ards that promote those risks and owner liabilities associated with them, and, generally, the reasons that dams fail. Improving a dam owner’s under-standing of realistic risks and possible reasons for failure is an essential first step in any overall effort to improve dam safety and preserve the benefits of dam ownership. Hazards as Sources of Risks The dam structure itself can be a source of risk due to possible construction flaws and weaknesses that develop because of aging. The site immedi-ately surrounding the structure may also increase the structural risk if the dam is not positioned or anchored properly or if excessive reservoir seep-age erodes the foundation or abutments. The physical hazards that can cause dam fail-ure are translated into high risks when people or properties are threatened. These high risks are exacerbated by a number of important fac-tors. For instance, in Oklahoma and most other states, people are often allowed to build within a dam’s inundation zone, thereby greatly com-pounding the associated risk. Natural hazards such as floods, earthquakes, and landslides are also important contributors to risk. These have now become even greater hazards because development has placed people and property in their way. Failure to adjust to these events has been costly both to dam owners and to the public in general. Human behavior is another element of dam fail-ure risk; simple mistakes, operational misman-agement, negligence, unnecessary oversights, or destructive intent can interact with other hazards to compound the possibility of failure. Figure 3.1 Embankment Dam failure in Cleveland county 20 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Floding from high precipitation : Of the natural events that can impact dams, floods are the most significant. A floodplain map of the U.S. (Figure 3.2) gives the estimated percentage of land resting within a floodplain. Floods are the most frequent and costly natural events that lead to disaster in the U.S. Therefore, flood potentials must be included in risk analyses for dam failure. Flash floods can happen anywhere in Oklahoma, even on small drainages. A common safety factor for dam design is to construct them to withstand a “probable maximum flood” (PMF) assumed to occur on the upstream watershed. A PMF is the flood that may be expected from the most severe combination of critical meteorologic and hydro-logic conditions that are reasonably possible in the region. However, dams are often built in areas where estimates of the PMF are based on short precipitation and runoff records. As a result, spill-way capacity may often be underestimated. Flooding from dam failure : When a dam fails as a result of a flood, more people and prop-erty are generally placed in jeopardy than during Thus, a broad range of natural and human hazards, taken separately or in combination, in-crease the probability of dam failure and injury to people and property. The following discussion of some of the most significant hazards that lead to public risk illus-trates the interrelationships among events that can lead to dam failure. Natural Hazards That Threaten Dams The most important natural hazards threaten-ing dams include: • flooding from high precipitation • flooding from dam failure • earthquakes • landslides Figure 3.2 Floodplain map of the U.S. Oklahoma Water Resources Board - Dam Safety Program - March 2011 21 natural floods. The Rapid City, South Dakota, flood of 1970, which killed 242 people, caused a dam failure which added significantly to the loss of life. When a natural flood occurs near a dam, the probability of failure and loss of life almost always increases. The sudden surge of water generated by a dam failure usually far exceeds that expected from a natural 100-year floodplain estimate; therefore, residences and businesses that would escape natural flooding may still be at extreme risk from flooding due to dam failure. Hence, it is important to inform residents and business per-sonnel of the full risk to which they are exposed so that they can respond accordingly. To compound the risk even further, when one dam fails, the sudden surge of water may well be powerful enough to destroy another dam downstream. Upstream dams may seem too far away to be a real threat, but inundation zones and surge crests can extend many miles down-stream, especially if the reservoir behind the collapsed dam held a large quantity of water. Earthquakes : Pose significant threats to dam safety. While very common in Oklahoma, it is rare that earthquakes here are substantial enough to harm a dam. Nevertheless, dam own-ers should be aware of the history of seismic activity in their locality and develop their emer-gency procedures accordingly. Both earthen and concrete dams can be dam-aged by ground motions caused by seismic activity. Cracks or seepage can develop, leading to immediate or delayed failure. Dams, such as those in California, located near relatively young, active faults are of particular concern, but dams (especially older concrete and earthen structures) located where relatively low-scale seismic events may occur are also at risk. Recent detailed seismic analyses have indicated a much broader area of seismic activity sufficient to damage dams than previously considered; the seismic risk is essentially nationwide. Landslides: Rock slides and landslides may affect dams directly by blocking a spillway or by eroding and weakening abutments. Indirectly, a large landslide into a reservoir behind a dam can cause an overflow wave that will exceed the capacity of the spillway and lead to failure. A landslide (or mudslide) can form a natural dam across a stream which can then be overtopped and fail. In turn, failure of such a natural dam could then cause the overtopping of a down-stream dam or by itself cause damage equivalent to the failure of a human built dam. In addition, large increases in sediment caused by such events can materially reduce storage capacity in reservoirs and thus increase a downstream dam’s vulnerability to flooding. Sedimentation can also restrict the operation of low level gates and water outlets; damage to gates and outlets can lead to failure. Hazards from Human Activity Human activity must also be considered when analyzing the risks posed by dams. The “high hazard” designation does not imply structural weakness or an unsafe dam. In Oklahoma, the hazard classification of dams is based on the potential for loss of life and economic loss in the area downstream of the dam, not on its structural safety (Table 3.1). Thus, dams that may be of very sound construction are labeled “high hazard” if failure could result in catastrophic loss of life. Risk may well increase through time because few governmental entities have found the means to limit settlement below dams. The hazard level of more dams is rising to “high” or “significant” as development occurs in potential inundation zones below dams previously rated “low hazard.” Because of short-term revenue needs or other pressures, governments often permit develop-ment in hazardous areas despite long-term dan-ger and the risk of high future disaster costs. Diversion of development away from potential inundation zones is a sure means of reducing 22 Oklahoma Water Resources Board - Dam Safety Program - March 2011 risk, but is not always a policy suitable to the immediate needs of local government. Perhaps the ultimate irony for a dam owner is to have de-veloped and implemented a safety program only to have development permitted in the potential inundation zone so that the hazard rating and owner’s liability increase. All sorts of other human behavior should be in-cluded in risk analyses; vandalism, for example, cannot be excluded and is in fact a problem faced by many dam owners. Vegetated surfaces of a dam embankment, mechanical equipment, manhole covers and rock riprap are particularly susceptible to damage by people. Every precau-tion should be taken to limit access to a dam by unauthorized persons and vehicles. Dirt bikes (motorcycles) and off-road vehicles, in par-ticular, can severely degrade the vegetation on embankments. Worn areas lead to erosion and more serious problems. Mechanical equipment and associated control mechanisms should be protected from tamper-ing, whether purposeful or inadvertent. Buildings housing mechanical equipment should be sturdy, have protected windows, and heavy-duty doors, and be secured with padlocks. Detachable controls, such as handles and wheels, should be removed when not in use and stored inside the padlocked building. Other controls should be secured with locks and heavy chains where possible. Manhole covers are often removed and sometimes thrown into reservoirs or spillways by vandals. Rock used as riprap around dams is sometimes thrown into the reservoirs, spillways, stilling basins, pipe-spillway risers, and elsewhere. Riprap is often displaced by fishermen to form benches. The best way to prevent this abuse is to use rock too large and heavy to move easily, or to slush-grout the riprap. Otherwise, the rock must be regularly replenished and other dam-ages repaired. Regular visual inspection can easily detect such human impacts. Owners should be aware of their responsibility for the safety of people using their facility even though their entry may not be authorized. “No Trespassing” signs should be posted, and fences and warning signs erected around dangerous areas. As discussed in Chapter 10, liability insurance can be purchased for protection in the event of accidents. Site-Specific Structural Risk Developing site-specific risk analyses involves consideration of a number of hazards. Such analyses are helpful in stimulating better aware-ness, planning, and design. In some cases dam structure analyses are quantitative. Hence, pre-cise conclusions about engineering and design can be made. Probabilistic analyses can also be important and useful; however, exact quantita-tive and probabilistic tools are not yet applicable in many situations and do not fully supplement or replace qualitative analyses such as informed perception and judgment of the risks. Judgment and engineering experience should play an im-portant role in reaching useful conclusions in any site-specific analysis of structural risk. TABLE 3.1. Table of Hazard-Potential Clasification Hazard -Potential Classification Description Low Dams assigned the low hazard-potential classification are those where failure would result in no probable loss of human life and low economic losses. Significant Dams assigned the significant hazard-potential classification are those dams where failure would result in no probable loss of human life but can cause economic loss or disruption of lifeline facilities. High Dams assigned the high hazard-potential classification are those where failure will probably cause loss of human life. Oklahoma Water Resources Board - Dam Safety Program - March 2011 23 As mentioned in Chapter 2, structural risks tend to result from design and construction prob-lems related to the dam materials, construction practice, and hydrology. The complexity of the hazard is such that structural design and causes of dam failure are significant areas of research in engineering. Indeed, better design criteria have been developed and safer dams are being built, but there is no basis for complacency. Dams continue to age, people continue to move into inundation zones, and enough hazards exist that the net risk to the public will remain high despite design improvements. Sources of Dam Failure There are many complex reasons, both struc-tural and non-structural, for dam failure. Many sources of failure can be traced to decisions made during the design and construction process and to inadequate maintenance or op-erational mismanagement. Failures have also resulted from the natural hazards previously mentioned. However, from your perspective as owner, the structure of a dam is the starting point for thorough understanding of the poten-tials for failure. Thre Categories of Structural Failure : Three categories of structural failure alluded to in Chapter 2 are: • overtopping by flood • foundation defects • piping and seepage Overtopping may develop from many sources, but often evolves from inadequate spillway design. Alternatively, even an adequate spillway may become clogged with debris. In either situ-ation, water pours over other parts of the dam, such as abutments or the toe, and erosion and failure follow. Concrete dams are more susceptible to founda-tion failure than overtopping, whereas earthen dams suffer from seepage and piping. Overall, these three events have about the same incidence. A more specific analysis of the po-tential sources of failure has to take into account types of dams. Similarly, the characteristics of the type of dam being monitored will point to problems requiring more careful attention by the owner when developing a safety program. Failures : Embankment or Earthen Dams: The major reason for failure of fill or embankment dams is piping or seepage. Other hydrologic failures are significant as well, including overtopping and erosion from water flows. All earthen dams exhibit some seep-age; however, as discussed earlier, this seepage can and must be controlled in velocity and amount. Seepage occurs through the structure and, if uncontrolled, can erode material from the down-stream slope or foundation backward toward the upstream slope. This “piping” phenomenon can lead to a complete failure of the structure. Piping action can be recognized by an increased seepage flow rate, the discharge of muddy or discolored water below the dam, sinkholes on or near the embankment, and a whirlpool in the reservoir (see Inset 3.1). Hydrologic failures of earthen dams result from the uncontrolled flow of water over the dam, around it, adjacent to it, or from the erosive action of water on the dam’s foundation. Earthen dams are particularly susceptible to hydrologic failure since most sediment erodes at relatively low water flow velocities. Once erosion has begun during overtopping, it is almost impossible to stop. In a very special case, a well-vegetated earthen em-bankment may withstand limited overtopping if water flows over the top and down the face as evenly distributed sheet and does not become concentrated in a single channel. Concrete Dams: Failure of concrete dams (see Inset 3.2) is primarily associated with foundation problems. Overtopping is also a significant cause again primarily when spillways are built with in-adequate capacity. Other causes include failure to let concrete set properly and earthquakes. 24 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Age and Its Relation to Failure Foundation failures occur relatively early in the life of a dam, whereas other causes generally take much longer to materialize. Thus, it is not surpris-ing that a very large percentage of all dam failures occur during initial filling, since that is when design or construction flaws, or latent site defects, appear. As dams age, maintenance becomes more critical. Lack of maintenance will result in dete-rioration and eventually, failure. Oklahoma dams are aging as shown in Table 3.2, and problems as described above are slowly becoming apparent. INset 3.1. Examples of Earthen-Dam Failures Southfork, Pensylvania The famous Johnstown disaster, caused by the failure of the South Fork Dam in 1889, in which 2,209 people were killed, is an example of the overtopping of an earthen dam. Heavy rainfall in the upper drainage basin of the dam filled the reservoir and caused overtopping. It was later calculated that, if a spillway had been built according to specifications and if the original outlet pipes had been available for full capacity discharge, there would have been no overtop-ping. Teton Dam, Idaho The Teton Dam failure in 1976 was attributed to (1) internal erosion (piping) of the core of the dam deep in the right foundation key trench, with the eroded soil particles finding exits through channels in and along the interface of the dam with the highly pervious abutment rock and talus to points at the right groin of the dam; (2) destruction of the exit avenues and their removal by the outrush of reservoir water, (3) the existence of openings through inadequately sealed rock joints which may have developed through cracks in the core zone in the key trench; (4) the development of piping through the main body of the dam that quickly led to complete failure; and (5) the design of the dam did not adequately take into account the foundation conditions and the characteristics of the soil used for filling the key trench. Baldwin Hills and St. Francis Dams , California The Baldwin Hills Dam failed in 1963 follow-ing displacement of its foundation. Foundation problems were ultimately traced to seismic activity along nearby faults. The failure of the large St. Francis Dam (part of the water supply system for Los Angeles) in 1928 was also attrib-uted to a variety of problems related to founda-tion pressures, seepage around the foundation, and faulty operation. Source: Jansen, 1980. Oklahoma Water Resources Board - Dam Safety Program - March 2011 25 Inset 3.2. Examples of Concrete-Dam Failures Austin, Pensylvania An example of a foundation problem can be found in the failure of the Austin, Pennsylvania Dam in September, 1911. Evidently, the reservoir was filled before the concrete had set sufficiently. Eventual failure near the base occurred because of weakness in the foundation or in the bond between the foundation and the concrete. . Walnut Grove , Arizona In 1890, the Walnut Grove dam on the Hassayompa River failed due to overtopping, killing about 150 people. The failure was blamed on inadequate capacity of the spillway and poor construction and workmanship. A spillway 6 x 26 feet had been blasted out of rock on one abutment, but, with a drainage area above the dam site of about 500 square miles, the spillway did not have nearly enough discharge capacity. Source: Jansen, 1980. Dates Percent of Dams Constructed Prior to 1950 10.7 1950 – 1959 15.9 1960 – 1969 44.9 1970 – 1979 21.0 1980 – 1989 5.2 1990 – 1999 1.8 2000 – Present 0.6 TABLE 3.2. Ages of dams in Oklahoma Oklahoma Water Resources Board - Dam Safety Program - March 2011 27 Objectives of a Safety Program The pressing issue of dam failure points up the need for a safety program. You, the owner, should base your program on an evaluation of your dam’s structural and operational safety. Your program should iden-tify problems and recommend remedial repairs, operational restrictions and modifications, or further analyses and studies to determine solu-tions. Components of a safety program that address the spectrum of possible actions to be taken over the short and long-term include: • assessing the condition of the dam and its components • conducting preliminary and detailed inspections • identifying repairs and continuing maintenance needs • establishing periodic and continuous monitoring capabilities over the long-term • establishing an emergency action plan to help minimize adverse impacts should the dam fail • establishing operations procedures which recognize dam failure hazards and risks • documenting the safety program so that the information established is available at times of need and can be readily updated Develop your safety program in phases, begin- CHAPTER 4: Developing a Personal Safety Program4 ning with collection and review of existing information, proceeding to detailed inspections and analyses, and culminating with formal documentation. You can accomplish much of the preliminary work personally, with the as-sistance of state and local agencies. However, depending upon the number and seriousness of problems identified by the initial assessment, you may require the professional assistance of qualified engineers and contractors. Guidelines for Assessing Existing Conditions The guidelines for assessing existing conditions involve a sequence of steps that will enable you, the owner, to secure the information you will need to determine whether subsequent detailed investigations, repairs, and maintenance are required. The steps include: • reviewing existing data • visiting the site • inspecting the dam • assessing significance of observed conditions • deciding what to do next Reviewing Existing Data : First and foremost, collect and review available information on the dam such as plans of its design, construction, 28 Oklahoma Water Resources Board - Dam Safety Program - March 2011 and operation. Maps of the site, watershed, and the downstream channel reaches are also valuable. Review the design of the dam and its appurtenant structures to assess its actual per-formance compared to that intended. Review engineering records originating during construction to verify that structures were con-structed as designed. Collect records of subse-quent construction modifications, as well as op-eration records that document the performance of the dam and reservoir. Review any previous emergency action plan to determine if it is up-to-date and workable. Incorporate all these records into a notebook or file; they are most important in establishing a safety program and serve as the basis for its supporting documenta-tion. (For help with the development of such documentation, refer to Chapters 5 through 10.) If no records exist, a detailed examination of the structure is appropriate. Visiting the Dam Site: Undoubtedly you know it well and have visited it many times, but in this visit there are particular things for you to look for. Take a fresh look at the dam structure and its surroundings from the view of their potential hazard. Inspecting the Dam: Also, take a detailed and systematic look at all components of the dam and reservoir system. The description of the site’s components in Chapter 2 should aid this inspection. (The descriptions are general, so bear in mind that dams and their components come in various shapes and sizes and differ greatly in detail). Features to inspect include : • access roads and highways • upstream slope • crest • downstream slope • left and right abutments • spillways • outlets • drains • reservoir area (exposed and submerged) area immediately downstream of the dam • downstream areas for change in hazard classification What to look for : • obvious deterioration • cracks and slumps • boiling seepage • less than obvious internal corrosion • weathering • settlement • foundation-rock deterioration • dissolution A dam can look stable and still be susceptible to failure from gradual deterioration of its internal structure. Regular and very detailed inspections (Chapter 5) and follow-up monitoring (Chapter 6) and maintenance (Chapter 7) are needed to ensure maximum safety. Oklahoma Water Resources Board - Dam Safety Program - March 2011 29 Assesing Significance of Observed Conditions: Chapter 5 presents detailed information on conducting inspections and assessing the significance of conditions you observe. Typically, eroded areas, seepage, slides, and outflow draw the most attention. Deciding What to Do Next: Your dam safety program is now off to a good start. Available infor-mation on design and construction of the dam and later structural modifications provides perspective on its existing condition relative to that intended. If no documentation exists, then development of equivalent details should be a first priority. Inspec-tion and documentation assistance is available from several sources, including the Oklahoma Water Re-sources Board, the state agency responsible for dam safety. Professional engineering consultants can also perform detailed inspections, testing, and analyses, and create documentation (Chapter 10). Procedural Guidelines – A Source Book This chapter provides an overview of how to establish a safety program. Subsequent chapters detail technical and procedural steps of the pro-gram components. They include: • detailed inspection guidelines (Chapter 5) • monitoring and instrumentation guidelines (Chapter 6) • maintenance guidelines (Chapter 7) • emergency action guidelines (Chapter 8) • operations guidelines (Chapter 9) These program components can be visualized as a sequence of initial and continuing activities to insure dam safety. The flowchart illustrates the cyclical nature of the program and the need for continuing vigilance. Emergency action can, it is hoped, be avoided, but a well thought out plan of action (Chapter 8) in case of imminent or actual failure can greatly reduce damage and loss of life. Figure 4.1 Flow Chart of Dam Safety Pro gram Components Documenting the Safety Program It is important to document a safety program in order to make the best use of reliable information about the dam. The procedural guidelines that follow can serve as an outline or table of contents for a safety program report. The operations plan (Chapter 9) presents a detailed outline of the infor-mation that should be included in the documen-tation. The chapters that follow suggest forms for inspections, monitoring, etc., which can be used to record information. It is helpful to maintain all the material in a single notebook or file that is easily accessible so that it can be updated and is available when needed. Store a duplicate copy of the report at a different location. INSPECTION OPERATIONS MONITORING rEPAIRS & mAINTENANCE EMERGENCY ACTION Oklahoma Water Resources Board - Dam Safety Program - March 2011 31 CHAPTER 5: Inspection Guidelines 5 Introduction An effective inspection program is essen-tial for identifying problems and pro-viding safe maintenance of a dam. An inspection program should involve four types of inspections: (1) periodic technical inspec-tions; (2) periodic maintenance inspections; (3) downstream development inspections; (4) informal observations by project personnel as they operate the dam. Technical inspections must be performed by professional engineers familiar with the design and construction of dams and should include assessments of structure safety. Maintenance inspections are performed more frequently than technical inspections in order to detect, at an early stage, any developments that may be detrimental to the dam. They involve assessing operational capability as well as structural stability. Maintenance inspections are included as part of the more comprehensive technical inspection. Downstream hazard verification inspections are performed by the dam owner to determine if there has been any construction of homes, buildings, or other structures downstream of their dam which could affect the hazard classification of their dam. This is a particular problem for low hazard-potential dams. If a house, other inhabited structure, or other construction is built downstream of a dam this could result in the need to reclassify to a higher hazard-potential class. This has important implications for the dam owner as it could result in a change how often the dam must be inspected and require structural changes to the dam. Structural changes could include the amount of freeboard that must be maintained and the amount of water the spillway must be able to pass. Informal inspections are actually a continuing effort by the dam owner’s on-site project personnel (dam tenders, powerhouse operators, maintenance workers) performed in the course of their normal duties. The continued effectiveness of these inspections requires education of new personnel. Regular visual inspections are among the most economical means you, the owner, can use to ensure the safety and long life of your dam and its immediate environment. Visual inspection is a straightforward procedure that can be used by any properly trained person to make a reasonably accurate assessment of a dam’s condition. Technical and maintenance inspections involve careful examination of the surface and all parts of the structure, including its adjacent environment, by a professional engineer. The equipment required is not expensive, and the inspection usually can be completed in less than one day (see Appendix A). Hazard verification and informal inspections can be performed by the dam owner or their operator. A dam owner, by applying the maximum prudent effort, can identify any changes in previously noted conditions that may indicate a safety problem. Quick, corrective action to conditions requiring attention will promote the safety and extend the useful life of your dam, while possibly preventing costly future repairs. 32 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Recording Inspection Observations : An accurate and detailed description of conditions during each inspection will enable meaningful comparison of conditions observed at different times. The inspector should record all measure-ments and observed details required for an ac-curate picture of a dam’s current condition and possible problems. Using the forms discussed in Chapter 9, and given in the appendices, will help record the details. This information has three elements: Location : Accurately describe the location of any questionable area or condition so that it can be evaluated for changes over time or re-examined by experts. Photographs should be taken of the upstream and downstream embankments, outlet and conduit structures, emergency spillway, and toe of the dam, as well as photographs of any specific problem areas. Record the location along the dam, as well as the distance above the toe or below the crest. Similarly, document the location of problems in the outlet or spillway. Extent or area: The length, width, and depth (or height) of any suspected problem area should be determined. Organizing for Inspection The following discussion is concerned primarily with technical inspections. All inspections should be organized and systematic. Inspectors should use equipment appropriate for the task, record observations accurately, and survey the structure and site comprehensively. It is essential that documentation be developed and maintained in order to ensure adequate follow-up and repair (Appendix A). Chapter 9 further discusses what form this documenta-tion should take. Technical inspections are to be conducted annually for high hazard-potential dams and once every three years for significant hazard-potential dams. Inspection reports are to be submitted to the Oklahoma Water Resources Board, Dam Safety Program. Owners of low hazard-potential dams are only required to submit a downstream hazard-potential verifica-tion and maintenance inspection once every five years. Descriptive detail : Give a brief yet detailed description of any anomalous condition. Some items to include are: • quantity of drain outflows • quantity of seepage from point and area sources • color or quantity of sediment in water • depth of deterioration in concrete • length, displacement, and depth of cracks • extent of moist, wet, or saturated areas • adequacy of protective cover • adequacy of surface drainage • steepness or configuration of slopes • apparent deterioration rate • changes in conditions Coverage : An inspection is conducted by walking along and over a dam as many times as is required to observe the entire structure. From any given location, a person can usually gain a detailed view for 10 to 30 feet in each direction, depending upon the smoothness of the surface Oklahoma Water Resources Board - Dam Safety Program - March 2011 33 or the type of material (grass, concrete, riprap, brush) on the surface. On the downstream slope, a zigzag inspection path will ensure that any cracking is detected. Sequence: The following inspection sequence ensures that systematic coverage of an entire site is obtained: • upstream slope • crest • downstream slope • seepage areas Following a consistent sequence lessens the chance of an important condition being overlooked. Reporting inspection results in the same sequence is recommended to ensure consistent records. Inspection forms are included in Appendix A. The forms should be supplemented with additional details specific to a given dam. Record keping: The inspector should fill out a dated report for each inspection, which should be filed along with any photographs taken (which should also be dated). In addition to inspection observations, monitoring measurements and weather conditions (especially recent rains, extended dry spells, and snow cover) should also be systematically included in the inspec-tion record. A sketch of the dam with problem areas noted is helpful. Immediately following an inspection, observations should be compared with previous records to see if there are any trends that may indicate developing problems. If a questionable change or trend is noted, and failure is not imminent, you, the owner, should consult a professional engineer experienced in dam safety. Reacting quickly to questionable conditions will ensure the safety and long life of a dam and possibly prevent costly repairs or expensive litigation. Crucial inspection times: There are at least six special times when an inspection is recom-mended regardless of the regular schedule: (1) Prior to a predicted major rainstorm: check spillway, outlet channel, and riprap. (2) During or after a severe rainstorm: check spillway, outlet channel, and riprap. (3) During or after a severe windstorm: check riprap performance during the storm (if possible) and again after the storm has subsided. (4) Following an earthquake in the area: make a complete inspection immediately after the event and weekly inspections for the next several months to detect any delayed effects. (5) During construction, repairs, or modification of the dam. (6) During and immediately after the first reservoir filling: schedule a regular program of frequent complete inspections during the period a reservoir is first being filled to ensure that design and site conditions are as predicted. An inspection and filing schedule are frequently prescribed by the design engineer. Embankment Dams and Structures Embankment dams constitute the majority of structures in place in the U.S. The major features include: • upstream slope • downstream slope • crest • seepage areas • spillway Many of the principles and guidelines presented in that section are also aplicable to concrete structures . ! • inlet • outlet • spillway 34 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Upstream Slope : Typically, major problems encountered on an upstream slope are: • cracks • slides • cave-ins or sinkholes • severe erosion The first three conditions may indicate serious problems within the embankment. Severe erosion obviously can weaken the structure. An upstream slope should receive a close inspection because riprap, vegetative cover, and high water levels can hide problems. (When walking on riprap, take caution to avoid personal injury.) Slope protection is designed to prevent erosion of the embankment slopes, crest, and groin areas. Inadequate slope protection usually results in deterioration of the embankment from erosion, and in the worst cases, can lead to dam failure. The inspector should look for inadequate slope protection, including eroded vegetative cover and displaced riprap. The two primary types of slope protection used on embankment dams include vegetative cover (grass) and riprap (rock). Grass cover is usually used on most embankment surfaces, while riprap is commonly used on the shoreline of the upstream slope. Soil, cement, concrete, asphalt, articulated concrete blocks, and other types of slope protection also may be used. The type of slope protection selected depends upon economics, how the dam is used, and the prevailing conditions found at the site. A good growth of grass on an embankment provides excellent protection against erosion caused by rainfall and runoff. Deep rooted grass that can tolerate repeated wetting and drying cycles should be used on embankments. A lack of vegetative cover or insufficient vegetative cover will result in rapid deterioration of the embankment by erosion. A lack of riprap, or improperly designed riprap along the shoreline can result in erosion of the shoreline soils if riprap is needed to protect the soil against wave action. It should be noted that not all dams will require riprap shoreline protection. A crisscross path should be used when inspecting the slope so that cracks and slides can be easily identified. In many instances, sighting along the waterline alignment will indicate a change in the uniformity Figure 5.1. Drawin g of typical Embankment Dam Features . Oklahoma Water Resources Board - Dam Safety Program - March 2011 35 of the slope; an inspector should stand at one end of the dam and sight along the waterline, checking for straightness and uniformity. If a crack is seen, the crest and downstream slope in its immediate area should be carefully inspected. Cracks indicate possible foundation movement, embankment failure, or a surface slide. Locating them can be difficult. Cracks more than one foot deep usually are not produced by drying and are likely cause for concern. A line of recently dislodged riprap on an upstream slope could indicate a crack below the riprap. Slides can be almost as difficult to detect as cracks. When a dam is constructed, the slopes may not be uniformly graded. Familiarity with the slope configuration at the end of construction can help identify subsequent slope movements. Moreover, the appearance of slides may be subtle; for example, they may produce only about two feet of settlement or bulging in a distance of 100 feet or more, yet that would still be a significant amount of settlement. Dated photographs are particularly helpful in detecting such changes. Sinkholes or cave-ins result from internal erosion of the dam—a very serious condition for earthen embankments. The internal erosion, or piping, may be reflected by turbid seepage water on exit. Surface soil may be eroded by wave action, rain runoff, and animal burrowing. Such erosion, if allowed to continue, can lessen the thickness of the embankment and weaken the structure. Animal burrows on the upstream slope can also indicate a serious problem on smaller dams. Beavers, nutria, and other burrowing animals can create pathways for seepage. See Chapter 7. To ensure adequate inspection, prevent potential seepage paths, and keep the upstream slope free from obscuring weeds, brush, or trees. Downstream Slope : The downstream slope should be inspected carefully because it is the area where evidence of developing problems appears most frequently. To ensure adequate Figure 5.2. figure of various problems with an embankment dam . no trash rack: principal spillway intake structure partially blocked with debris THE PROBLEM DAM ruts from traffic with gullies forming Settlement Surface crack Trees and brush Animal burrows Emergency spillway too small Emergency spillway partially blocked Downstream channel partially blocked Sloughed area springs and excessive seepage 36 Oklahoma Water Resources Board - Dam Safety Program - March 2011 inspection, keep this area free from obscuring weeds, brush, or trees. On the downstream slope, some of the more threatening conditions that could be identified are: • cracks • slides • seepage Notify the designated dam-safety authorities immediately if any of these conditions (Fig. 5.3) are noted on the downstream slope. Cracks can indicate settlement, drying and shrinkage, or the development of a slide. Whatever the cause, cracks should be monitored and changes in length and width noted. Drying cracks may appear and disappear seasonally and normally will not show vertical displacement as will settlement cracks or slide cracks. Slides require immediate detailed evaluation. Early warning signs include a bulge in the embankment near the toe of a dam or vertical displacement in the upper portion of an embankment. Seepage is discussed separately. If a downstream slope is covered with heavy brush or vegetation, a more concentrated search must be made and may require cleaning off the vegetation. In addition, the downstream slope should be inspected for animal burrows, excessive vegetative cover, and for erosion, especially at the contacts with the abutments. Figures 5.1 & 5.2 show potential problems with the downstream slope, causes, possible consequences, and recommended action. Crest: A dam’s crest usually provides the pri-mary access for inspection and maintenance. Because surface water will pond on a crest unless that surface is well maintained, this part of a dam usually requires periodic re-grading. However, problems found on the crest should not be simply graded over or covered up. On the crest, some of the more threatening conditions that may be identified are: • longitudinal cracking • transverse cracking • misalignment • sinkholes Longitudinal cracking (Figure 5.3) can indicate localized instability, differential settlement, movement between adjacent sections of the embankment, or any combination of the three. Longitudinal cracking is typically characterized by a single crack or a close, parallel system of cracks along the crest, more or less parallel to the axis of the dam. These cracks, which are usually continuous over their length and usually greater than one foot deep, can be differentiated from drying cracks, which are usually intermittent, erratic in pattern, shallow, very narrow, and numerous. Longitudinal cracking may precede vertical displacement as a dam attempts to adjust to a position of greater stability. Frequently, longitudinal cracking occurs at the edge of the crest with either slope. Vertical displacements on the crest are usually accompanied by displacements on the upstream or downstream face of a dam. Vertical Displacement Figure 5.3. Longitudinal cracks Oklahoma Water Resources Board - Dam Safety Program - March 2011 37 Transverse cracking (Figure 5.4) can indicate differential settlement or movement between adjacent segments of a dam. Transverse cracking usually manifests as a single crack or a close, parallel system of cracks that extend across the crest more or less perpendicular to the length of the dam. This type of cracking is usually greater than one foot in depth. If this condition is seen or suspected, notify the Oklahoma Dam Safety Program office immediately. Transverse cracking poses a definite threat to the safety and integrity of a dam. If a crack should progress to a point below the reservoir water-surface elevation, seepage could progress along the crack and through the embankment, causing severe erosion and—if not corrected—leading to failure of the dam. Misalignment can indicate relative movement between adjacent portions of a dam—generally perpendicular to its axis. Excessive settlement of dam material, the foundation, or both can also cause misalignment. Most problems are usually detectable during close inspection. Misalignment may, however, only be detectable by viewing a dam from either abutment. If on close inspection the crest appears to be straight for the length of the structure, alignment can be further checked by standing away from the dam on either abutment and then sighting along the upstream and downstream edges of the crest. On curved dams, alignment can be checked by standing at either end of a short segment of the dam and sighting along the crest’s upstream and downstream edges, noting any curvature or misalignment in that section. Leaning utility poles or poles used for highway barriers can also indicate movement. Sinkholes can indicate internal collapse, piping, or the presence of animal dens. The formation or progression of a sinkhole is dangerous because it poses a threat to inspectors or vehicles traversing the crest. A sinkhole collapse can also lead to a flow path through a dam, which can create an uncontrolled breach. The crest should be inspected for animal burrows, low areas, vegetative cover, erosion, sloping of the crest, narrowing of the crest, and traffic ruts. Seepage Areas: As discussed previously, al-though all dams have some seepage, seepage in any area on or near a dam can be dangerous, and all seepage should be treated as a potential problem. Wet areas downstream from dams are not usually natural springs, but seepage areas (Figure 5.5). Seepage must be controlled in both velocity and quantity. High-velocity flows through a dam can cause progressive erosion and, ultimately, failure. Saturated areas of an embankment or abutment can move in massive slides and thus also lead to failure. Seepage can emerge anywhere on the downstream face of a dam, beyond the toe, or on the downstream abutments at elevations below normal reservoir levels. A potentially dangerous condition exists when seepage appears on the downstream face above the toe of a dam (Figure 5.6). If seepage is found on the top half of the downstream slope, the problem should be immediately corrected. Seepage Figure 5.4. transverse cracks . Initial Transverse Cracking Progression of Transverse Crack to a Point Below the Waterlines 38 Oklahoma Water Resources Board - Dam Safety Program - March 2011 on the downstream slope can cause a slide or failure of the dam by internal erosion (piping). Evidence of seepage may vary from a soft, wet area to a flowing spring and may appear initially as only an area where vegetation is lush and dark green in color. Cattails, reeds, mosses, and other marsh vegetation often become established in seepage areas. Downstream abutment areas should always be inspected closely for signs of seepage, as should the area of contact between an embankment and a conduit spillway, drain, or other appurtenant structures and outlets. Slides in the embankment or an abutment may be the result of seepage causing soil saturation and high pore pressures. Since seepage can be present but not readily visible, an intensive search should be made of all downstream areas where seepage water might emerge. Even in short grass cover, seepage may not be visible and must be walked on to be found. Ideally, an inspection for seepage should be made when a reservoir is full. Concrete Dams and Structures From a safety standpoint, the principal advantage of concrete over earthen dams is their relative freedom from failure by erosion during overtopping as well as from embankment slides and piping failures. Although concrete dams comprise a minority of all dams, they are commonly of greater height and storage capacity than earthen structures. Thus, they often represent a potentially greater hazard to life and property. It is important that concrete-dam owners be aware of the principal modes of failure of such dams and that they be able to discern between conditions which threaten the safety of the dam and those that merely indicate a need for maintenance. Concrete dams fail for reasons that are significantly different from earth dams. These include: • structural cracks • foundation and abutment weakness • deterioration due to alkali-aggregate reaction If any of these conditions are discovered during inspection, an owner should immediately address the problem with his/her engineer. Structural cracks occur when portions of the dam are overstressed; they result from inadequate design, poor construction, foundation settlement, or faulty materials. Structural cracks are often irregular, may run at an angle to the major axes of the dam and may exhibit abrupt changes in direction. These cracks can also be noticeably displaced, radially, transversely, or vertically. Concrete dams transfer a substantial load to the abutments and foundation. Although the concrete of a dam may endure, the natural abutments or foundation may crack, crumble, or move in a massive slide. If that occurs, support for the dam is lost and it fails. Impending failure of the foundation or abutments may be difficult to detect because initial movements are often very small. Figure 5.5. area of sepa ge near toe of dam . Oklahoma Water Resources Board - Dam Safety Program - March 2011 39 Severe deterioration can result from a chemical reaction between alkali present in cements and certain forms of silica present in some aggregates. This chemical reaction produces by-products of silica gels, which cause expansion and loss of strength within concrete. An alkali reaction is characterized by certain observable conditions such as cracking (usually a random pattern on a fairly large scale), and by excessive internal and overall expansion. Additional indications include the presence of a gelatinous exudation or whitish amorphous deposits on the surface and a chalky appearance in freshly fractured concrete. The alkali-aggregate reaction takes place in the presence of water. Surfaces exposed to the elements or dampened by seepage will deteriorate most rapidly. Once suspected, the condition can be confirmed by a series of tests performed on core samples drilled from a dam. Although the deterioration is gradual, an alkali-aggregate reaction cannot be economically corrected by any means now known. Continued deterioration may require total replacement of a structure. Inspection of a concrete dam is similar to that of an earthen dam. However, the following additional items should be considered: • access and safety • monitoring • outlet system • cracks at construction and expansion joints • shrinkage cracks • deterioration due to spalling • minor leakage Access and safety are important because the faces of concrete dams are often nearly vertical, and sites are commonly steep-walled rock canyons. Access to the downstream face, toe area, and abutments of such dams may be difficult and require special safety equipment, such as safety ropes or a boatswain’s chair. Concrete dams pose a special problem for the dam owner because of the difficulty in gaining close access to the steep surfaces. Regular inspection with a pair of powerful binoculars can initially identify areas where change is occurring. When changes are noted, a detailed, close-up inspection should be conducted. Close inspection of the upstream face may also require a boatswain’s chair or a boat. Figure 5.6. sepa ge throu gh embankment dam . Uncontrolled Seepage Through and Embankment Reservoir water surface sepage surface foundation sepage sepage 40 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Monitoring helps detect structural problems in concrete dams such as cracks in the dam, abutments, or foundation. Cracks may develop slowly at first, making it difficult to determine if they are widening or otherwise changing over time. If a structural crack is discovered, it should be monitored for changes in width, length, and offset, and a network of monitoring instruments should be installed and read regularly. Outlet-system deterioration is a problem for all dams, but the frequency of such damage may be higher in concrete dams because of their greater average hydraulic pressure. Thus, outlet-system inspection should be emphasized for large concrete dams. Cracks at construction joints exist because concrete dams are built in segments, while expansion joints—referred to as “designed” cracks—are built into dams to accommodate volumetric changes which occur in the structures after concrete placement. These joints are typically constructed so that no bond or reinforcing, except non-bonded water stops and dowels, extend across the joints. Shrinkage cracks often occur when, during original construction, irregularities or pockets in the abutment contact are filled with concrete and not allowed to cure fully prior to placement of adjacent portions of the dam. Subsequent shrinkage of the concrete may lead to irregular cracking at or near the abutment. Shrinkage cracks are also caused by temperature variation. During winter months, the upper portion of a dam may become significantly colder than those portions in direct contact with reservoir water. This temperature differential can result in cracks which extend from the crest for some distance down each face of the dam. These cracks will probably occur at construction or expansion joints, if any. Shrinkage cracks can be a sign that certain portions of the dam are not carrying the design load. In such cases, the total compression load must be carried by a smaller proportion of the structure. It may be necessary to restore load-carrying capability by grouting affected areas. This work requires the assistance of an engineer. Spalling is the process by which concrete chips and breaks away as a result of freezing and thawing, corrosion of the reinforcement, or movement. Almost every concrete dam in colder climates experiences continued minor deterioration due to spalling. Because it usually affects only the surface of a structure, it is not ordinarily considered dangerous. However, if allowed to continue, spalling can result in structural damage, particularly if a dam is thin in cross-section. Repair is also necessary when reinforcing steel becomes exposed. The method of repairing spalled areas depends upon the depth of the deterioration. In severe situations, engineering assistance is required. Minor leakage through concrete dams, although unsightly, is not usually dangerous unless accompanied by structural cracking. The effect may be to promote deterioration due to freezing and thawing. However, increases in seepage could indicate that, through chemical action, materials are being leached from the dam and carried away by the flowing water. Dam owners should note that decreases in seepage can also occur as mineral deposits are formed in portions of the seepage channel. In either case, the condition is not inherently dangerous and detailed study is required to determine if repair is necessary for other than cosmetic reasons. Spillways As detailed in Chapter 2, the main function of a spillway is a safe exit for excess water in a reservoir. If a spillway is too small, a dam could be overtopped and fail. Similarly, defects in a spillway can cause failure by rapid erosion. A spillway should always be kept free of obstructions, have the ability to resist erosion, and be protected from deterioration. Because dams represent a substantial investment and spillways make up a major part of dam costs, Oklahoma Water Resources Board - Dam Safety Program - March 2011 41 a conscientious annual maintenance program should be pursued not only to protect the public but to minimize costs as well. The primary problems encountered with spillways include: • inadequate capacity • obstructions • erosion • deterioration • cracks • open joints • undermining of the spillway outlet • deterioration of spillway gates Inadequate capacity is determined by several factors, such as the drainage area served, the magnitude or intensity of storms in the watershed, the storage capacity of the reservoir, and the speed with which rainwater flows into and fills the reservoir. An inadequate spillway can cause the water in a reservoir to overtop the dam. Obstruction of a spillway is commonly due to excessive growth of grass and weeds, thick brush, trees, debris, fences across channels to prevent migration of fish, or landslide deposits. An obstructed spillway can have a substantially reduced discharge capacity which can lead to overtopping of the dam. Grass is usually not considered an obstruction; however, tall weeds, brush, and young trees should periodically be cleared from spillways. Similarly, any substantial amount of soil deposited in a spillway—whether from sloughing, landslide or sediment transport—should be immediately removed. Timely removal of large rocks is especially important, since they can obstruct flow and encourage erosion. Erosion of a spillway may occur during a large storm when large amounts of water flow for many hours. Severe damage of a spillway or complete washout can result if the spillway cannot resist erosion. If a spillway is excavated out of a rock formation or lined with concrete, erosion is usually not a problem. However, if a spillway is excavated in sandy soil, deteriorated granite, clay, or silt deposits, protection from erosion is very important. Deterioration of a spillway can greatly affect its performance. Generally, resistance to deterioration can be increased if a spillway channel has a mild slope, or if it is covered with a layer of grass or riprap with bedding material. Examples of spillway deterioration may include collapse of side slopes, cracking or undermining of concrete lining, erosion of the approach section, chute channel, stilling basin, and discharge channel. These problems can cause water to flow under and around the protective material and lead to severe erosion. Remedial action must be taken as soon as any sign of deterioration has been detected. Cracks in an earthen spillway channel are usually not regarded as a functional problem. However, missing rocks in a riprap lining can be considered a crack in the protective cover, and must be repaired at once. Cracks in concrete lining of a spillway are commonly encountered. These cracks may be caused by uneven foundation settlement, shrinkage, slab displacement, or excessive earth or water pressure. Large cracks will allow water to wash out fine material below or behind the concrete slab, causing erosion, more cracks, and even severe displacement of the slab. The slab may even be dislodged and washed away by the flow. A severely cracked concrete spillway should be examined by and repaired under the supervision of an engineer. Open or displaced joints can occur from excessive and uneven settlement of the foundation or the sliding of a concrete slab. In some cases, a construction joint is too wide or has been left unsealed. Sealants deteriorate and wash away. Water can flow through the joints, undermining the slabs, which in turn could result in collapse of the spillway slabs. Pressures resulting from water flowing over the open slabs could also result in lifting and displacement of slabs. Hence, all joints need to be sealed and kept sealed. 42 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Undermining of the spillway outlet is the erosion of foundation material and may weaken support and cause further cracks. Pressure induced by water flowing over displaced joints may wash away part of a wall or slab, or cause extensive undermining. Undermining of a spillway causes erosion at a spillway outlet, whether it is a pipe or overflow spillway, and is one of the most common spillway problems. Severe undermining of the outlet can displace sections of pipe, cause slides in the downstream embankment of the dam, and eventually lead to complete failure of a dam. Water must be conveyed safely from the reservoir to a point downstream of the dam without endangering the spillway itself or the embankment. Often the spillway outlet is adequately protected for normal flow conditions, but not for extreme turbulent flows. It is easy to misestimate the energy and force of flowing water and the resistance of outlet material (earth, rock, concrete, etc). The required level of protection is difficult to establish by visual inspection but can usually be determined by hydraulic calculations performed by a professional engineer. Structures that completely control erosion at a spillway outlet are usually expensive, but often necessary. Less expensive protection can also be effective, but require extensive periodic maintenance as areas of erosion and deterioration develop. The following four factors, often interrelated, contribute to erosion at the spillway outlet: 1. Flows emerging from the outlet are above the stream channel. If outlet flows emerge at the correct elevation, tailwater in the stream channel can absorb a substantial amount of the high velocity. The flow and the hydraulic energy will be contained in the stilling basin. 2. Flows emerging from the spillway are generally free of sediment and therefore have substantial sediment-carrying capacity. In taking on sediment, moving water will scour soil material from the channel and leave eroded areas. Such erosion is difficult to design for and requires protection of the outlet for a safe distance downstream from the dam. 3. Flows leaving the outlet at high velocity can create negative pressures that can cause material to come loose and separate from the floor and walls of the outlet channel. This process is called cavitation when it occurs on concrete or metal surfaces. Venting can sometimes be used to relieve negative pressures. 4. Water leaking through pipe joints or flowing along a pipe from the reservoir may weaken the soil structure around the pipe. Inadequate compaction adjacent to such structures during construction can compound this problem. Deterioration of spillway gates can result in an inability of the gates to function during storm events. Causes of structural deterioration include, but are not limited to: 1. Corrosion can seriously weaken a structure or impair its operation. The effect of corrosion on the strength, stability, and serviceability of gates must be evaluated. A loss of cross section in a member causes a reduction in strength and stiffness that leads to increased stress levels and deformation without any change in the imposed loading. Flexure, shear, and buckling strength may be affected. A buildup of corrosion products can be damaging at connection details. For example, corrosion buildup in a tainter gate trunnion can lead to extremely high hoist loads. Localized pitting corrosion can form notches that may serve as fracture initiation sites, which could significantly reduce the member’s fatigue life. 2. Fracture usually initiates at a discontinuity that serves as a local stress raiser. Structural connections that are welded, bolted, or riveted are sources of discontinuities and stress concentrations. Oklahoma Water Resources Board - Dam Safety Program - March 2011 43 3. Fatigue is the process of cumulative damage caused by repeated cyclic loading. Fatigue damage generally occurs at stress-concentrated regions where the localized stress exceeds the yield stress of the material. Fatigue is particularly a concern with spillway gates with vibration problems. 4. Proper operation and maintenance of spillway gates are necessary to prevent structural deterioration. The following items are possible causes of structural deterioration. a. Weld repairs are often sources of future cracking or fracture problems, particularly if the existing steel had poor weldability. b. If moving connections are not lubricated properly, the bushings will wear and result in misalignment of the gate, resulting in wear of other parts and unforeseen loads. c. Malfunctioning limit switches could result in detrimental loads and wear. d. A coating system or cathodic protection that is not maintained can result in detrimental corrosion of metal components. 5. Unforeseen loading of a gate can result in deformed members or fracture. When structural members become deformed or buckled, they may have significantly reduced strength or otherwise impair the performance of the gate. Dynamic loading may be caused by hydraulic flow at the seals. Other unusual loadings may occur from malfunctioning limit switches or debris trapped at interfaces between moving parts. Unusual loads may also develop on gates supported by walls that are settling or moving. These unusual loads can cause overstressing and lead to deterioration. Mark Harrison , Oklahoma Conservation Comission 44 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Procedure for Inspection of the Spillway Spillway inspection is an important part of a dam safety program. Its basic objective is to detect any sign of obstruction, erosion, deterioration, misalignment, or cracking. An inspection of an earth spillway should determine whether side slopes have sloughed and whether there is excessive vegetation in the channel, and should look for signs of erosion and rodent activity. The inspector should also use a probe to determine the hardness and moisture content of the soil, note the location of particularly wet or soft spots, and see if the stilling basin or drop structure is properly protected with rocks or riprap. Because some erosion is unavoidable during spilling, an owner should also determine whether such erosion might endanger the embankment itself. If the spillway is installed with a sill or wall, a dam owner should also determine if there are any cracks or misalignment in the sill or wall and check for erosion beneath the sill or wall or downstream from it. Hairline cracks are usu-ally harmless. Large cracks should be carefully inspected and their location, width, length, and orientation noted. Deterioration should be de-termined. The concrete should be examined for exposure of reinforcing bars. Spillway surfaces exposed to freeze-thaw cycles often suffer from surface spalling. Chemical ac-tion, corrosion of the reinforcement, movement, contamination, and unsound aggregates can also cause spalling. If spalling is extensive, the spalled area should be sketched or photographed, show-ing its length, width, and depth. The problem should be examined closely to see if the remaining concrete has deteriorated or if reinforcing bars are exposed. The concrete should be tapped with a tapping device or rock hammer to determine if voids exist below the surface. Shallow spalling should be examined from time to time to determine if it is becoming worse. Deep spalling should be repaired as soon as possible by an experienced contractor. Walls of spillways are usually equipped with weep (or drain) holes. Occasionally spillway chute slabs are also equipped with weep holes. If all such holes are dry, the soil behind the wall or below the slab is probably dry as well. If some holes are draining while others are dry, the dry holes may be plugged by mud or mineral deposits. Plugged weep holes increase the chances for failure of retaining walls or chute slabs. The plugged holes should be probed to determine causes of blockage, and soil or deposits cleaned out to restore drainage. If that work is not successful, rehabilitate the drain sys-tem as soon as possible under the supervision of a professional engineer. Spillway retaining walls and chute slabs are normally constructed in sections. Between ad-joining sections, gaps or joints must be tightly sealed with flexible materials such as tar, epox-ies, or other chemical compounds. Sometimes rubber or plastic diaphragm materials or cop-per foil are used to obtain water tightness. Dur-ing inspection, one should note the location, Oklahoma Water Resources Board - Dam Safety Program - March 2011 45 length, and depth of any missing sealant, and probe open gaps to determine if soil behind the wall or below the slab has been undermined. Misalignment of spillway retaining walls or chute slabs may be caused by foundation settlement or earth or water pressure. The inspector should carefully look at the upstream or downstream end of a spillway near the wall to determine if it has been tipped inward or outward. Relative displace-ment or offset between neighboring sections can be readily identified at joints. The horizontal as well as vertical displacement should be measured. A fence on top of the retaining wall is usually erected in a straight line at the time of construc-tion; thus any curve or distortion of the fence line may indicate wall deformation. At the time of construction, the entire spillway chute should form a smooth surface. Thus, measurement of relative movement between neighboring chute slabs at joints will give a good indication of slab displacement. Misalignment or displacement of walls or the slab is often accompanied by cracks. A clear description of crack patterns should be recorded or photos taken to help in understanding the nature of the displacement. The folowing areas should be in-spected on al gates in spilways : • main framing members and lifting and support assemblies • locations susceptible to fracture or weld-related cracking • corrosion-susceptible areas—normal waterline, abrasion areas, crevices, areas where water could stand • lifting connections and chains or cables • trunnions • intersecting welds • previous cracks repaired by welding • locations of previous repairs or where damage has been reported • seal plates Inlets, Outlets, and Drains A dam’s inlet and outlet works, including internal drains, are essential to its operation. Items for inspection and special attention include: • reservoir pool levels • lake drains and internal drains • corrosion • trash racks on pipe spillways • cavitation • areas on gates and spillways as listed immediately above Reservoir pool level drawdown should not exceed about 1 foot per week for slopes composed of clay or silt materials except in an emergency. Very flat slopes or slopes with free-draining upstream soils can, however, withstand more rapid drawdown rates. Pool levels can be controlled by spillway gates, drain-and-release structures, or flashboards. Flashboards, sometimes used to permanently or temporarily raise the pool level of water supply reservoirs, should not be installed or allowed unless there is sufficient freeboard remaining to safely accommodate a design flood Conditions causing or requiring temporary or permanent adjustment of the pool level include: • A problem that requires lowering of the pool. Drawdown is a temporary solution until the problem is solved. • Release of water downstream to supplement stream flow during dry conditions. • Fluctuations in the service area’s demand for water. • Repair of boat docks in the winter and growth of aquatic vegetation along the shoreline. • Requirements for recreation, hydropower, or waterfowl and fish management. ! 46 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Lake drains : A lake drain should always be operable so that the pool level can be drawn down in case of an emergency or for necessary repair. Lake-drain valves or gates that have not been operated for a long time can present a special problem for owners. If the valve cannot be closed after it is opened, the impoundment could be completely drained. An uncontrolled and rapid drawdown could also cause more serious problems such as slides along the saturated upstream slope of the embankment or downstream flooding. Therefore, when a valve or gate is operated, it should be inspected and all appropriate parts lubricated and repaired. It is also prudent to advise downstream residents of large or prolonged discharges. Testing a valve or gate without risking complete drainage entails physically blocking the drain inlet upstream from the valve. Some drains have been designed with this capability and have dual valves or gates, or slots for stop logs (sometimes called bulkheads) upstream from the valve. Otherwise, divers can be hired to inspect the drain inlet and may be able to construct a temporary block at the inlet. Since that could be dangerous, safety precautions are needed. Other problems may be encountered when operating a lake drain. Sediment can build up and block the drain inlet, or debris can enter the valve chamber, hindering its function. The likelihood of these problems is greatly decreased if the valve or gate is operated and maintained on a schedule prepared by a professional engineer. Corrosion is a common problem of pipe spillways and other conduits made of metal. Exposure to moisture, acid conditions, or salt will accelerate corrosion. In particular, acid runoff from strip mine areas will cause rapid corrosion of steel pipes. In such areas, pipes made of noncorrosive materials such as concrete or plastic should be used. Metal pipes which have been coated to resist accelerated corrosion are also available. The coating can be of epoxy, aluminum, zinc (galvanization), asbestos or mortar. Coatings applied to pipes in service are generally not very effective because of the difficulty of establishing a bond. Similarly, bituminous coating cannot be expected to last more than one to two years on flow ways. Of course, corrosion of metal parts of operating mechanisms can be effectively treated and prevented by keeping those parts greased and/or painted. Corrosion can also be controlled or arrested by installing cathodic protection. A sacrificial metallic anode made out of a material such as magnesium is buried in the soil and is connected to the metal pipe by wire. An electric potential is established which causes the magnesium to corrode and not the pipe. Trash on pipe spilways : Many dams have pipe and riser spillways. As with concrete spillways, pipe inlets that become plugged with debris or trash reduce spillway capacity. As a result, the potential for overtopping is greatly increased, particularly if there is only one outlet. A plugged principal spillway will cause more frequent and greater than normal flow in the emergency spillway which is de-signed for infrequent flows of short duration Figure 5.7. Keeping the trash rack fre of debris reduces the chance of overtoppin g the dam . Oklahoma Water Resources Board - Dam Safety Program - March 2011 47 and thus result in serious and unnecessary damage. For these reasons trash collectors or trash racks should be installed at the inlets to pipe spillways and lake drains (Figure 5.7). A well-designed trash rack will stop large debris that could plug a pipe but allow unrestricted passage of water and smaller debris. Some of the most effective racks have submerged openings which allow water to pass beneath the trash into the riser inlet as the pool level rises. Openings that are too small will stop small debris such as twigs and leaves, which in turn will cause a progression of larger items to build up, eventually completely blocking the inlet. Trash rack openings should be at least 6 inches across, regardless of the pipe size. The larger the principal spillway conduit, the larger the trash rack opening should be. The largest possible openings should be used, up to a maximum of about 2 feet. A trash rack should be properly attached to the riser inlet and strong enough to withstand the forces of fast-flowing debris, heavy debris, and ice. It is a common occurrence for vandals to throw riprap stone into the riser. The size of the trash rack openings should not be decreased to prevent this. Instead, use riprap that is larger than the trash rack openings or too large to handle. Maintenance should include periodic checking of the trash rack for rusted and broken sections and repair as needed. The rack should be checked frequently during and after storms to ensure that it is functioning properly and to remove accumulated debris. Cavitation : When water flows through an out-let system and passes restrictions (e.g., valves), the pressure may drop. If localized water pres-sures drop below the vapor pressure of water, a partial vacuum is created and the water actually boils, causing shock-waves which can damage the outlet pipes and control valves. This process can be a serious problem for large dams where discharge velocities are high. Testing the Outlet System All valves should be fully opened and closed at least once a year. This not only limits corrosion buildup on control stems and gate guides, but also provides an opportunity to check for smooth operation of the system. Jerky or erratic operation could signal problems, and indicate the need for more detailed inspection. The full range of gate settings should be checked. The person performing the inspection should slowly open the valve, checking for noise and vibration. Certain valve settings may result in greater turbulence. The inspector should also listen for noise like gravel being rapidly transported through the system. This sound would indicate some cavitation and henceforth, those gate settings should be avoided. The operation of all mechanical and electrical systems, backup electric motors, power generators, power and lighting wiring associated with the outlet should all be checked. Inspecting the Outlet System Accessible portions of the outlet, such as the outfall structure and control, can be inspected easily and regularly. However, severe problems are commonly associated with deterioration or failure of portions of the system either buried in the dam or normally under water. • Outlet pipes 30 inches or greater in diameter can be inspected internally, provided the system has an upstream valve allowing the pipe to be emptied. Tapping the conduit interior with a hammer can help locate voids behind the pipe. This type of inspection should be performed at least once a year. • Small-diameter outlet pipes can be inspected by remote TV camera if necessary. The camera is channeled through the conduit and transmits a picture back to an equipment 48 Oklahoma Water Resources Board - Dam Safety Program - March 2011 truck. This type of inspection is expensive and usually requires the services of an engineer. However, if no other method of inspection is possible, inspection by TV is recommended at least once every five years. • Outlet intake structures, wet wells, and outlet pipes with only downstream valves are the most difficult dam appurtenances to inspect because they are usually under water. These should be inspected whenever the reservoir is drawn down or at five-year intervals. If a definite problem is suspected, or if the reservoir remains full over extended periods, divers should be hired to perform an underwater inspection. General Areas Other areas requiring inspection include: • mechanical and electrical systems • the reservoir surface and shoreline • the upstream watershed • downstream floodplains Mechanical equipment includes spillway gates, sluice gates or valves for lake drains or water supply pipes, stop logs, sump pumps, flashboards, relief wells, emergency power sources, siphons, and other devices. All mechanical and ass
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Title | Dam Safety Guidance Manual 2011 |
OkDocs Class# | W1700.5 D154s 2011 |
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ODL electronic copy | Downloaded from agency website: www.owrb.ok.gov/hazard/dam/pdf.../DamSafetyGuidanceManual2011.pdf |
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Full text | Dam Safety Guidance Manual Published by the Oklahoma Water Resources Board Dam Safety Program 2011 For Oklahoma Dam Owners FPO Mark Harrison, Oklahoma Conservation Commission Oklahoma Water Resources Board - Dam Safety Program - March 2011 1 Acknowledgement Dam safety has attracted a great deal of attention in recent years, and in preparing this manual, information from a number of sources was used. The National Dam Safety Program, instituted in response to several major dam failures in the early 1970’s, fo-cused on the problem nationwide. Federal Emergency Management Agency (FEMA) has taken the lead in providing assistance to states in promoting dam safety. The National Dam Safety Program Act of 1996 continues to reinforce the commitment by the Federal Government to dam safety. Special recognition is given to the Federal Emergency Management Agency (FEMA) and the Association of State Dam Safety Officials (ASDSO) for their leadership in developing effective dam safety programs and policies for the furtherance of dam safety. Their diligence in assisting the U.S. dam safety community was an important factor in the issuance of the FEMA grant. The cooperation of the owners of dams within Oklahoma is essential to the success of the State’s dam safety effort. The ultimate purpose of such a program is the protection of the lives and property of citizens of Oklahoma. Cover and other photographs courtesy of Oklahoma Conservation Commission. This publication was funded by a grant from the Federal Emergency Management Agency (FEMA), National Dam Safety Program Grant Agreement 2010-RC-50-0006. Cover Photo: Kadashan No. 2 in Wagoner County Courtesy Mark Harrison, Oklahoma Conservation Commission 2 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Contents DAM SAFETY: AN OWNER’S GUIDANCE MANUAL................................4 Introduction to Dam Safety......................................................4 Hazards, Risk, Failures...............................................................5 Inspection Guidelines................................................................5 Instrumentation and Monitoring Guidelines.........................5 Maintenance Guidelines............................................................6 Emergency Action Plan Guidelines.........................................6 Operation Plan Guidelines........................................................7 Measures to Reduce the Consequences of Dam Failure..................................................7 Water Retention Ability...........................................................15 Seepage Through a Dam.........................................................16 Seepage Around a Dam...........................................................16 Release of Water........................................................................16 Principal or Mechanical Spillway...........................................17 Drawdown Facility...................................................................17 Emergency (Auxiliary) Spillway.............................................17 CHAPTER 3: HAZARDS, RISKS, FAILURES.........................................19 General.......................................................................................19 Hazards as Sources of Risks....................................................19 Natural Hazards That Threaten Dams..................................20 Hazards from Human Activity...............................................21 Site-Specific Structural Risk....................................................22 Sources of Dam Failure............................................................23 Three Categories of Structural Failure..................................23 Failures.......................................................................................23 Age and Its Relation to Failure...............................................25 CHAPTER 4: DEVELOPING A PERSONAL SAFETY PROGRAM..........27 Objectives of a Safety Program...............................................27 Guidelines for Assessing Existing Conditions......................27 Procedural Guidelines – A Source Book...............................29 Documenting the Safety Program..........................................29 CHAPTER 5: INSPECTION GUIDELINES.............................................31 Introduction..............................................................................31 Organizing for Inspection.......................................................32 Embankment Dams and Structures.......................................33 Upstream Slope.........................................................................34 Downstream Slope...................................................................35 Crest...........................................................................................36 Seepage Areas............................................................................37 Concrete Dams and Structures...............................................38 Spillways....................................................................................40 Procedure for Inspection of the Spillway..............................44 Inlets, Outlets, and Drains......................................................45 Inspecting the outlet system...................................................47 General Areas............................................................................48 DAM SAFETY: AN OWNER’S GUIDANCE MANUAL CHAPTER 1: AN APPROACH TO DAM SAFETY....................................9 General.........................................................................................9 Urgency for Safety......................................................................9 Dam Ownership and Safety......................................................9 Role of the Dam Owner in Dam Safety.................................10 The Role of Consultants in Dam Safety................................11 Role of the Oklahoma Water Resources Board....................12 CHAPTER 2: INTRODUCTON TO DAMS.............................................13 General.......................................................................................13 The Watershed System.............................................................13 Types of Dams...........................................................................14 Components..............................................................................14 Construction Materials............................................................14 MARK HARRISON, OKLAHOMA CONSERVATION COMMISSION Oklahoma Water Resources Board - Dam Safety Program - March 2011 3 Contents CHAPTER 6: INSTRUMENTATION AND MONITORING GUIDELINES...........................................71 General.......................................................................................71 Reasons for Instrumentation..................................................71 Instrument Types and Usage..................................................72 Visual Observations.................................................................72 Movements................................................................................72 Pore Pressure and Uplift Pressure..........................................74 Water Level and Flow...............................................................74 Water Quality............................................................................74 Temperature..............................................................................74 Crack and Joint Size.................................................................75 Seismic Activity........................................................................75 Weather......................................................................................75 Stress and Strain.......................................................................75 Automated Data-Acquisition Systems...................................75 Frequency of Monitoring........................................................76 CHAPTER 7: MAINTENANCE INSPECTION GUIDELINES..................79 General.......................................................................................79 Maintenance Priorities.............................................................79 Immediate Maintenance..........................................................79 Required Maintenance at Earliest Possible Date..................79 Continuing Maintenance........................................................80 Specific Maintenance Items....................................................80 Earthwork Maintenance and Repair......................................80 Riprap Maintenance and Repair.............................................82 Controlling Vegetation............................................................84 Controlling Livestock..............................................................85 Controlling Animal Damage..................................................85 Controlling Damage From Traffic.........................................86 Mechanical Maintenance.........................................................86 Electrical Maintenance............................................................88 Cleaning.....................................................................................88 Concrete Maintenance.............................................................89 Metal Component Maintenance.............................................89 CHAPTER 8: EMERGENCY ACTION PLAN GUIDELINES....................91 General.......................................................................................91 Contents of Guidelines............................................................92 Description of the Project.......................................................92 Notification Flowchart.............................................................92 Emergency Detection, Evaluation, and Classifications.......93 Responsibilities.........................................................................93 Preparedness.............................................................................94 Inundation Maps......................................................................94 Implementation........................................................................94 CHAPTER 9: GUIDELINES FOR OPERATIONS....................................97 General.......................................................................................97 Plan Guidelines.........................................................................97 Background Data......................................................................98 Operating Instructions and Records......................................98 Schedule of Routine Tasks.......................................................99 Record Keeping.......................................................................100 CHAPTER 10: REDUCING THE CONSEQUENCES OF A DAM FAILURE.....................................................101 Supplements to a Dam Safety Program...............................101 Liability....................................................................................101 Measures to Reduce the Consequences of Dam Failures............................................103 Insurance.................................................................................104 Government Assistance.........................................................104 DAM SAFETY: AN OWNER’S GUIDANCE MANUAL apendix a.....................................................................106 iNSPECTION EQUIPMENT AND CHECKLIST.............107 APPENDIX B: REPORT FORM.......................................111 APPENDIX C: GLOSSARY...............................................113 REFERENCES.....................................................................121 Oklahoma Water Resources Board - Dam Safety Program - March 2011 4 DAM SAFETY: An Owner’s Guidance Manual Introduction to Dam Safety The need for dam safety is urgent. Across the United States thousands of dams are now in place with many more built each year. Dams—essential elements of the national infrastructure—supply water for households and businesses and cooling water for power plants, offer opportunities for recreation, and help control floods. Should a dam fail, many lives and many dollars’ worth of property are at risk. The legal and moral responsibility for dam safety rests with the dam owner. Existing dams are aging and new ones are being built in hazardous areas. At the same time, devel-opment continues in potential inundation zones downstream. More people are at risk from dam failure than ever, despite better engineering and construction methods, and continued deaths and property losses from dam failures are to be expected. Society and individuals alike may profit from dam operations. Dam ownership, however, is neither justified nor effective if one cannot assure the safety of citizens and property. The costs of dam safety are small in comparison to the consequences following a dam failure, particularly in today’s litigious society. Liability due to dam failure can easily offset years of profitability. You can directly influence the safety of a dam by developing a safety program which includes inspection, monitoring through instrumentation, maintenance of the structure, and proactive emergency planning. A high-quality safety program is attuned to the dam structure and its im-mediate environment and depends on the owner’s knowledge of the dam and how it works. Lakes in Oklahoma and in other parts of the country may either be human-built or exist because of geologic activities such as landslides, erosion, or glaciation. The majority of dams are human structures constructed of earthfill or concrete. It is important that you, as a dam owner, be aware of the different types of dams, their essential components and their function, as well as the important physical conditions likely that influence them. As in the case with buildings, highways, and other works that we construct, dams require an on-going maintenance program to insure their continued useful life. This fact has not always been appreciated. Often there is a tendency to neglect them once construction is completed. As is the case with buildings, highways, and other works that we construct, dams require an on-going maintenance program to ensure their continued useful life. This fact has not always been fully appreciated. Often there is a tendency to neglect them once construction is completed. Oklahoma Water Resources Board - Dam Safety Program - March 2011 5 National statistics show that dam failure is an all too common problem. It is imperative that you, as a dam owner or operator, familiarize yourself with the risks and hazards of dam ownership. Risk has greatly increased for existing dams as developers have been allowed to construct be-low dams within their inundation zone and new dams are being built in areas where the geology may be inappropriate. Other risks include natural phenomena such as floods, earthquakes, and landslides. These hazards threaten dam structures and their surroundings. Floods that exceed the capacity of a dam’s spillway and then erode the dam or abutments are particularly hazardous. Seismic activity which appears to be on the rise in Oklahoma may also cause cracking or seepage. Similarly, debris from landslides may block a dam’s spillway and cause an overflow event that erodes the abutments and ultimately weakens the structure. Hazards, Risk, Failures The three major categories of dam failure are overtopping by flood, foundation defects, and piping. For earthen dams, the major reason for failure has been piping or seepage. For concrete dams, the major reasons for failure have been associated with foundations. Overtopping has been a significant cause of dam failure, primar-ily where a spillway was inadequate. Inspection Guidelines An effective inspection program is essential to identification of problems and for safe main-tenance of a dam. The program should involve three types of inspections: (1) periodic technical inspections; (2) periodic maintenance inspec-tions; and (3) informal observations by project personnel as they operate the dam. Technical inspections involve specialists familiar with the design and construction of dams and include assessments of structure safety. Maintenance inspections are performed more frequently than technical inspections in order to detect, at an early stage, any detrimental developments in the dam. These involve assessment of operational capabil-ity as well as structural stability. The third type of inspection is actually a continuing effort by on-site project personnel (dam tenders, powerhouse operators, maintenance personnel) performed in the course of their normal duties. A fact sheet on Dam Inspection Guidelines is available online at www.owrb.ok.gov/damsafety. Instrumentation and Monitoring Guidelines A dam’s instrumentation furnishes data for determining if the structure is functioning as intended and continuing surveillance to warn of any unsafe developments. Monitoring physi-cal phenomena that can lead to a dam failure may draw on a wide spectrum of instruments and procedures ranging from very simple to very complex. Any program of dam- safety instrumentation must involve proper design consistent with other project components. The program must be based on prevailing geotech-nical conditions at the dam, and must include consideration of the hydrologic and hydraulic factors present before and after the project is in operation. Instrumentation designed for moni-toring potential deficiencies at existing dams must take into account the threat to life and property that the dam presents. Thus, the extent and nature of the instrumentation depends not only on the complexity of the dam and the size of the reservoir, but also on the potential for deaths and property losses downstream. An instrumentation program should involve instruments and evaluation methods that are as simple and straight forward as the project will allow. The involvement of qualified personnel in the design, installation, consistent and regular monitoring, and evaluation of an instrumenta-tion system is of prime importance to the suc-cess of the program. Specific information that instrumentation can provide includes: 6 Oklahoma Water Resources Board - Dam Safety Program - March 2011 • warning of a problem, i.e. settlement , movement, seepage, stability • definition and analysis of a problem, such as locating areas of concern • proof that behavior of the dam is as expected • evaluating any remedial actions Maintenance Guidelines A good maintenance pro-gram will protect a dam against deterioration and prolong its life. A poorly maintained dam will deteriorate, and may fail. Nearly all the components of a dam and the materials used for its construction are susceptible to damaging deterioration if not properly maintained. A good maintenance pro-gram protects both you and the general public. The cost of a proper maintenance program is small compared to the cost of major repairs or the loss of life and property and resultant litiga-tion. You should develop a basic maintenance program based primarily on systematic and frequent inspections. Inspections, as noted in Chapter 5, should be carried out monthly and after major floods or earthquakes. During each inspection, fill out a checklist of items requiring maintenance. An Inspection Checklist is avail-able online at www.owrb.ok.gov/damsafety. Emergency Action Plan Guidelines History has shown that dams sometimes fail and that often these failures cause loss of life, injuries and extensive property damage. You should prepare for this possibility by developing an emergency action plan which provides a systematic means to: • identify potential problems that could threaten a dam • determine who would be at risk should a failure occur • expedite effective response actions to prevent failure • develop a notification plan for evacuating people to reduce loss of life and property damage should failure occur You are responsible for preparing a plan cov-ering these measures and listing actions that you and operating personnel should take. You should be familiar with the local government officials and agencies responsible for warn-ing and evacuating the public. An Emergency Action Guide is available online at www.owrb. ok.gov/damsafety. It is important that you make full use of others who are concerned with dam safety. Emergency plans will be more effective if they integrate the actions of others who can expedite response. People and organizations with whom you should consult in preparing an emergency action plan include numerous local participants, state and federal agencies. An essential part of the emergency action plan is a list of agencies and persons to be notified in the event of a potential failure. Possible inclu-sions for this list should be obtained from and coordinated with local law enforcement agencies and local disaster emergency services. These are key institutions that can activate public warn-ing and evacuation procedures or that might be able to assist you, the dam owner, in delaying or preventing failure. Certain key elements must be included in every notification plan. Information about potential inundation (flooding) areas and travel times for the breach (flood) wave is essential. Inunda-tion maps are especially useful in local warning and evacuation planning, including identifying evacuation routes. Oklahoma Water Resources Board - Dam Safety Program - March 2011 7 Operation Plan Guidelines Establishing an operation procedure or plan calls for detailed: • data on the physical characteristics of dam and reservoir • descriptions of dam components • operating instructions for operable mechanisms • instructions for inspections Measures to Reduce the Consequences of Dam Failure Liabilities that are determined following a dam failure strongly affect organizations and individuals, governments and dam owners alike. Establishing liability is the legal means de-veloped by society to recover damages due to some intentional or negligent wrong (in this case, a lack of dam safety) and represents another perspective on the dam safety problem. A thorough understanding of this legal process can help you decide the steps necessary to reduce liability. You can directly and indirectly influence the use of a variety of measures that will serve to reduce the consequences of dam failure. For example, insurance against the costs that will accrue after a failure will save you money by spreading costs to multiple dam owners. Some land use measures instituted by gov-ernments represent better means of mitigating future disasters. Land use measures that restrict living or de-veloping in inundation zones radically improve safety and are among the most effective ways to save lives and preserve property over the long term; however, such steps are not always acceptable to the local population or government. Thus, increasing public awareness and governmental planning are vital measures that must be considered as ways to reduce the consequences of dam failure. • instrumentation and monitoring guidelines • guidelines for maintenance • guidelines for emergency operations • bibliographic references Establish a schedule for both day-to-day tasks and tasks performed less frequently throughout the year. The schedule should formalize inspec-tion and maintenance procedures so that even an inexperienced person can determine when a task is to be done. Oklahoma Water Resources Board - Dam Safety Program - March 2011 9 CHAPTER 1: An Approach to Dam Safety 1 public and private agencies, and private citizens. Typical reasons for building dams include water storage for human consumption, agricultural production, power generation, flood control, reduction of soil erosion, industrial use, and recreation. Thus, dam owners serve society by meeting important state needs and may also personally profit from dam operations. How-ever, those are not sufficient reasons for build-ing or owning a dam if the owner cannot keep people and property safe in potential inunda-tion zones. Both financially and morally, successful dam ownership and the maintenance of safety standards go hand in hand. Investment in dam safety should be accepted as an integral part of project costs and not viewed as an expendable item that can be eliminated if a budget becomes tight (Jansen, 1980). The potential cost and sta-tistical likelihood of dam failure to both life and property are simply too high to ignore. As national needs for water intensify and its value increases, more dams are being built. At the same time, many existing dams are reach-ing or passing their design life spans and, for various reasons, people continue to settle near dams. As builders use poorer sites for dams or as areas below a dam develop, the job of protect-ing life and property becomes more difficult. Therefore, as dam construction continues and the population grows, exposure of the public to dam failure hazards increases and the overall safety problem becomes more difficult. General This manual is a safety guide for the dam owner. The continuing need for dam safety is critical because of the thousands of dams now in place and the many new ones being built each year. Although these dams are essential elements of the national infrastructure, the risks to the public posed by their possible failure are great; a large and growing number of lives and valuable property are at stake. Though many are concerned about dam safety, the legal and moral responsibility essentially rests with the dam owner. Urgency for Safety The critical need for dam safety is clear. World and national statistics on dam failures show an unaccept-able record of deaths and property losses. The record for U.S. losses from major dam failures in recent years, shown in Table 1 is also discouraging. Actual national losses are much higher than indicated be-cause the statistics shown exclude small dam failures and combinations of dam failure with natural flood-ing events. Two examples are dams that failed near Hearne, Texas in May 2004 and the Johnstown, Pennsylvania, disaster of 1889 which is still regarded as one of the nation’s great catastrophes. The poten-tial for future similar catastrophes due to dam failure remains strong. Only a cooperative effort in dam safety involving owners and communities can lessen this potential. Dam Ownership and Safety This manual can be applied to dams owned and operated by a wide range of organizations and people, including state and local governments, 10 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Name & Location of Dam Date of Failure Number of Lives Lost Damages Mohegan Park, CT 3/1963 6 $3 million. Little Deer Creek, UT 6/1963 1 Summer cabins damaged. Baldwin Hills, CA 12/1963 5 41 houses destroyed, 986 houses damaged, 100 apartment buildings damaged. Swift, MT 6/1964 19 Unknown. Lower Two Medicine, MT 6/1968 9 Unknown. Lee Lake, MA 3/1968 2 6 houses destroyed, 20 houses damaged, 1 manufacturing plant damaged or destroyed. Buffalo Creek, WV 2/1972 125 546 houses destroyed, 538 houses damaged. Lake “O” Hills, AR 4/1972 1 Unknown. Canyon Lake, SD 6/1972 33 Unable to assess damage because dam failure accompanied damage caused by natural flooding. Bear Wallow, NC 2/1976 4 1 house destroyed. Teton, ID 6/1976 11 771 houses destroyed, 3,002 houses damaged, 246 businesses damaged or destroyed. Laurel Run, PA 7/1977 40 6 houses destroyed, 19 houses damaged. Sandy Run & 5 others, PA 7/1977 5 Unknown. Kelly Barnes, GA 11/1977 39 9 houses, 18 house trailers, & 2 college buildings destroyed; 6 houses, 5 college buildings damaged. Lawn Lake, CO 7/1982 3 18 bridges destroyed, 117 businesses & 108 houses damaged. Campgrounds, fisheries, power plant damaged. D.M.A.D., UT 6/1983 1 Unknown. Nix Lake Dam, TX 3/1989 1 Unknown. Silver Lake, MI 5/2003 0 $102,000,000. Big Bay Lake, MS 3/2004 0 98 houses, 2 churches, fire station, bridge, $2.2 million Kaloko Res., HI 3/2006 7 Unknown. Source: Graham, 1983, 2004 TABLE 1.1. Loss of Life and Property Dama ge From Notable U.S. Dam Failures , 1963-2006 Governments across the nation have shown increasing concern for this problem and have enacted laws, statutes, and regulations that in-crease the dam owner’s responsibility. In most states, including Oklahoma, owners are held strictly liable for losses or damages resulting from dam failure. Concurrently, liability insur-ance costs have risen rapidly. Role of the Dam Owner in Dam Safety An owner should be aware of and use both direct and indirect means of achieving dam safety. The owner can monitor and work on factors directly in his control (for example, structural integrity), which are detailed below. However, the owner may also seek to influence governmental policy and work for positive change in statutes and laws that affect dam safety (example, zoning laws). Such indirect influence by an owner could con-tribute significantly to reducing the likelihood and consequences of dam failure and, thus, to overall community safety. Liability, insurance coverage, and the roles of the state and federal governments should all be well-understood by an owner. Liability can apply not only to the individual dam owner, but also to any company or organization that possesses the dam, or any person who operates or maintains it and, potentially, even those who live around a lake. If an unsafe condition existed prior to a new dam owner’s term of ownership, the new owner cannot be relieved of liability should the Oklahoma Water Resources Board - Dam Safety Program - March 2011 11 dam fail during this term. Thus, the potential owner must carefully inspect the structural integrity of any dam prior to purchase and then inspect, maintain, and repair it thereafter. Legally, the dam owner must do what is neces-sary to avoid injuring persons or property which usually applies to circumstances and situations which a reasonable person could anticipate. In order to meet your responsibility to maintain the dam in a reasonable and safe condition, you, the owner, should conduct regular inspections of the dam and maintain or repair deficient items. Regular inspections by qualified professionals are necessary to identify and correct any problems. A dam owner should have a thorough understand-ing of the dam’s physical and social environment. This would include: knowledge of natural and technological hazards that threaten it, an under-standing of the developing human settlement pat-terns around the dam, and an under-standing of any events that can lead to structural failure. It is a good idea for every owner of a dam to pause and consider what lies below their dam. Several questions need to be asked. • What is the nature of the land use downstream: wooded or agricultural land, scattered homes, roads, villages, urban? • How many structures are located within a half mile, a mile or several miles of the dam? • How are downstream structures located with regard to the watercourse or floodplain, with respect to both distances from the watercourse or river and elevation above it? • What is the first-floor elevation of homes located downstream. Are they only a few feet above the level of the water surface, or are they on bluffs high above it and out of danger? • Is the valley below the dam characterized by steep hills, or is there a broad floodplain? This is an important consideration, as it determines whether water released in a dam failure or during flooding would soon spread out and lose its force or whether a destructive wall of water would travel a long distance downstream. Owning a dam brings many different concerns and possible rewards, but in the end success will largely be measured by a continuing record of safety. Owners can also influence the safety of dams in more direct ways. They can and should de-velop their own safety programs, which should include such important elements as inspection, monitoring through instrumentation, mainte-nance, emergency action planning, and proper operation. Such programs are directly related to a specific dam’s structure and its immediate environment and depend on the owner’s knowl-edge of the dam and how it works. The Role of Consultants in Dam Safety A dam is a special kind of structure, simple in concept but with many complicated com-ponents. There is no such thing as a standard dam design; furthermore, each dam site is unique. The existence of a dam necessitates the involvement of many specialists to analyze, design, build, inspect, and repair it. This wide variety of consultants will include civil, geo-technical, mechanical, and electrical engineers, geologists and hydrologists. As owner, you should know more about your dam than anyone else. A consultant can advise you on such important items as: • the design and construction of a new dam • the overall stability of the dam under normal and flood conditions • any repairs or maintenance needed by the dam and appurtenant works • the severity of any problems and indicate in what order to repair them • cost estimates for repair work • adequacy of the spillway to pass the design flood 12 Oklahoma Water Resources Board - Dam Safety Program - March 2011 • an assessment of downstream hazards • the dam owner’s preparation and procedures to deal with emergency conditions Hazardous conditions at the dam should be reported verbally and in writing to the dam owner and the OWRB. A written report from the owner’s consultant is essential for every inspection. It is uncommon that a dam owner has all of the technical skills needed to monitor the condition of the dam. Thus, the role of the consulting engineer is critical in dam safety Role of the Oklahoma Water Resources Board The Oklahoma Water Resources Board is responsible for administrating state dam safety laws. The staff of the OWRB has four primary areas of activity in the dam safety program: (1) review and approval of plans and specifications of new dams, (2) review of plans and specifications for repairs, modification, or rehabilitation work, (3) periodic inspections of construction work on new and existing dams, and (4) review of inspection reports and approval of emergency action plans. Oklahoma Water Resources Board - Dam Safety Program - March 2011 13 CHAPTER 2: Introduction to Dams 2 General The purpose of a dam is to impound (store) water for any of several reasons, e.g., flood control, water supply for hu-mans or livestock, irrigation, energy generation, recreation, or pollution control. This manual primarily concentrates on earthen dams, which constitute the majority of structures in place and under development in Oklahoma. The Watershed System Water from rainfall or snowmelt naturally runs downhill into a stream valley and then into larger streams or other bodies of water. The “watershed system” refers to the drainage process through which rainfall or snowmelt is collected into a particular stream valley during natural runoff (directed by gravity). Dams con-structed across such a valley then impound the runoff water and release it at a controlled rate. During periods of high runoff, water stored in the reservoir typically increases, and overflow through a spillway may occur. During periods of low water flow is normally controlled. Hence, with the insertion of a dam into a watershed very high runoffs (floods) and very low runoffs (drought periods) are generally avoided. Mark Harrison, Oklahoma Conservation Commission 14 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Types of Dams Dams may either be human-built or result from natural phenomena, such as landslides or glacial deposition. The majority of dams are human struc-tures normally constructed of earthfill or concrete. Naturally occurring lakes may also be modified by adding a spillway to allow for safe, efficient release of excess water from the resulting reservoir. Dam owners should be aware of the different types of dam’s essential components of a dam how the components function, and important physical conditions likely to affect a dam. Human-built dams may be classified according to the type of construction materials used, the methods used in construction, their slope or cross-section, the way they resist the forces of the water pressure behind them, the means of controlling seepage, and oc-casionally, their purpose. Components : The components of a typical dam are illustrated in Figure 2.1. Nearly all dams possess the features shown or variations of those features. Definitions of the terms are given in the Glos-sary. The various dam components are discussed in greater detail later on. Construction Materials : The materials used for construction of dams include earth, rock, tailings from mining or milling, concrete, masonry, steel, and any combination of those materials. 1. Embankment Dams: Embankment dams, the most common type in use today, have the general shape shown in Figure 2.1. Their side slopes typically have a grade of two to one (horizontal to vertical) or flatter. Their capacity for water retention is due to the low permeability of the entire mass (in the case of a homogeneous embankment) or of a zone of low-permeability material (in the case of a zoned embankment dam). Material used for embankment dams include natural soil or rock obtained from borrow areas or nearby quarries, or waste materials obtained from mining or milling operations. If the natural material has a high permeability, then a zone of very low permeability material must be included in the dam to retain water. 2. Concrete Dams: Concrete dams may be categorized into gravity and arch dams according to the designs used to resist the stress due to reservoir water pressure. A concrete gravity dam (shown in Figure 2.2) is the most common form of concrete dam. In it, the mass weight of the concrete and friction resist the reservoir water pressure. A buttress dam is a specific type of gravity dam in which the large mass of concrete is reduced, and the forces are diverted to the dam foundation through vertical or sloping buttresses. Gravity Figure 2.1. Embankment dam (U.S. Army Corps of Enginers). Oklahoma Water Resources Board - Dam Safety Program - March 2011 15 dams are constructed of non-reinforced vertical blocks of concrete with flexible seals in the joints between the blocks. Concrete arch dams are typically rather thin in cross-section. The reservoir water forces acting on an arch dam are carried laterally into the abutments. The shape of the arch may resemble a segment of a circle or an ellipse, and the arch may be curved in the vertical plane as well. Such dams are usually built from a series of thin vertical blocks that are keyed together, with water stops between the blocks. Variations of arch dams include multi-arch dams, in which more than one curved section is used, and arch gravity dams, which combine some features of the two types. A recently developed method for constructing concrete gravity dams involves the use of a relatively weak concrete mix which is placed and compacted in a manner similar to that used for earthfill dams. Roller-compacted concrete has the advantages of decreased cost and time. In addition, there are no joints where seepage could occur. 3. Other Types: Various construction techniques could be used in a single dam. For example, a dam could include an earthen or rock fill embankment as well as a portion made of concrete. In such a case, the concrete section would normally contain the spillway or other outlet works. A recent design for low-head dams (with a minimal height of water behind the dam uses inflatable rubber or plastic materials anchored at the bottom by a concrete slab. Some dams are constructed for special purposes, such as diversion of water, or permit construction of other facilities in river valleys. These dams are called diversion dams and cofferdams, respectively. Water Retention Ability Because the purpose of a dam is to retain water effectively and safely, its water-retention ability is of prime importance. Water may pass from the reservoir to the downstream side of a dam by: • seeping through the dam • seeping through the abutments • seeping under the dam • overtopping the dam • passing through the outlet works • passing through or over a primary spillway • passing over an emergency spillway The first three ways water pass from a reser-voir are considered undesirable, particularly if the seepage is not limited in area or volume. Overtopping of an embankment dam is also very undesirable because the embankment mate-rial may be eroded away. Additionally, only a few concrete dams have been designed to be overtopped. Water normally leaves a dam by passing through an outlet works or spillway. Water should pass over an emergency spillway only during periods of very high reservoir levels and high water inflow. Figure 2.2. CONCRETE GRAVITY dam (U.S. Army Corps of Engineers ). 16 Oklahoma Water Resources Board - Dam Safety Program - March 2011 tion, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is undesirable. To minimize this possibility, dams are constructed with internal impermeable barriers and internal drainage facilities such as drainpipes or filter sys-tems, or other drainage systems such as toe, blanket, or chimney drains. Flow through a dam foundation may be diminished by grouting known or suspected highly permeable material, constructing a cutoff wall or trench below a dam, or constructing an upstream impermeable blanket. Figure 2.3 illustrates a cutoff trench. In summary, the overall water retention ability of a dam depends on its permeability, the abutments, the foundation, and the efforts made to reduce that permeability or restrict the flow of water through these components. Should high permeability oc-cur, seepage can lead to piping, which will likely result in failure. Release of Water Intentional release of water, as stated earlier, is confined to water releases through a service spillway or outlet works or over emergency spillways. Principal or Mechanical Spilway : The principal or mechanical spillway maintains the normal water level in the reservoir. Its function is to pass expected flood flows past the dam safely and without erosion. It may consist of a pipe through the dam or a system of gates that discharge water into a concrete spillway. Either method uses the Seepage Through a Dam: All embankment dams and most concrete dams allow some seep-age. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is impor-tant to control the quantity of seepage by using low permeability materials in construction and by channeling and restricting the flow so that embankment materials do not erode. Seepage through a concrete dam is usually minimal and is almost always through joints between blocks, or through cracks or deteriorated concrete which may have developed. Maintenance of these joints and cracks is therefore essential. The seepage water should be collected and channelized, so that its quantity can be measured and erosion minimized. Seepage Around a Dam: Seepage around the ends of a dam through the abutment materials or under a dam, through the dam foundation material, may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it less stable. Seepage through abutments or foundations can dissolve the constituents of certain rocks such as limestone, dolomite, or gypsum so that any cracks or joints in the rock become progressively larger and in turn allow more seepage. Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When the entrance point is reached, water may then flow without restric- Figure 2.3. Embankment dam WITH A CUT-OFF TRENCH (U.S. Army Corps of Enginers ). Oklahoma Water Resources Board - Dam Safety Program - March 2011 17 overflow principle. When the reservoir reaches a certain level, water flows into a standpipe or riser pipe (Figure 2.4) or over a gate. Intake structures for spillways must have systems that prevent clog-ging by trash or debris. Drawdown Facility : All dams should have some type of drawdown facility which can: • quickly lower the water level if failure of the dam is imminent. • serve the operational purposes of the reservoir. • lower the water level for dam repairs. • periodically raise and lower the pool level to kill weeds and mosquitoes. The valve regulating the drawdown facility should be on the upstream end of the conduit to minimize the risk to the dam posed by a pos-sible internal rupture of the pipe. Emergency (Auxiliary ) Spilway : As the name implies an emergency spillway functions during emergency conditions to prevent over-topping of a dam. A typical emergency spillway is an excavated channel in earth (Figure 2.5) or rock near one abutment of a dam. An emergency spillway should always discharge away from the toe of a dam to avoid its erosion. Furthermore, the spillway should be constructed in such a manner that the spillway itself will not seriously erode when it is in use. Obviously erosional fail-ure of the spillway could be as catastrophic as failure of the dam itself. An emergency spillway should be sized to convey the so-called “design flood”, the rare, large-magnitude flood used to establish design criteria. The spillways of many existing dams are now considered undersized because standards for the design flood have increased over the years. Figure 2.4. Principal Spilway Figure 2.5. Emergency Spilway . Mark Harrison, Oklahoma Conservation Commission Oklahoma Water Resources Board - Dam Safety Program - March 2011 19 CHAPTER 3: Hazards, Risks, and Failures 3 General Dam failures are severe threats to life and property and are now being recorded and documented much more thoroughly than in the past. Recorded losses have been high. Statistics on losses of life and property fully justify the need for dam owners to better understand the risks to the public posed by dams, the kinds of haz-ards that promote those risks and owner liabilities associated with them, and, generally, the reasons that dams fail. Improving a dam owner’s under-standing of realistic risks and possible reasons for failure is an essential first step in any overall effort to improve dam safety and preserve the benefits of dam ownership. Hazards as Sources of Risks The dam structure itself can be a source of risk due to possible construction flaws and weaknesses that develop because of aging. The site immedi-ately surrounding the structure may also increase the structural risk if the dam is not positioned or anchored properly or if excessive reservoir seep-age erodes the foundation or abutments. The physical hazards that can cause dam fail-ure are translated into high risks when people or properties are threatened. These high risks are exacerbated by a number of important fac-tors. For instance, in Oklahoma and most other states, people are often allowed to build within a dam’s inundation zone, thereby greatly com-pounding the associated risk. Natural hazards such as floods, earthquakes, and landslides are also important contributors to risk. These have now become even greater hazards because development has placed people and property in their way. Failure to adjust to these events has been costly both to dam owners and to the public in general. Human behavior is another element of dam fail-ure risk; simple mistakes, operational misman-agement, negligence, unnecessary oversights, or destructive intent can interact with other hazards to compound the possibility of failure. Figure 3.1 Embankment Dam failure in Cleveland county 20 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Floding from high precipitation : Of the natural events that can impact dams, floods are the most significant. A floodplain map of the U.S. (Figure 3.2) gives the estimated percentage of land resting within a floodplain. Floods are the most frequent and costly natural events that lead to disaster in the U.S. Therefore, flood potentials must be included in risk analyses for dam failure. Flash floods can happen anywhere in Oklahoma, even on small drainages. A common safety factor for dam design is to construct them to withstand a “probable maximum flood” (PMF) assumed to occur on the upstream watershed. A PMF is the flood that may be expected from the most severe combination of critical meteorologic and hydro-logic conditions that are reasonably possible in the region. However, dams are often built in areas where estimates of the PMF are based on short precipitation and runoff records. As a result, spill-way capacity may often be underestimated. Flooding from dam failure : When a dam fails as a result of a flood, more people and prop-erty are generally placed in jeopardy than during Thus, a broad range of natural and human hazards, taken separately or in combination, in-crease the probability of dam failure and injury to people and property. The following discussion of some of the most significant hazards that lead to public risk illus-trates the interrelationships among events that can lead to dam failure. Natural Hazards That Threaten Dams The most important natural hazards threaten-ing dams include: • flooding from high precipitation • flooding from dam failure • earthquakes • landslides Figure 3.2 Floodplain map of the U.S. Oklahoma Water Resources Board - Dam Safety Program - March 2011 21 natural floods. The Rapid City, South Dakota, flood of 1970, which killed 242 people, caused a dam failure which added significantly to the loss of life. When a natural flood occurs near a dam, the probability of failure and loss of life almost always increases. The sudden surge of water generated by a dam failure usually far exceeds that expected from a natural 100-year floodplain estimate; therefore, residences and businesses that would escape natural flooding may still be at extreme risk from flooding due to dam failure. Hence, it is important to inform residents and business per-sonnel of the full risk to which they are exposed so that they can respond accordingly. To compound the risk even further, when one dam fails, the sudden surge of water may well be powerful enough to destroy another dam downstream. Upstream dams may seem too far away to be a real threat, but inundation zones and surge crests can extend many miles down-stream, especially if the reservoir behind the collapsed dam held a large quantity of water. Earthquakes : Pose significant threats to dam safety. While very common in Oklahoma, it is rare that earthquakes here are substantial enough to harm a dam. Nevertheless, dam own-ers should be aware of the history of seismic activity in their locality and develop their emer-gency procedures accordingly. Both earthen and concrete dams can be dam-aged by ground motions caused by seismic activity. Cracks or seepage can develop, leading to immediate or delayed failure. Dams, such as those in California, located near relatively young, active faults are of particular concern, but dams (especially older concrete and earthen structures) located where relatively low-scale seismic events may occur are also at risk. Recent detailed seismic analyses have indicated a much broader area of seismic activity sufficient to damage dams than previously considered; the seismic risk is essentially nationwide. Landslides: Rock slides and landslides may affect dams directly by blocking a spillway or by eroding and weakening abutments. Indirectly, a large landslide into a reservoir behind a dam can cause an overflow wave that will exceed the capacity of the spillway and lead to failure. A landslide (or mudslide) can form a natural dam across a stream which can then be overtopped and fail. In turn, failure of such a natural dam could then cause the overtopping of a down-stream dam or by itself cause damage equivalent to the failure of a human built dam. In addition, large increases in sediment caused by such events can materially reduce storage capacity in reservoirs and thus increase a downstream dam’s vulnerability to flooding. Sedimentation can also restrict the operation of low level gates and water outlets; damage to gates and outlets can lead to failure. Hazards from Human Activity Human activity must also be considered when analyzing the risks posed by dams. The “high hazard” designation does not imply structural weakness or an unsafe dam. In Oklahoma, the hazard classification of dams is based on the potential for loss of life and economic loss in the area downstream of the dam, not on its structural safety (Table 3.1). Thus, dams that may be of very sound construction are labeled “high hazard” if failure could result in catastrophic loss of life. Risk may well increase through time because few governmental entities have found the means to limit settlement below dams. The hazard level of more dams is rising to “high” or “significant” as development occurs in potential inundation zones below dams previously rated “low hazard.” Because of short-term revenue needs or other pressures, governments often permit develop-ment in hazardous areas despite long-term dan-ger and the risk of high future disaster costs. Diversion of development away from potential inundation zones is a sure means of reducing 22 Oklahoma Water Resources Board - Dam Safety Program - March 2011 risk, but is not always a policy suitable to the immediate needs of local government. Perhaps the ultimate irony for a dam owner is to have de-veloped and implemented a safety program only to have development permitted in the potential inundation zone so that the hazard rating and owner’s liability increase. All sorts of other human behavior should be in-cluded in risk analyses; vandalism, for example, cannot be excluded and is in fact a problem faced by many dam owners. Vegetated surfaces of a dam embankment, mechanical equipment, manhole covers and rock riprap are particularly susceptible to damage by people. Every precau-tion should be taken to limit access to a dam by unauthorized persons and vehicles. Dirt bikes (motorcycles) and off-road vehicles, in par-ticular, can severely degrade the vegetation on embankments. Worn areas lead to erosion and more serious problems. Mechanical equipment and associated control mechanisms should be protected from tamper-ing, whether purposeful or inadvertent. Buildings housing mechanical equipment should be sturdy, have protected windows, and heavy-duty doors, and be secured with padlocks. Detachable controls, such as handles and wheels, should be removed when not in use and stored inside the padlocked building. Other controls should be secured with locks and heavy chains where possible. Manhole covers are often removed and sometimes thrown into reservoirs or spillways by vandals. Rock used as riprap around dams is sometimes thrown into the reservoirs, spillways, stilling basins, pipe-spillway risers, and elsewhere. Riprap is often displaced by fishermen to form benches. The best way to prevent this abuse is to use rock too large and heavy to move easily, or to slush-grout the riprap. Otherwise, the rock must be regularly replenished and other dam-ages repaired. Regular visual inspection can easily detect such human impacts. Owners should be aware of their responsibility for the safety of people using their facility even though their entry may not be authorized. “No Trespassing” signs should be posted, and fences and warning signs erected around dangerous areas. As discussed in Chapter 10, liability insurance can be purchased for protection in the event of accidents. Site-Specific Structural Risk Developing site-specific risk analyses involves consideration of a number of hazards. Such analyses are helpful in stimulating better aware-ness, planning, and design. In some cases dam structure analyses are quantitative. Hence, pre-cise conclusions about engineering and design can be made. Probabilistic analyses can also be important and useful; however, exact quantita-tive and probabilistic tools are not yet applicable in many situations and do not fully supplement or replace qualitative analyses such as informed perception and judgment of the risks. Judgment and engineering experience should play an im-portant role in reaching useful conclusions in any site-specific analysis of structural risk. TABLE 3.1. Table of Hazard-Potential Clasification Hazard -Potential Classification Description Low Dams assigned the low hazard-potential classification are those where failure would result in no probable loss of human life and low economic losses. Significant Dams assigned the significant hazard-potential classification are those dams where failure would result in no probable loss of human life but can cause economic loss or disruption of lifeline facilities. High Dams assigned the high hazard-potential classification are those where failure will probably cause loss of human life. Oklahoma Water Resources Board - Dam Safety Program - March 2011 23 As mentioned in Chapter 2, structural risks tend to result from design and construction prob-lems related to the dam materials, construction practice, and hydrology. The complexity of the hazard is such that structural design and causes of dam failure are significant areas of research in engineering. Indeed, better design criteria have been developed and safer dams are being built, but there is no basis for complacency. Dams continue to age, people continue to move into inundation zones, and enough hazards exist that the net risk to the public will remain high despite design improvements. Sources of Dam Failure There are many complex reasons, both struc-tural and non-structural, for dam failure. Many sources of failure can be traced to decisions made during the design and construction process and to inadequate maintenance or op-erational mismanagement. Failures have also resulted from the natural hazards previously mentioned. However, from your perspective as owner, the structure of a dam is the starting point for thorough understanding of the poten-tials for failure. Thre Categories of Structural Failure : Three categories of structural failure alluded to in Chapter 2 are: • overtopping by flood • foundation defects • piping and seepage Overtopping may develop from many sources, but often evolves from inadequate spillway design. Alternatively, even an adequate spillway may become clogged with debris. In either situ-ation, water pours over other parts of the dam, such as abutments or the toe, and erosion and failure follow. Concrete dams are more susceptible to founda-tion failure than overtopping, whereas earthen dams suffer from seepage and piping. Overall, these three events have about the same incidence. A more specific analysis of the po-tential sources of failure has to take into account types of dams. Similarly, the characteristics of the type of dam being monitored will point to problems requiring more careful attention by the owner when developing a safety program. Failures : Embankment or Earthen Dams: The major reason for failure of fill or embankment dams is piping or seepage. Other hydrologic failures are significant as well, including overtopping and erosion from water flows. All earthen dams exhibit some seep-age; however, as discussed earlier, this seepage can and must be controlled in velocity and amount. Seepage occurs through the structure and, if uncontrolled, can erode material from the down-stream slope or foundation backward toward the upstream slope. This “piping” phenomenon can lead to a complete failure of the structure. Piping action can be recognized by an increased seepage flow rate, the discharge of muddy or discolored water below the dam, sinkholes on or near the embankment, and a whirlpool in the reservoir (see Inset 3.1). Hydrologic failures of earthen dams result from the uncontrolled flow of water over the dam, around it, adjacent to it, or from the erosive action of water on the dam’s foundation. Earthen dams are particularly susceptible to hydrologic failure since most sediment erodes at relatively low water flow velocities. Once erosion has begun during overtopping, it is almost impossible to stop. In a very special case, a well-vegetated earthen em-bankment may withstand limited overtopping if water flows over the top and down the face as evenly distributed sheet and does not become concentrated in a single channel. Concrete Dams: Failure of concrete dams (see Inset 3.2) is primarily associated with foundation problems. Overtopping is also a significant cause again primarily when spillways are built with in-adequate capacity. Other causes include failure to let concrete set properly and earthquakes. 24 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Age and Its Relation to Failure Foundation failures occur relatively early in the life of a dam, whereas other causes generally take much longer to materialize. Thus, it is not surpris-ing that a very large percentage of all dam failures occur during initial filling, since that is when design or construction flaws, or latent site defects, appear. As dams age, maintenance becomes more critical. Lack of maintenance will result in dete-rioration and eventually, failure. Oklahoma dams are aging as shown in Table 3.2, and problems as described above are slowly becoming apparent. INset 3.1. Examples of Earthen-Dam Failures Southfork, Pensylvania The famous Johnstown disaster, caused by the failure of the South Fork Dam in 1889, in which 2,209 people were killed, is an example of the overtopping of an earthen dam. Heavy rainfall in the upper drainage basin of the dam filled the reservoir and caused overtopping. It was later calculated that, if a spillway had been built according to specifications and if the original outlet pipes had been available for full capacity discharge, there would have been no overtop-ping. Teton Dam, Idaho The Teton Dam failure in 1976 was attributed to (1) internal erosion (piping) of the core of the dam deep in the right foundation key trench, with the eroded soil particles finding exits through channels in and along the interface of the dam with the highly pervious abutment rock and talus to points at the right groin of the dam; (2) destruction of the exit avenues and their removal by the outrush of reservoir water, (3) the existence of openings through inadequately sealed rock joints which may have developed through cracks in the core zone in the key trench; (4) the development of piping through the main body of the dam that quickly led to complete failure; and (5) the design of the dam did not adequately take into account the foundation conditions and the characteristics of the soil used for filling the key trench. Baldwin Hills and St. Francis Dams , California The Baldwin Hills Dam failed in 1963 follow-ing displacement of its foundation. Foundation problems were ultimately traced to seismic activity along nearby faults. The failure of the large St. Francis Dam (part of the water supply system for Los Angeles) in 1928 was also attrib-uted to a variety of problems related to founda-tion pressures, seepage around the foundation, and faulty operation. Source: Jansen, 1980. Oklahoma Water Resources Board - Dam Safety Program - March 2011 25 Inset 3.2. Examples of Concrete-Dam Failures Austin, Pensylvania An example of a foundation problem can be found in the failure of the Austin, Pennsylvania Dam in September, 1911. Evidently, the reservoir was filled before the concrete had set sufficiently. Eventual failure near the base occurred because of weakness in the foundation or in the bond between the foundation and the concrete. . Walnut Grove , Arizona In 1890, the Walnut Grove dam on the Hassayompa River failed due to overtopping, killing about 150 people. The failure was blamed on inadequate capacity of the spillway and poor construction and workmanship. A spillway 6 x 26 feet had been blasted out of rock on one abutment, but, with a drainage area above the dam site of about 500 square miles, the spillway did not have nearly enough discharge capacity. Source: Jansen, 1980. Dates Percent of Dams Constructed Prior to 1950 10.7 1950 – 1959 15.9 1960 – 1969 44.9 1970 – 1979 21.0 1980 – 1989 5.2 1990 – 1999 1.8 2000 – Present 0.6 TABLE 3.2. Ages of dams in Oklahoma Oklahoma Water Resources Board - Dam Safety Program - March 2011 27 Objectives of a Safety Program The pressing issue of dam failure points up the need for a safety program. You, the owner, should base your program on an evaluation of your dam’s structural and operational safety. Your program should iden-tify problems and recommend remedial repairs, operational restrictions and modifications, or further analyses and studies to determine solu-tions. Components of a safety program that address the spectrum of possible actions to be taken over the short and long-term include: • assessing the condition of the dam and its components • conducting preliminary and detailed inspections • identifying repairs and continuing maintenance needs • establishing periodic and continuous monitoring capabilities over the long-term • establishing an emergency action plan to help minimize adverse impacts should the dam fail • establishing operations procedures which recognize dam failure hazards and risks • documenting the safety program so that the information established is available at times of need and can be readily updated Develop your safety program in phases, begin- CHAPTER 4: Developing a Personal Safety Program4 ning with collection and review of existing information, proceeding to detailed inspections and analyses, and culminating with formal documentation. You can accomplish much of the preliminary work personally, with the as-sistance of state and local agencies. However, depending upon the number and seriousness of problems identified by the initial assessment, you may require the professional assistance of qualified engineers and contractors. Guidelines for Assessing Existing Conditions The guidelines for assessing existing conditions involve a sequence of steps that will enable you, the owner, to secure the information you will need to determine whether subsequent detailed investigations, repairs, and maintenance are required. The steps include: • reviewing existing data • visiting the site • inspecting the dam • assessing significance of observed conditions • deciding what to do next Reviewing Existing Data : First and foremost, collect and review available information on the dam such as plans of its design, construction, 28 Oklahoma Water Resources Board - Dam Safety Program - March 2011 and operation. Maps of the site, watershed, and the downstream channel reaches are also valuable. Review the design of the dam and its appurtenant structures to assess its actual per-formance compared to that intended. Review engineering records originating during construction to verify that structures were con-structed as designed. Collect records of subse-quent construction modifications, as well as op-eration records that document the performance of the dam and reservoir. Review any previous emergency action plan to determine if it is up-to-date and workable. Incorporate all these records into a notebook or file; they are most important in establishing a safety program and serve as the basis for its supporting documenta-tion. (For help with the development of such documentation, refer to Chapters 5 through 10.) If no records exist, a detailed examination of the structure is appropriate. Visiting the Dam Site: Undoubtedly you know it well and have visited it many times, but in this visit there are particular things for you to look for. Take a fresh look at the dam structure and its surroundings from the view of their potential hazard. Inspecting the Dam: Also, take a detailed and systematic look at all components of the dam and reservoir system. The description of the site’s components in Chapter 2 should aid this inspection. (The descriptions are general, so bear in mind that dams and their components come in various shapes and sizes and differ greatly in detail). Features to inspect include : • access roads and highways • upstream slope • crest • downstream slope • left and right abutments • spillways • outlets • drains • reservoir area (exposed and submerged) area immediately downstream of the dam • downstream areas for change in hazard classification What to look for : • obvious deterioration • cracks and slumps • boiling seepage • less than obvious internal corrosion • weathering • settlement • foundation-rock deterioration • dissolution A dam can look stable and still be susceptible to failure from gradual deterioration of its internal structure. Regular and very detailed inspections (Chapter 5) and follow-up monitoring (Chapter 6) and maintenance (Chapter 7) are needed to ensure maximum safety. Oklahoma Water Resources Board - Dam Safety Program - March 2011 29 Assesing Significance of Observed Conditions: Chapter 5 presents detailed information on conducting inspections and assessing the significance of conditions you observe. Typically, eroded areas, seepage, slides, and outflow draw the most attention. Deciding What to Do Next: Your dam safety program is now off to a good start. Available infor-mation on design and construction of the dam and later structural modifications provides perspective on its existing condition relative to that intended. If no documentation exists, then development of equivalent details should be a first priority. Inspec-tion and documentation assistance is available from several sources, including the Oklahoma Water Re-sources Board, the state agency responsible for dam safety. Professional engineering consultants can also perform detailed inspections, testing, and analyses, and create documentation (Chapter 10). Procedural Guidelines – A Source Book This chapter provides an overview of how to establish a safety program. Subsequent chapters detail technical and procedural steps of the pro-gram components. They include: • detailed inspection guidelines (Chapter 5) • monitoring and instrumentation guidelines (Chapter 6) • maintenance guidelines (Chapter 7) • emergency action guidelines (Chapter 8) • operations guidelines (Chapter 9) These program components can be visualized as a sequence of initial and continuing activities to insure dam safety. The flowchart illustrates the cyclical nature of the program and the need for continuing vigilance. Emergency action can, it is hoped, be avoided, but a well thought out plan of action (Chapter 8) in case of imminent or actual failure can greatly reduce damage and loss of life. Figure 4.1 Flow Chart of Dam Safety Pro gram Components Documenting the Safety Program It is important to document a safety program in order to make the best use of reliable information about the dam. The procedural guidelines that follow can serve as an outline or table of contents for a safety program report. The operations plan (Chapter 9) presents a detailed outline of the infor-mation that should be included in the documen-tation. The chapters that follow suggest forms for inspections, monitoring, etc., which can be used to record information. It is helpful to maintain all the material in a single notebook or file that is easily accessible so that it can be updated and is available when needed. Store a duplicate copy of the report at a different location. INSPECTION OPERATIONS MONITORING rEPAIRS & mAINTENANCE EMERGENCY ACTION Oklahoma Water Resources Board - Dam Safety Program - March 2011 31 CHAPTER 5: Inspection Guidelines 5 Introduction An effective inspection program is essen-tial for identifying problems and pro-viding safe maintenance of a dam. An inspection program should involve four types of inspections: (1) periodic technical inspec-tions; (2) periodic maintenance inspections; (3) downstream development inspections; (4) informal observations by project personnel as they operate the dam. Technical inspections must be performed by professional engineers familiar with the design and construction of dams and should include assessments of structure safety. Maintenance inspections are performed more frequently than technical inspections in order to detect, at an early stage, any developments that may be detrimental to the dam. They involve assessing operational capability as well as structural stability. Maintenance inspections are included as part of the more comprehensive technical inspection. Downstream hazard verification inspections are performed by the dam owner to determine if there has been any construction of homes, buildings, or other structures downstream of their dam which could affect the hazard classification of their dam. This is a particular problem for low hazard-potential dams. If a house, other inhabited structure, or other construction is built downstream of a dam this could result in the need to reclassify to a higher hazard-potential class. This has important implications for the dam owner as it could result in a change how often the dam must be inspected and require structural changes to the dam. Structural changes could include the amount of freeboard that must be maintained and the amount of water the spillway must be able to pass. Informal inspections are actually a continuing effort by the dam owner’s on-site project personnel (dam tenders, powerhouse operators, maintenance workers) performed in the course of their normal duties. The continued effectiveness of these inspections requires education of new personnel. Regular visual inspections are among the most economical means you, the owner, can use to ensure the safety and long life of your dam and its immediate environment. Visual inspection is a straightforward procedure that can be used by any properly trained person to make a reasonably accurate assessment of a dam’s condition. Technical and maintenance inspections involve careful examination of the surface and all parts of the structure, including its adjacent environment, by a professional engineer. The equipment required is not expensive, and the inspection usually can be completed in less than one day (see Appendix A). Hazard verification and informal inspections can be performed by the dam owner or their operator. A dam owner, by applying the maximum prudent effort, can identify any changes in previously noted conditions that may indicate a safety problem. Quick, corrective action to conditions requiring attention will promote the safety and extend the useful life of your dam, while possibly preventing costly future repairs. 32 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Recording Inspection Observations : An accurate and detailed description of conditions during each inspection will enable meaningful comparison of conditions observed at different times. The inspector should record all measure-ments and observed details required for an ac-curate picture of a dam’s current condition and possible problems. Using the forms discussed in Chapter 9, and given in the appendices, will help record the details. This information has three elements: Location : Accurately describe the location of any questionable area or condition so that it can be evaluated for changes over time or re-examined by experts. Photographs should be taken of the upstream and downstream embankments, outlet and conduit structures, emergency spillway, and toe of the dam, as well as photographs of any specific problem areas. Record the location along the dam, as well as the distance above the toe or below the crest. Similarly, document the location of problems in the outlet or spillway. Extent or area: The length, width, and depth (or height) of any suspected problem area should be determined. Organizing for Inspection The following discussion is concerned primarily with technical inspections. All inspections should be organized and systematic. Inspectors should use equipment appropriate for the task, record observations accurately, and survey the structure and site comprehensively. It is essential that documentation be developed and maintained in order to ensure adequate follow-up and repair (Appendix A). Chapter 9 further discusses what form this documenta-tion should take. Technical inspections are to be conducted annually for high hazard-potential dams and once every three years for significant hazard-potential dams. Inspection reports are to be submitted to the Oklahoma Water Resources Board, Dam Safety Program. Owners of low hazard-potential dams are only required to submit a downstream hazard-potential verifica-tion and maintenance inspection once every five years. Descriptive detail : Give a brief yet detailed description of any anomalous condition. Some items to include are: • quantity of drain outflows • quantity of seepage from point and area sources • color or quantity of sediment in water • depth of deterioration in concrete • length, displacement, and depth of cracks • extent of moist, wet, or saturated areas • adequacy of protective cover • adequacy of surface drainage • steepness or configuration of slopes • apparent deterioration rate • changes in conditions Coverage : An inspection is conducted by walking along and over a dam as many times as is required to observe the entire structure. From any given location, a person can usually gain a detailed view for 10 to 30 feet in each direction, depending upon the smoothness of the surface Oklahoma Water Resources Board - Dam Safety Program - March 2011 33 or the type of material (grass, concrete, riprap, brush) on the surface. On the downstream slope, a zigzag inspection path will ensure that any cracking is detected. Sequence: The following inspection sequence ensures that systematic coverage of an entire site is obtained: • upstream slope • crest • downstream slope • seepage areas Following a consistent sequence lessens the chance of an important condition being overlooked. Reporting inspection results in the same sequence is recommended to ensure consistent records. Inspection forms are included in Appendix A. The forms should be supplemented with additional details specific to a given dam. Record keping: The inspector should fill out a dated report for each inspection, which should be filed along with any photographs taken (which should also be dated). In addition to inspection observations, monitoring measurements and weather conditions (especially recent rains, extended dry spells, and snow cover) should also be systematically included in the inspec-tion record. A sketch of the dam with problem areas noted is helpful. Immediately following an inspection, observations should be compared with previous records to see if there are any trends that may indicate developing problems. If a questionable change or trend is noted, and failure is not imminent, you, the owner, should consult a professional engineer experienced in dam safety. Reacting quickly to questionable conditions will ensure the safety and long life of a dam and possibly prevent costly repairs or expensive litigation. Crucial inspection times: There are at least six special times when an inspection is recom-mended regardless of the regular schedule: (1) Prior to a predicted major rainstorm: check spillway, outlet channel, and riprap. (2) During or after a severe rainstorm: check spillway, outlet channel, and riprap. (3) During or after a severe windstorm: check riprap performance during the storm (if possible) and again after the storm has subsided. (4) Following an earthquake in the area: make a complete inspection immediately after the event and weekly inspections for the next several months to detect any delayed effects. (5) During construction, repairs, or modification of the dam. (6) During and immediately after the first reservoir filling: schedule a regular program of frequent complete inspections during the period a reservoir is first being filled to ensure that design and site conditions are as predicted. An inspection and filing schedule are frequently prescribed by the design engineer. Embankment Dams and Structures Embankment dams constitute the majority of structures in place in the U.S. The major features include: • upstream slope • downstream slope • crest • seepage areas • spillway Many of the principles and guidelines presented in that section are also aplicable to concrete structures . ! • inlet • outlet • spillway 34 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Upstream Slope : Typically, major problems encountered on an upstream slope are: • cracks • slides • cave-ins or sinkholes • severe erosion The first three conditions may indicate serious problems within the embankment. Severe erosion obviously can weaken the structure. An upstream slope should receive a close inspection because riprap, vegetative cover, and high water levels can hide problems. (When walking on riprap, take caution to avoid personal injury.) Slope protection is designed to prevent erosion of the embankment slopes, crest, and groin areas. Inadequate slope protection usually results in deterioration of the embankment from erosion, and in the worst cases, can lead to dam failure. The inspector should look for inadequate slope protection, including eroded vegetative cover and displaced riprap. The two primary types of slope protection used on embankment dams include vegetative cover (grass) and riprap (rock). Grass cover is usually used on most embankment surfaces, while riprap is commonly used on the shoreline of the upstream slope. Soil, cement, concrete, asphalt, articulated concrete blocks, and other types of slope protection also may be used. The type of slope protection selected depends upon economics, how the dam is used, and the prevailing conditions found at the site. A good growth of grass on an embankment provides excellent protection against erosion caused by rainfall and runoff. Deep rooted grass that can tolerate repeated wetting and drying cycles should be used on embankments. A lack of vegetative cover or insufficient vegetative cover will result in rapid deterioration of the embankment by erosion. A lack of riprap, or improperly designed riprap along the shoreline can result in erosion of the shoreline soils if riprap is needed to protect the soil against wave action. It should be noted that not all dams will require riprap shoreline protection. A crisscross path should be used when inspecting the slope so that cracks and slides can be easily identified. In many instances, sighting along the waterline alignment will indicate a change in the uniformity Figure 5.1. Drawin g of typical Embankment Dam Features . Oklahoma Water Resources Board - Dam Safety Program - March 2011 35 of the slope; an inspector should stand at one end of the dam and sight along the waterline, checking for straightness and uniformity. If a crack is seen, the crest and downstream slope in its immediate area should be carefully inspected. Cracks indicate possible foundation movement, embankment failure, or a surface slide. Locating them can be difficult. Cracks more than one foot deep usually are not produced by drying and are likely cause for concern. A line of recently dislodged riprap on an upstream slope could indicate a crack below the riprap. Slides can be almost as difficult to detect as cracks. When a dam is constructed, the slopes may not be uniformly graded. Familiarity with the slope configuration at the end of construction can help identify subsequent slope movements. Moreover, the appearance of slides may be subtle; for example, they may produce only about two feet of settlement or bulging in a distance of 100 feet or more, yet that would still be a significant amount of settlement. Dated photographs are particularly helpful in detecting such changes. Sinkholes or cave-ins result from internal erosion of the dam—a very serious condition for earthen embankments. The internal erosion, or piping, may be reflected by turbid seepage water on exit. Surface soil may be eroded by wave action, rain runoff, and animal burrowing. Such erosion, if allowed to continue, can lessen the thickness of the embankment and weaken the structure. Animal burrows on the upstream slope can also indicate a serious problem on smaller dams. Beavers, nutria, and other burrowing animals can create pathways for seepage. See Chapter 7. To ensure adequate inspection, prevent potential seepage paths, and keep the upstream slope free from obscuring weeds, brush, or trees. Downstream Slope : The downstream slope should be inspected carefully because it is the area where evidence of developing problems appears most frequently. To ensure adequate Figure 5.2. figure of various problems with an embankment dam . no trash rack: principal spillway intake structure partially blocked with debris THE PROBLEM DAM ruts from traffic with gullies forming Settlement Surface crack Trees and brush Animal burrows Emergency spillway too small Emergency spillway partially blocked Downstream channel partially blocked Sloughed area springs and excessive seepage 36 Oklahoma Water Resources Board - Dam Safety Program - March 2011 inspection, keep this area free from obscuring weeds, brush, or trees. On the downstream slope, some of the more threatening conditions that could be identified are: • cracks • slides • seepage Notify the designated dam-safety authorities immediately if any of these conditions (Fig. 5.3) are noted on the downstream slope. Cracks can indicate settlement, drying and shrinkage, or the development of a slide. Whatever the cause, cracks should be monitored and changes in length and width noted. Drying cracks may appear and disappear seasonally and normally will not show vertical displacement as will settlement cracks or slide cracks. Slides require immediate detailed evaluation. Early warning signs include a bulge in the embankment near the toe of a dam or vertical displacement in the upper portion of an embankment. Seepage is discussed separately. If a downstream slope is covered with heavy brush or vegetation, a more concentrated search must be made and may require cleaning off the vegetation. In addition, the downstream slope should be inspected for animal burrows, excessive vegetative cover, and for erosion, especially at the contacts with the abutments. Figures 5.1 & 5.2 show potential problems with the downstream slope, causes, possible consequences, and recommended action. Crest: A dam’s crest usually provides the pri-mary access for inspection and maintenance. Because surface water will pond on a crest unless that surface is well maintained, this part of a dam usually requires periodic re-grading. However, problems found on the crest should not be simply graded over or covered up. On the crest, some of the more threatening conditions that may be identified are: • longitudinal cracking • transverse cracking • misalignment • sinkholes Longitudinal cracking (Figure 5.3) can indicate localized instability, differential settlement, movement between adjacent sections of the embankment, or any combination of the three. Longitudinal cracking is typically characterized by a single crack or a close, parallel system of cracks along the crest, more or less parallel to the axis of the dam. These cracks, which are usually continuous over their length and usually greater than one foot deep, can be differentiated from drying cracks, which are usually intermittent, erratic in pattern, shallow, very narrow, and numerous. Longitudinal cracking may precede vertical displacement as a dam attempts to adjust to a position of greater stability. Frequently, longitudinal cracking occurs at the edge of the crest with either slope. Vertical displacements on the crest are usually accompanied by displacements on the upstream or downstream face of a dam. Vertical Displacement Figure 5.3. Longitudinal cracks Oklahoma Water Resources Board - Dam Safety Program - March 2011 37 Transverse cracking (Figure 5.4) can indicate differential settlement or movement between adjacent segments of a dam. Transverse cracking usually manifests as a single crack or a close, parallel system of cracks that extend across the crest more or less perpendicular to the length of the dam. This type of cracking is usually greater than one foot in depth. If this condition is seen or suspected, notify the Oklahoma Dam Safety Program office immediately. Transverse cracking poses a definite threat to the safety and integrity of a dam. If a crack should progress to a point below the reservoir water-surface elevation, seepage could progress along the crack and through the embankment, causing severe erosion and—if not corrected—leading to failure of the dam. Misalignment can indicate relative movement between adjacent portions of a dam—generally perpendicular to its axis. Excessive settlement of dam material, the foundation, or both can also cause misalignment. Most problems are usually detectable during close inspection. Misalignment may, however, only be detectable by viewing a dam from either abutment. If on close inspection the crest appears to be straight for the length of the structure, alignment can be further checked by standing away from the dam on either abutment and then sighting along the upstream and downstream edges of the crest. On curved dams, alignment can be checked by standing at either end of a short segment of the dam and sighting along the crest’s upstream and downstream edges, noting any curvature or misalignment in that section. Leaning utility poles or poles used for highway barriers can also indicate movement. Sinkholes can indicate internal collapse, piping, or the presence of animal dens. The formation or progression of a sinkhole is dangerous because it poses a threat to inspectors or vehicles traversing the crest. A sinkhole collapse can also lead to a flow path through a dam, which can create an uncontrolled breach. The crest should be inspected for animal burrows, low areas, vegetative cover, erosion, sloping of the crest, narrowing of the crest, and traffic ruts. Seepage Areas: As discussed previously, al-though all dams have some seepage, seepage in any area on or near a dam can be dangerous, and all seepage should be treated as a potential problem. Wet areas downstream from dams are not usually natural springs, but seepage areas (Figure 5.5). Seepage must be controlled in both velocity and quantity. High-velocity flows through a dam can cause progressive erosion and, ultimately, failure. Saturated areas of an embankment or abutment can move in massive slides and thus also lead to failure. Seepage can emerge anywhere on the downstream face of a dam, beyond the toe, or on the downstream abutments at elevations below normal reservoir levels. A potentially dangerous condition exists when seepage appears on the downstream face above the toe of a dam (Figure 5.6). If seepage is found on the top half of the downstream slope, the problem should be immediately corrected. Seepage Figure 5.4. transverse cracks . Initial Transverse Cracking Progression of Transverse Crack to a Point Below the Waterlines 38 Oklahoma Water Resources Board - Dam Safety Program - March 2011 on the downstream slope can cause a slide or failure of the dam by internal erosion (piping). Evidence of seepage may vary from a soft, wet area to a flowing spring and may appear initially as only an area where vegetation is lush and dark green in color. Cattails, reeds, mosses, and other marsh vegetation often become established in seepage areas. Downstream abutment areas should always be inspected closely for signs of seepage, as should the area of contact between an embankment and a conduit spillway, drain, or other appurtenant structures and outlets. Slides in the embankment or an abutment may be the result of seepage causing soil saturation and high pore pressures. Since seepage can be present but not readily visible, an intensive search should be made of all downstream areas where seepage water might emerge. Even in short grass cover, seepage may not be visible and must be walked on to be found. Ideally, an inspection for seepage should be made when a reservoir is full. Concrete Dams and Structures From a safety standpoint, the principal advantage of concrete over earthen dams is their relative freedom from failure by erosion during overtopping as well as from embankment slides and piping failures. Although concrete dams comprise a minority of all dams, they are commonly of greater height and storage capacity than earthen structures. Thus, they often represent a potentially greater hazard to life and property. It is important that concrete-dam owners be aware of the principal modes of failure of such dams and that they be able to discern between conditions which threaten the safety of the dam and those that merely indicate a need for maintenance. Concrete dams fail for reasons that are significantly different from earth dams. These include: • structural cracks • foundation and abutment weakness • deterioration due to alkali-aggregate reaction If any of these conditions are discovered during inspection, an owner should immediately address the problem with his/her engineer. Structural cracks occur when portions of the dam are overstressed; they result from inadequate design, poor construction, foundation settlement, or faulty materials. Structural cracks are often irregular, may run at an angle to the major axes of the dam and may exhibit abrupt changes in direction. These cracks can also be noticeably displaced, radially, transversely, or vertically. Concrete dams transfer a substantial load to the abutments and foundation. Although the concrete of a dam may endure, the natural abutments or foundation may crack, crumble, or move in a massive slide. If that occurs, support for the dam is lost and it fails. Impending failure of the foundation or abutments may be difficult to detect because initial movements are often very small. Figure 5.5. area of sepa ge near toe of dam . Oklahoma Water Resources Board - Dam Safety Program - March 2011 39 Severe deterioration can result from a chemical reaction between alkali present in cements and certain forms of silica present in some aggregates. This chemical reaction produces by-products of silica gels, which cause expansion and loss of strength within concrete. An alkali reaction is characterized by certain observable conditions such as cracking (usually a random pattern on a fairly large scale), and by excessive internal and overall expansion. Additional indications include the presence of a gelatinous exudation or whitish amorphous deposits on the surface and a chalky appearance in freshly fractured concrete. The alkali-aggregate reaction takes place in the presence of water. Surfaces exposed to the elements or dampened by seepage will deteriorate most rapidly. Once suspected, the condition can be confirmed by a series of tests performed on core samples drilled from a dam. Although the deterioration is gradual, an alkali-aggregate reaction cannot be economically corrected by any means now known. Continued deterioration may require total replacement of a structure. Inspection of a concrete dam is similar to that of an earthen dam. However, the following additional items should be considered: • access and safety • monitoring • outlet system • cracks at construction and expansion joints • shrinkage cracks • deterioration due to spalling • minor leakage Access and safety are important because the faces of concrete dams are often nearly vertical, and sites are commonly steep-walled rock canyons. Access to the downstream face, toe area, and abutments of such dams may be difficult and require special safety equipment, such as safety ropes or a boatswain’s chair. Concrete dams pose a special problem for the dam owner because of the difficulty in gaining close access to the steep surfaces. Regular inspection with a pair of powerful binoculars can initially identify areas where change is occurring. When changes are noted, a detailed, close-up inspection should be conducted. Close inspection of the upstream face may also require a boatswain’s chair or a boat. Figure 5.6. sepa ge throu gh embankment dam . Uncontrolled Seepage Through and Embankment Reservoir water surface sepage surface foundation sepage sepage 40 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Monitoring helps detect structural problems in concrete dams such as cracks in the dam, abutments, or foundation. Cracks may develop slowly at first, making it difficult to determine if they are widening or otherwise changing over time. If a structural crack is discovered, it should be monitored for changes in width, length, and offset, and a network of monitoring instruments should be installed and read regularly. Outlet-system deterioration is a problem for all dams, but the frequency of such damage may be higher in concrete dams because of their greater average hydraulic pressure. Thus, outlet-system inspection should be emphasized for large concrete dams. Cracks at construction joints exist because concrete dams are built in segments, while expansion joints—referred to as “designed” cracks—are built into dams to accommodate volumetric changes which occur in the structures after concrete placement. These joints are typically constructed so that no bond or reinforcing, except non-bonded water stops and dowels, extend across the joints. Shrinkage cracks often occur when, during original construction, irregularities or pockets in the abutment contact are filled with concrete and not allowed to cure fully prior to placement of adjacent portions of the dam. Subsequent shrinkage of the concrete may lead to irregular cracking at or near the abutment. Shrinkage cracks are also caused by temperature variation. During winter months, the upper portion of a dam may become significantly colder than those portions in direct contact with reservoir water. This temperature differential can result in cracks which extend from the crest for some distance down each face of the dam. These cracks will probably occur at construction or expansion joints, if any. Shrinkage cracks can be a sign that certain portions of the dam are not carrying the design load. In such cases, the total compression load must be carried by a smaller proportion of the structure. It may be necessary to restore load-carrying capability by grouting affected areas. This work requires the assistance of an engineer. Spalling is the process by which concrete chips and breaks away as a result of freezing and thawing, corrosion of the reinforcement, or movement. Almost every concrete dam in colder climates experiences continued minor deterioration due to spalling. Because it usually affects only the surface of a structure, it is not ordinarily considered dangerous. However, if allowed to continue, spalling can result in structural damage, particularly if a dam is thin in cross-section. Repair is also necessary when reinforcing steel becomes exposed. The method of repairing spalled areas depends upon the depth of the deterioration. In severe situations, engineering assistance is required. Minor leakage through concrete dams, although unsightly, is not usually dangerous unless accompanied by structural cracking. The effect may be to promote deterioration due to freezing and thawing. However, increases in seepage could indicate that, through chemical action, materials are being leached from the dam and carried away by the flowing water. Dam owners should note that decreases in seepage can also occur as mineral deposits are formed in portions of the seepage channel. In either case, the condition is not inherently dangerous and detailed study is required to determine if repair is necessary for other than cosmetic reasons. Spillways As detailed in Chapter 2, the main function of a spillway is a safe exit for excess water in a reservoir. If a spillway is too small, a dam could be overtopped and fail. Similarly, defects in a spillway can cause failure by rapid erosion. A spillway should always be kept free of obstructions, have the ability to resist erosion, and be protected from deterioration. Because dams represent a substantial investment and spillways make up a major part of dam costs, Oklahoma Water Resources Board - Dam Safety Program - March 2011 41 a conscientious annual maintenance program should be pursued not only to protect the public but to minimize costs as well. The primary problems encountered with spillways include: • inadequate capacity • obstructions • erosion • deterioration • cracks • open joints • undermining of the spillway outlet • deterioration of spillway gates Inadequate capacity is determined by several factors, such as the drainage area served, the magnitude or intensity of storms in the watershed, the storage capacity of the reservoir, and the speed with which rainwater flows into and fills the reservoir. An inadequate spillway can cause the water in a reservoir to overtop the dam. Obstruction of a spillway is commonly due to excessive growth of grass and weeds, thick brush, trees, debris, fences across channels to prevent migration of fish, or landslide deposits. An obstructed spillway can have a substantially reduced discharge capacity which can lead to overtopping of the dam. Grass is usually not considered an obstruction; however, tall weeds, brush, and young trees should periodically be cleared from spillways. Similarly, any substantial amount of soil deposited in a spillway—whether from sloughing, landslide or sediment transport—should be immediately removed. Timely removal of large rocks is especially important, since they can obstruct flow and encourage erosion. Erosion of a spillway may occur during a large storm when large amounts of water flow for many hours. Severe damage of a spillway or complete washout can result if the spillway cannot resist erosion. If a spillway is excavated out of a rock formation or lined with concrete, erosion is usually not a problem. However, if a spillway is excavated in sandy soil, deteriorated granite, clay, or silt deposits, protection from erosion is very important. Deterioration of a spillway can greatly affect its performance. Generally, resistance to deterioration can be increased if a spillway channel has a mild slope, or if it is covered with a layer of grass or riprap with bedding material. Examples of spillway deterioration may include collapse of side slopes, cracking or undermining of concrete lining, erosion of the approach section, chute channel, stilling basin, and discharge channel. These problems can cause water to flow under and around the protective material and lead to severe erosion. Remedial action must be taken as soon as any sign of deterioration has been detected. Cracks in an earthen spillway channel are usually not regarded as a functional problem. However, missing rocks in a riprap lining can be considered a crack in the protective cover, and must be repaired at once. Cracks in concrete lining of a spillway are commonly encountered. These cracks may be caused by uneven foundation settlement, shrinkage, slab displacement, or excessive earth or water pressure. Large cracks will allow water to wash out fine material below or behind the concrete slab, causing erosion, more cracks, and even severe displacement of the slab. The slab may even be dislodged and washed away by the flow. A severely cracked concrete spillway should be examined by and repaired under the supervision of an engineer. Open or displaced joints can occur from excessive and uneven settlement of the foundation or the sliding of a concrete slab. In some cases, a construction joint is too wide or has been left unsealed. Sealants deteriorate and wash away. Water can flow through the joints, undermining the slabs, which in turn could result in collapse of the spillway slabs. Pressures resulting from water flowing over the open slabs could also result in lifting and displacement of slabs. Hence, all joints need to be sealed and kept sealed. 42 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Undermining of the spillway outlet is the erosion of foundation material and may weaken support and cause further cracks. Pressure induced by water flowing over displaced joints may wash away part of a wall or slab, or cause extensive undermining. Undermining of a spillway causes erosion at a spillway outlet, whether it is a pipe or overflow spillway, and is one of the most common spillway problems. Severe undermining of the outlet can displace sections of pipe, cause slides in the downstream embankment of the dam, and eventually lead to complete failure of a dam. Water must be conveyed safely from the reservoir to a point downstream of the dam without endangering the spillway itself or the embankment. Often the spillway outlet is adequately protected for normal flow conditions, but not for extreme turbulent flows. It is easy to misestimate the energy and force of flowing water and the resistance of outlet material (earth, rock, concrete, etc). The required level of protection is difficult to establish by visual inspection but can usually be determined by hydraulic calculations performed by a professional engineer. Structures that completely control erosion at a spillway outlet are usually expensive, but often necessary. Less expensive protection can also be effective, but require extensive periodic maintenance as areas of erosion and deterioration develop. The following four factors, often interrelated, contribute to erosion at the spillway outlet: 1. Flows emerging from the outlet are above the stream channel. If outlet flows emerge at the correct elevation, tailwater in the stream channel can absorb a substantial amount of the high velocity. The flow and the hydraulic energy will be contained in the stilling basin. 2. Flows emerging from the spillway are generally free of sediment and therefore have substantial sediment-carrying capacity. In taking on sediment, moving water will scour soil material from the channel and leave eroded areas. Such erosion is difficult to design for and requires protection of the outlet for a safe distance downstream from the dam. 3. Flows leaving the outlet at high velocity can create negative pressures that can cause material to come loose and separate from the floor and walls of the outlet channel. This process is called cavitation when it occurs on concrete or metal surfaces. Venting can sometimes be used to relieve negative pressures. 4. Water leaking through pipe joints or flowing along a pipe from the reservoir may weaken the soil structure around the pipe. Inadequate compaction adjacent to such structures during construction can compound this problem. Deterioration of spillway gates can result in an inability of the gates to function during storm events. Causes of structural deterioration include, but are not limited to: 1. Corrosion can seriously weaken a structure or impair its operation. The effect of corrosion on the strength, stability, and serviceability of gates must be evaluated. A loss of cross section in a member causes a reduction in strength and stiffness that leads to increased stress levels and deformation without any change in the imposed loading. Flexure, shear, and buckling strength may be affected. A buildup of corrosion products can be damaging at connection details. For example, corrosion buildup in a tainter gate trunnion can lead to extremely high hoist loads. Localized pitting corrosion can form notches that may serve as fracture initiation sites, which could significantly reduce the member’s fatigue life. 2. Fracture usually initiates at a discontinuity that serves as a local stress raiser. Structural connections that are welded, bolted, or riveted are sources of discontinuities and stress concentrations. Oklahoma Water Resources Board - Dam Safety Program - March 2011 43 3. Fatigue is the process of cumulative damage caused by repeated cyclic loading. Fatigue damage generally occurs at stress-concentrated regions where the localized stress exceeds the yield stress of the material. Fatigue is particularly a concern with spillway gates with vibration problems. 4. Proper operation and maintenance of spillway gates are necessary to prevent structural deterioration. The following items are possible causes of structural deterioration. a. Weld repairs are often sources of future cracking or fracture problems, particularly if the existing steel had poor weldability. b. If moving connections are not lubricated properly, the bushings will wear and result in misalignment of the gate, resulting in wear of other parts and unforeseen loads. c. Malfunctioning limit switches could result in detrimental loads and wear. d. A coating system or cathodic protection that is not maintained can result in detrimental corrosion of metal components. 5. Unforeseen loading of a gate can result in deformed members or fracture. When structural members become deformed or buckled, they may have significantly reduced strength or otherwise impair the performance of the gate. Dynamic loading may be caused by hydraulic flow at the seals. Other unusual loadings may occur from malfunctioning limit switches or debris trapped at interfaces between moving parts. Unusual loads may also develop on gates supported by walls that are settling or moving. These unusual loads can cause overstressing and lead to deterioration. Mark Harrison , Oklahoma Conservation Comission 44 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Procedure for Inspection of the Spillway Spillway inspection is an important part of a dam safety program. Its basic objective is to detect any sign of obstruction, erosion, deterioration, misalignment, or cracking. An inspection of an earth spillway should determine whether side slopes have sloughed and whether there is excessive vegetation in the channel, and should look for signs of erosion and rodent activity. The inspector should also use a probe to determine the hardness and moisture content of the soil, note the location of particularly wet or soft spots, and see if the stilling basin or drop structure is properly protected with rocks or riprap. Because some erosion is unavoidable during spilling, an owner should also determine whether such erosion might endanger the embankment itself. If the spillway is installed with a sill or wall, a dam owner should also determine if there are any cracks or misalignment in the sill or wall and check for erosion beneath the sill or wall or downstream from it. Hairline cracks are usu-ally harmless. Large cracks should be carefully inspected and their location, width, length, and orientation noted. Deterioration should be de-termined. The concrete should be examined for exposure of reinforcing bars. Spillway surfaces exposed to freeze-thaw cycles often suffer from surface spalling. Chemical ac-tion, corrosion of the reinforcement, movement, contamination, and unsound aggregates can also cause spalling. If spalling is extensive, the spalled area should be sketched or photographed, show-ing its length, width, and depth. The problem should be examined closely to see if the remaining concrete has deteriorated or if reinforcing bars are exposed. The concrete should be tapped with a tapping device or rock hammer to determine if voids exist below the surface. Shallow spalling should be examined from time to time to determine if it is becoming worse. Deep spalling should be repaired as soon as possible by an experienced contractor. Walls of spillways are usually equipped with weep (or drain) holes. Occasionally spillway chute slabs are also equipped with weep holes. If all such holes are dry, the soil behind the wall or below the slab is probably dry as well. If some holes are draining while others are dry, the dry holes may be plugged by mud or mineral deposits. Plugged weep holes increase the chances for failure of retaining walls or chute slabs. The plugged holes should be probed to determine causes of blockage, and soil or deposits cleaned out to restore drainage. If that work is not successful, rehabilitate the drain sys-tem as soon as possible under the supervision of a professional engineer. Spillway retaining walls and chute slabs are normally constructed in sections. Between ad-joining sections, gaps or joints must be tightly sealed with flexible materials such as tar, epox-ies, or other chemical compounds. Sometimes rubber or plastic diaphragm materials or cop-per foil are used to obtain water tightness. Dur-ing inspection, one should note the location, Oklahoma Water Resources Board - Dam Safety Program - March 2011 45 length, and depth of any missing sealant, and probe open gaps to determine if soil behind the wall or below the slab has been undermined. Misalignment of spillway retaining walls or chute slabs may be caused by foundation settlement or earth or water pressure. The inspector should carefully look at the upstream or downstream end of a spillway near the wall to determine if it has been tipped inward or outward. Relative displace-ment or offset between neighboring sections can be readily identified at joints. The horizontal as well as vertical displacement should be measured. A fence on top of the retaining wall is usually erected in a straight line at the time of construc-tion; thus any curve or distortion of the fence line may indicate wall deformation. At the time of construction, the entire spillway chute should form a smooth surface. Thus, measurement of relative movement between neighboring chute slabs at joints will give a good indication of slab displacement. Misalignment or displacement of walls or the slab is often accompanied by cracks. A clear description of crack patterns should be recorded or photos taken to help in understanding the nature of the displacement. The folowing areas should be in-spected on al gates in spilways : • main framing members and lifting and support assemblies • locations susceptible to fracture or weld-related cracking • corrosion-susceptible areas—normal waterline, abrasion areas, crevices, areas where water could stand • lifting connections and chains or cables • trunnions • intersecting welds • previous cracks repaired by welding • locations of previous repairs or where damage has been reported • seal plates Inlets, Outlets, and Drains A dam’s inlet and outlet works, including internal drains, are essential to its operation. Items for inspection and special attention include: • reservoir pool levels • lake drains and internal drains • corrosion • trash racks on pipe spillways • cavitation • areas on gates and spillways as listed immediately above Reservoir pool level drawdown should not exceed about 1 foot per week for slopes composed of clay or silt materials except in an emergency. Very flat slopes or slopes with free-draining upstream soils can, however, withstand more rapid drawdown rates. Pool levels can be controlled by spillway gates, drain-and-release structures, or flashboards. Flashboards, sometimes used to permanently or temporarily raise the pool level of water supply reservoirs, should not be installed or allowed unless there is sufficient freeboard remaining to safely accommodate a design flood Conditions causing or requiring temporary or permanent adjustment of the pool level include: • A problem that requires lowering of the pool. Drawdown is a temporary solution until the problem is solved. • Release of water downstream to supplement stream flow during dry conditions. • Fluctuations in the service area’s demand for water. • Repair of boat docks in the winter and growth of aquatic vegetation along the shoreline. • Requirements for recreation, hydropower, or waterfowl and fish management. ! 46 Oklahoma Water Resources Board - Dam Safety Program - March 2011 Lake drains : A lake drain should always be operable so that the pool level can be drawn down in case of an emergency or for necessary repair. Lake-drain valves or gates that have not been operated for a long time can present a special problem for owners. If the valve cannot be closed after it is opened, the impoundment could be completely drained. An uncontrolled and rapid drawdown could also cause more serious problems such as slides along the saturated upstream slope of the embankment or downstream flooding. Therefore, when a valve or gate is operated, it should be inspected and all appropriate parts lubricated and repaired. It is also prudent to advise downstream residents of large or prolonged discharges. Testing a valve or gate without risking complete drainage entails physically blocking the drain inlet upstream from the valve. Some drains have been designed with this capability and have dual valves or gates, or slots for stop logs (sometimes called bulkheads) upstream from the valve. Otherwise, divers can be hired to inspect the drain inlet and may be able to construct a temporary block at the inlet. Since that could be dangerous, safety precautions are needed. Other problems may be encountered when operating a lake drain. Sediment can build up and block the drain inlet, or debris can enter the valve chamber, hindering its function. The likelihood of these problems is greatly decreased if the valve or gate is operated and maintained on a schedule prepared by a professional engineer. Corrosion is a common problem of pipe spillways and other conduits made of metal. Exposure to moisture, acid conditions, or salt will accelerate corrosion. In particular, acid runoff from strip mine areas will cause rapid corrosion of steel pipes. In such areas, pipes made of noncorrosive materials such as concrete or plastic should be used. Metal pipes which have been coated to resist accelerated corrosion are also available. The coating can be of epoxy, aluminum, zinc (galvanization), asbestos or mortar. Coatings applied to pipes in service are generally not very effective because of the difficulty of establishing a bond. Similarly, bituminous coating cannot be expected to last more than one to two years on flow ways. Of course, corrosion of metal parts of operating mechanisms can be effectively treated and prevented by keeping those parts greased and/or painted. Corrosion can also be controlled or arrested by installing cathodic protection. A sacrificial metallic anode made out of a material such as magnesium is buried in the soil and is connected to the metal pipe by wire. An electric potential is established which causes the magnesium to corrode and not the pipe. Trash on pipe spilways : Many dams have pipe and riser spillways. As with concrete spillways, pipe inlets that become plugged with debris or trash reduce spillway capacity. As a result, the potential for overtopping is greatly increased, particularly if there is only one outlet. A plugged principal spillway will cause more frequent and greater than normal flow in the emergency spillway which is de-signed for infrequent flows of short duration Figure 5.7. Keeping the trash rack fre of debris reduces the chance of overtoppin g the dam . Oklahoma Water Resources Board - Dam Safety Program - March 2011 47 and thus result in serious and unnecessary damage. For these reasons trash collectors or trash racks should be installed at the inlets to pipe spillways and lake drains (Figure 5.7). A well-designed trash rack will stop large debris that could plug a pipe but allow unrestricted passage of water and smaller debris. Some of the most effective racks have submerged openings which allow water to pass beneath the trash into the riser inlet as the pool level rises. Openings that are too small will stop small debris such as twigs and leaves, which in turn will cause a progression of larger items to build up, eventually completely blocking the inlet. Trash rack openings should be at least 6 inches across, regardless of the pipe size. The larger the principal spillway conduit, the larger the trash rack opening should be. The largest possible openings should be used, up to a maximum of about 2 feet. A trash rack should be properly attached to the riser inlet and strong enough to withstand the forces of fast-flowing debris, heavy debris, and ice. It is a common occurrence for vandals to throw riprap stone into the riser. The size of the trash rack openings should not be decreased to prevent this. Instead, use riprap that is larger than the trash rack openings or too large to handle. Maintenance should include periodic checking of the trash rack for rusted and broken sections and repair as needed. The rack should be checked frequently during and after storms to ensure that it is functioning properly and to remove accumulated debris. Cavitation : When water flows through an out-let system and passes restrictions (e.g., valves), the pressure may drop. If localized water pres-sures drop below the vapor pressure of water, a partial vacuum is created and the water actually boils, causing shock-waves which can damage the outlet pipes and control valves. This process can be a serious problem for large dams where discharge velocities are high. Testing the Outlet System All valves should be fully opened and closed at least once a year. This not only limits corrosion buildup on control stems and gate guides, but also provides an opportunity to check for smooth operation of the system. Jerky or erratic operation could signal problems, and indicate the need for more detailed inspection. The full range of gate settings should be checked. The person performing the inspection should slowly open the valve, checking for noise and vibration. Certain valve settings may result in greater turbulence. The inspector should also listen for noise like gravel being rapidly transported through the system. This sound would indicate some cavitation and henceforth, those gate settings should be avoided. The operation of all mechanical and electrical systems, backup electric motors, power generators, power and lighting wiring associated with the outlet should all be checked. Inspecting the Outlet System Accessible portions of the outlet, such as the outfall structure and control, can be inspected easily and regularly. However, severe problems are commonly associated with deterioration or failure of portions of the system either buried in the dam or normally under water. • Outlet pipes 30 inches or greater in diameter can be inspected internally, provided the system has an upstream valve allowing the pipe to be emptied. Tapping the conduit interior with a hammer can help locate voids behind the pipe. This type of inspection should be performed at least once a year. • Small-diameter outlet pipes can be inspected by remote TV camera if necessary. The camera is channeled through the conduit and transmits a picture back to an equipment 48 Oklahoma Water Resources Board - Dam Safety Program - March 2011 truck. This type of inspection is expensive and usually requires the services of an engineer. However, if no other method of inspection is possible, inspection by TV is recommended at least once every five years. • Outlet intake structures, wet wells, and outlet pipes with only downstream valves are the most difficult dam appurtenances to inspect because they are usually under water. These should be inspected whenever the reservoir is drawn down or at five-year intervals. If a definite problem is suspected, or if the reservoir remains full over extended periods, divers should be hired to perform an underwater inspection. General Areas Other areas requiring inspection include: • mechanical and electrical systems • the reservoir surface and shoreline • the upstream watershed • downstream floodplains Mechanical equipment includes spillway gates, sluice gates or valves for lake drains or water supply pipes, stop logs, sump pumps, flashboards, relief wells, emergency power sources, siphons, and other devices. All mechanical and ass |
Date created | 2011-07-21 |
Date modified | 2012-05-14 |