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THE SAFETY OF DAMS 4 2 The Safety of Dams For centuries, dams have provided mankind with such essential benefits as water supply, flood control, recreation, hydropower, and irrigation. They are an integral part of society's infrastructure. In the last decade, several major dam failures have increased public awareness of the potential hazards caused by dams. In today's technical world, dam failures are rated as one of the major ''low- probability, high-loss'' events. The large number of dams that are 30 or more years old is a matter of great concern. Many of the older dams are characterized by increased hazard potential due to downstream development and increased risk due to structural deterioration or inadequate spillway capacity. The National Dam Inspection Program (PL 92-367) developed an inventory of about 68,000 dams that were classified according to their potential for loss of life and property damage (U. S. Army Corps of Engineers 1982b). About 8,800 "high hazard" dams (those whose failure would cause loss of life or substantial economic damage) were inspected and evaluated. Specific remedial actions have been recommended, ranging from more detailed investigations to immediate repair for correction of emergency conditions. The responsibility for the subsequent inspections, investigations, and any remedial work rests with the owners of the dams. In most states the actions or inactions of the dam owners will be monitored by a state agency responsible for supervision of the safety of dams. The National Dam Inspection Program provided a beginning to what is hoped to be a continuing effort to identify and alleviate the potential haz
THE SAFETY OF DAMS 5 ards presented by dams. Essential to the success of such an effort are understandings of the causes of dam failures and the effects of age; competent inspection and maintenance programs; thorough knowledge of individual site conditions as revealed by design, construction, and operating records, in addition to inspections and investigations; and an emergency action plan to minimize the consequences of dam failure. The remainder of this chapter presents a discussion of these elements. CAUSES OF DAM FAILURES Dam Failure Surveys A number of studies have been made of dam failures and accidents. The results of one survey, by the International Commission on Large Dams (ICOLD), were reported in its publication Lessons from Darn Incidents, USA. N. J. Schnitter (1979 Transactions of ICOLD Congress, New Delhi) summarized the survey data in the form illustrated by Figures 2-1 through 2-5. These data pertain only to dams more than 15 meters in height and include only failures resulting in water releases downstream. Figure 2-1 shows the relative importance of the three main causes of failures: overtopping, foundation defects, and piping. Overall, these three causes have about the same rate of incidence. Figure 2-2 gives the incidence of the causes of failure as a function of the dam's age at the time of failure. It can be seen that foundation failures occurred relatively early, while the other causes may take much longer to materialize. Figure 2-3 compares the heights of the failed dams to those of all dams built and shows that 50% of the failed dams are between 15 and 20 meters high. Figure 2-4 shows the relation between dams built and failed for the various dam types from 1900 to 1969. According to the bottom graph, gravity dams appear the safest, followed by arch and fill dams. Buttress dams have the poorest record but are also the ones used least. Figure 2-5 shows the improvement of the rate of failure over the 1900-1975 period. The upper graph is in semilogarithmic scale and gives the percentage of failed dams in relation to all dams in operation or at risk at a given time. The lower graph gives the proportion of the built dams that later failed and shows that modern fill and concrete dams are about equally safe. The United States Committee on Large Dams (USCOLD) made a survey of incidents to dams in the United States. Results of the initial study, which covered failures and accidents to dams through 1972, were published
THE SAFETY OF DAMS 6 jointly by the American Society of Civil Engineers (ASCE) and USCOLD in 1975 in Lessons from Dam Incidents, USA. These data were updated through subsequent USCOLD surveys of incidents occurring between 1972 and 1979. Table 2-1 was compiled from the information developed by the USCOLD surveys and includes accidents as well as failures. The USCOLD surveys pertained only to dams 15 or more meters in height. Figure 2-1 Cause of failure. Source: ICOLD (1973). Table 2-2 pertains only to concrete dams and lists the number of incidents in the USCOLD surveys for each principal type of such dams. Tables 2-1 and 2-2 list incidents by the earliest, or "triggering," principal cause as accurately as could be determined from the survey data. For instance, where failure was due to piping of embankment materials through a corroded outlet, the corrosion or deterioration was accepted as being the primordial cause of failure. Also, where a sliding failure was due to overtop
THE SAFETY OF DAMS 7 ping flows that eroded the foundation at the toe of a concrete dam, overtopping was listed as the cause of failure. Only one cause is listed for each incident. While only a few of the incidents were attributed to faulty construction, it is reasonable to expect that many of the other failures were due, at least in part, to inadequate construction or design investigations. However, the information on the specific eases is not sufficient to establish such inadequacies as the primordial causes. Failure Modes and Causes Table 2-3 pertains to embankment dams. It is of particular interest because it correlates failure modes and causes. As indicated, the modes and causes of failure are varied, multiple, and often complex and interrelated, i.e., Figure 2-2 Age at failure. Source: ICOLD (1973).
THE SAFETY OF DAMS 8 often the triggering cause may not truly have resulted in failure had the dam not had a secondary weakness. These causes illustrate the need for careful, critical review of all facets of a dam. Such a review should be based on a competent understanding of causes (and weaknesses), individually and collectively, and should be made periodically by experts in the field of dam engineering. Figure 2-3 Height of dams. Source: ICOLD (1973). Many dam failures could be cited to illustrate complex causes and the difficulty of identifying a simple, single root cause. For example, the 1976 Teton failure may be attributed to seepage failure (piping) (Jansen 1980). But several contributing physical (and institutional) causes may be identified (Independent Panel to Review the Cause of Teton Dam Failure 1976). In another example, a dam in Florida was lost due to a slope failure, trig
THE SAFETY OF DAMS 9 gered by seepage erosion of fine sandy soils at the embankment's toe. The soils lacked sufficient cohesion to support holes or cavities normally associated with piping and were removed from the surface of the dam toe by excessive seepage velocities and quantities. This undermining of the toe by seepage resulted in a structural failure, but the prime cause was the nature of the foundation soils. The complex interrelationship of failure modes and causes makes it extremely difficult to prepare summary tables such as Table 2-1. It also explains why different evaluators could arrive at different conclusions regarding prime causes. Certainly any such table should be accompanied by Figure 2-4 Dam types (Western Europe and USA, 1900-1969). Source: ICOLD (1979).
THE SAFETY OF DAMS 10 a commentary to provide the reader with a better understanding of the data. Thus in the following descriptions of each category of cause identified in Table 2-1, additional information is given about the involved incidents. Figure 2-5 Probability of failure (Western Europe and USA). Source: ICOLD (1979). Overtopping Overtopping caused about 26% of the reported failures and represents about 13% of all incidents. The principal reason for overtopping was inadequate spillway capacity. However, in 2 failure cases overtopping was
THE SAFETY OF DAMS 11 attributed to blockage of the spillways and in 2 others to settlement and erosion of the embankment crest, thus reducing the freeboard. In 1 of the latter cases the settlement was great enough to lower the elevation of the top of embankment below that of the spillway crest. Six concrete dams have failed due to overtopping and 3 others were involved in accidents. Two of the overtopping failures resulted from instability due to erosion of the rock foundation at the toe of dam, and 4 were due to the washout of an abutment or adjacent embankment structure. In 1 of these events a saddle spillway was first undermined and destroyed, and then the abutment ridge between the spillway and the dam was lost by erosion. In one instance of erosion of the rock at the toe of the dam, piping was suspected as a contributing cause. The 3 overtopping accidents reported for concrete dams involved erosion of the downstream foundation in only 1 case. In another instance the powerhouse and equipment were damaged, but the dam sustained no damage. TABLE 2-1 Causes of Dam Incidents Type of Dam Concrete Embankment Other* Totals Cause F A F A F A F A F&A Overtopping 6 3 18 7 3 27 10 37 Flow erosion 3 14 17 17 17 34 Slope protection 13 13 13 damage Embankment 23 14 23 14 37 leakage, piping Foundation 5 6 11 43 1 17 49 66 leakage, piping Sliding 2 5 28 7 28 35 Deformation 2 3 29 3 6 31 37 Deterioration 6 2 3 2 9 11 Earthquake 3 3 3 instability Faulty construction 2 3 2 3 5 Gate failures 1 2 1 3 2 5 7 TOTAL 19 19 77 163 7 103 182 285 * Steel, masonry-wood, or timber crib. F = failure. A = accident = an incident where failure was prevented by remedial work or operating procedures, such as drawing down the pool. SOURCE: Compiled from Lessons from Dam Incidents, USA, ASCE/USCOLD 1975, and supplementary survey data supplied by USCOLD.
THE SAFETY OF DAMS 12 TABLE 2-2 Causes of Concrete Dam Incidents Concrete Dam Type Arch Buttress Gravity Totals F A F A F A F A F&A Overtopping 2 1 1 3 2 6 3 9 Flow erosion 1 1 1 3 3 Foundation leakage, piping 1 1 2 2 5 5 6 11 Sliding 2 2 2 Deformation 2 2 2 Deterioration 3 2 1 6 6 Faulty construction 2 2 2 Gate failures 1 2 1 2 3 TOTAL 4 7 4 2 11 10 19 19 38 F = failure. A = accident. SOURCE: Compiled from Lessons from Dam Incidents, USA, ASCE/USCOLD 1975, and supplementary survey data supplied by USCOLD. In the third, structural cracking was believed to have been caused by the overtopping load on the structure, resulting in subsequent reservoir leakage through the dam. Flow Erosion This category includes all incidents caused by erosion except for overtop- ping, piping, and failure of slope protection. Flow erosion caused 17% of the failures and 12% of all reported incidents. Of the 17 failures, 14 were at embankment dams where, except in 2 cases, the spillways failed or were washed out. In 1 instance the gate structure rafted due to erosion of its foundation, and in another the embankment adjacent to the spillway weir was washed out. In the latter case, overtopping and/or poor compaction of the spillway-embankment interface was suspected but not confirmed. With respect to the 3 concrete dam failures, the spillways were destroyed in 2 instances and in the other, a small buttress dam, the entire dam was destroyed. The 17 reported accidents relating to flow erosion all involved embankment dams. In 1 case the downstream embankment slope was eroded, and in 2 other instances erosion of the outlets was involved. Two of the accidents actually were due to cavitation erosion in the tunnels. The remaining 12 accidents involved the loss or damage to spillway structures.
THE SAFETY OF DAMS 13 Slope Protection Damage Damage to slope protection was not reported to be involved in any failures; however, in 1 accident the undermining of riprap by wave action led to embankment erosion very nearly breaching the dam. The 13 reported accidents represent about 4% of all incidents. Of the 13 accidents, 6 involved concrete protection and the others riprap. In some of the latter cases the wave action pulled fill material through the riprap, and in the others rip-rap was either too small or not durable. Embankment Leakage and Piping Embankment leakage and piping accounted for 22% of the failures and 13% of all reported incidents. In 5 of the 37 incidents piping is known to have occurred along an outlet conduit or at the interface with abutment or concrete gravity structure. Foundation Leakage and Piping Foundation leakage and piping accounted for 17% of all failures and 24% of all reported incidents. It is the number one cause of all incidents. Six concrete dams, 1 steel dam, and 11 embankment dams were involved in the 18 failures. In at least 11 of the 49 accidents, which involved 6 concrete and 43 embankment dams, the leakage occurred in the abutments. Some reports cite inadequate grouting or relief wells and drains as causing the leakage and piping. In 1 event piping was caused by artesian pressures and not reservoir water. Sliding This category covers instability as represented by sliding in foundations or the embankment or abutment slopes. Sliding accounted for 6% of all failures and 12% of all incidents reported. Of the 6 failures, 1 was a concrete gravity structure where, during first filling, the structure's slide downstream of about 18 inches was preceded by a downstream abutment slide, followed by large quantities of water leaking from the ground just downstream of the dam. The reservoir was emptied successfully, but before repairs were accomplished, the reservoir filled again causing large sections of the dam to "overturn or open like a door." The 5 embankment failures occurred in the downstream slopes, 1 due to excessively steep slopes and the others probably due to excessive seepage forces. All of the 28 reported sliding accidents involved embankment dams. In 2 cases the slides occurred in abutment slopes, in 10 cases in the downstream
THE SAFETY OF DAMS 14 TABLE 2-3 Earth Dam Failures Form General Causes Preventive or Characteristics Corrective Measures Hydraulic Failures (30 % of all failures) Overtopping Flow over Inadequate Spillway designed embankment, spillway capacity. for maximum flood. washing out dam. Clogging of Maintenance, trash spillway with booms, clean debris. design. Insufficient Allowance for freeboard due to freeboard and settlement, settlement in skimpy design. design; increase crest height or add flood parapet. Wave erosion Notching of Lack of rijprap, Properly designed upstream face by too small riprap. riprap. waves, currents. Toe erosion Erosion of toe by Spillway too Training walls. outlet discharge. close to dam. Properly designed Inadequate riprap. riprap Gullying Rainfall erosion of Lack of sod or Sod, fine riprap; dam face. poor surface surface drains. drainage. Seepage Failures (40 % of all failures) Loss of water Excessive loss of Pervious Banker reservoir water from reservoir reservoir rim or with compacted and/or occasionally bottom. clay or chemical increased seepage or Pervious dam admix; grout increased foundation. seams, cavities. groundwater levels Pervious dam. Use foundation near reservoir. Leaking conduits. cutoff; grout; upstream blanket. Impervious core. Watertight joints; waterstops; grouting.
THE SAFETY OF DAMS 15 Form General Causes Preventive or Characteristics Corrective Measures Settlement cracks Remove in dam. compressible foundation, avoid sharp changes in abutment slope, compact soils at high moisture. Shrinkage cracks Use low-plasticity in dam. clays for core, adequate compaction. Seepage Progressive internal Settlement cracks Remove erosion or erosion of soil from in dam. compressible piping downstream side of foundation, avoid dam or foundation sharp changés, backward toward internal drainage the upstream side to with protective form an open filters. conduit or ''pipe.'' Often leads to a washout of a section of the dam. Shrinkage cracks Low-plasticity in dam. soil; adequate compaction; internal drainage with protective filters. Pervious seams in Foundation relief foundation. drain with filter; cutoff. Pervious seams, Construction roots, etc., in dam. control; core; internal drainage with protective filter. Concentration of Toe drain; internal seepage at face. drainage with filter. Boundary seepage Stub cutoff walls, along conduits, collars; good soil walls. compaction. Leaking conduits. Watertight joints; waterstops; materials. Animal burrows. Riprap, wire mesh.
THE SAFETY OF DAMS 16 Form General Causes Preventive or Characteristics Corrective Measures Structural Failures (30 % of all failures) Foundation slide Sliding of entire Soft or weak Flatten slope; dam, one face, or foundation. employ broad both faces in berms; remove opposite directions, weak material; with bulging of stabilize soil. foundation in the direction of movement. Excess water Drainage by pressure in deep drain confined sand or trenches with silt seams. protective filters; relief wells. Upstream slope Slide in upstream Steep slope. Flatten slope or face with little or employ berm at no bulging in toe. foundation below toe. Weak Increased embankment soil. compaction; better soil. Sudden Flatten slope, drawdown of rock berms; pond. operating rules. Downstream Slide in Steep slope. Flatten slope or slope downstream face. employ berm at toe. Weak soil. Increased compaction; better soil. Loss of soil Core; internal strength by drainage with seepage pressure protective filters; or saturation by surface drainage. seepage or rainfall. Flow slide Collapse and flow Loose Adequate of soil in either embankment soil compaction. upstream or at low cohesion, downstream triggered by direction shock, vibration, seepage, or foundation movements. SOURCE: Sowers (1961).
THE SAFETY OF DAMS 17 slope, in 11 cases in the upstream slope, and in 2 cases both in the upstream and downstream slopes. In 1 instance the slide was reported to have occurred in the foundation and to be due to very steep embankment slopes. Three reports did not indicate the location of the slope slides. Of the 11 slides in the upstream slope, 6 occurred during or immediately following reservoir drawdown. In several cases heavy rains preceded the slides, and 3 of the slides were known to have occurred in clay foundation layers. Deformation This category covers instability cases other than those involving sliding. Of the 6 failures, 3 involved timber crib dams where either the logs slipped out of their sockets or ice or flood flows breached the dam. The other 3 failures were embankment dams where, in one case, deformation of the outlet pipe permitted the outward leakage of the full-flowing pipe, causing piping of the embankment. In another case ice pressures displaced the intake riser of the outlet works. In the third embankment dam the concrete intake riser collapsed, with resulting leakage and piping along the conduit barrel. Of the 31 reported accidents, 29 occurred at embankment dams. However, in 19 of these cases the outlet or spillway was involved. In 5 instances the accident occurred in tunnels where serious leakage developed in 4 instances; in the other a complete blowout occurred. Excessive cracking, shearing, or collapse of outlet pipes occurred in 7 cases, in some instances due to differential settlement of the embankment. Failures of a valve structure, drop structure, and intake structure, the latter due to ice forces, were reported at 3 other embankment dams. One dam, a rockfill structure with a masonry shell, developed serious cracking in the shell due to differential settlements. In 3 instances differential settlements damaged spillway structures. In the 12 accidents where ancillary structures were not involved, differential settlement of the embankment led to transverse and/or longitudinal embankment cracks. Some leakage and piping occurred at the location of transverse cracks. Deterioration Two of the failures and 9 of the accidents were caused by deterioration. The 2 failures involved corrosion of outlet pipes, which allowed leakage and piping of embankment material into the outlet. The 9 accidents involved 3 embankment dams and 6 concrete dams. At the 3 embankment dams, leakage with piping of embankment material into the conduit was
THE SAFETY OF DAMS 18 caused by pipe corrosion in 2 cases and by concrete deterioration in the other. At 3 concrete dams the accidents were due to concrete deterioration caused by freeze-thaw damage. At another, alkali reactivity was the cause. Corrosion of the penstock and deterioration of timber bulkhead were listed as causes of the accidents at 2 concrete dams. Earthquake Instability Three incidents of earthquake instability were reportedâall considered to be accidents. Two of these were the Lower and Upper San Fernando (Van Norman) dams that were damaged during the 1971 San Fernando earthquake (Seed et al. 1973). These incidents are listed as accidents because reservoir water was not released downstream; however, essentially complete reconstruction of the dams was required. The other was the Hebgen Dam in Montana, which was damaged by the 1959 Madison Valley earthquake. Faulty Construction Faulty construction was listed as the cause of 2 failures and 3 accidents. The failures occurred in concrete gravity dams and in I case was attributed to the omission of reinforcing steel. The 3 accidents occurred at embankment dams and in 2 cases were caused by poor bonding between old and new embankment material, leading to seepage and slope failures (in 1 case during drawdown). At the third, poor concrete tunnel construction led to severe leakage through construction joints and spalled areas. Gate Failures Spillway gate failure was listed as the cause of failure of the dam in 2 cases. Gate or valve failure was the cause of 5 of the reported accidents, resulting in damage to downstream structures and/or loss of reservoir pool. Effects of Age and Aging Data published by ASCE/USCOLD (1975) and ICOLD (1973) show that older dams have failed or suffered serious accidents approaching failure more frequently than dams of recent vintage. This is largely attributed to better engineering and construction of modern dams, especially since about 1940. The records also show that failures and accidents have been more frequent during first filling and in the early years, mostly due to de
THE SAFETY OF DAMS 19 sign or construction flaws or latent site defects. Then follows an extended period of gradual aging with reduced frequency of failure and accidents during midlife. The frequency of accidents, but not failures, has then increased during later life, although some failures have occurred even after more than 100 years of satisfactory service. Weathering and mechanical and chemical agents can gradually lead to accident or failure unless subtle changes are detected and counteracted. The engineering properties of both the foundation and the materials composing a dam can be altered by chemical changes that occur with time. Dams constructed for the purposes of water quality control, sewage disposal, and for storing manufacturing and milling wastes, such as tailings dams, are particularly susceptible to changes from chemical action. Foundation shearing strengths and bearing capacities can be reduced and permeabilities can be increased by dissolution. Progression of solution channeling in limestone foundations is a widespread problem. The permeabilities of critically precise filter zones and drain elements can be reduced or obstructed by precipitates. The effectiveness of cement grout curtains can be reduced by softening, solutioning, and chemical attack. These time-related changes occur not only where chemical and industrial wastes are present in the stored water but also where the foundations are gypsiferous or calcareous or where the embankment zones have been constructed using deposits similarly constituted. If the mineral content of the stored water is very low, these changes can occur more rapidly. Concrete can gradually deteriorate and weaken from leaching and frost action. Alkali-aggregate reaction in concrete is irreversible and can gradually destroy the integrity of the structure (Jansen et al. 1973). Cracking of concrete in masonry dams should never be disregarded. Most cracks caused by shrinkage and temperature during the early period after construction do not penetrate deeply enough to be a threat to the dam's stability. However, sometimes cracking to significant depth can endanger the stability. This is because the monolithic behavior of the dam is affected, causing higher stress concentrations, and water pressure has freer access to the interior of the dam, causing higher pore pressures (principally uplift). Also, freeze-thaw damage to concrete is accelerated by the presence of cracks. The metal components of appurtenant structures, such as trash racks, pipe, gates, valves, and hoists, gradually corrode unless continuously maintained. Deterioration can be rapid in an acidic environment. Unless continuously wet in a freshwater environment, timber structures such as cribbing will eventually decay from water content cycling and insect infestation and attack by organisms. Low-quality riprap will soften and disintegrate, destroying its effectiveness for erosion and slope protection.
THE SAFETY OF DAMS 20 About one-half of the dams inventoried by the U.S. Army Corps of Engineers (1989b) under the National Dam Inspection Program (PL 92-367) were constructed prior to 1960. Many of these dams can be expected to possess some of the above symptoms of aging. Even the more recently built dams may show signs of deterioration. Therefore, it is essential that periodic inspections be made to detect such symptoms and that timely measures be taken to arrest and correct the deficiencies. FIELD INSPECTIONS An effective inspection program is essential to properly maintain a project in a safe condition. The program should involve three grades, or types, of inspections: (1) periodic technical inspections, (2) periodic maintenance inspections, and (3) informal observations by project personnel. Technical inspections are those involving specialists familiar with the design and construction of dams and include an assessment of the safety of project structures. Maintenance inspections are those performed at a greater frequency than technical inspections in order to detect at an early stage any significant developments in project conditions and involve consideration of operational capability as well as structural stability. The third type of inspection is actually a continuing effort performed by onsite project personnel (dam tenders, powerhouse operators, maintenance personnel) in the course of performing their normal duties. Technical Inspections Frequency of Inspections The frequency of technical inspections should depend on a number of factors. A dam that has not been properly inspected by experts for some years or a new or reconstructed dam should be inspected rather frequently in order to establish baseline data, information, and general familiarity. Initially, semiannual inspections would be prudent. These inspections are in addition to more frequent (say daily or weekly) visits by the regular caretakers or operators. It is advisable to have inspections made under variable operating conditions such as: ⢠Reservoir level down, so that the upstream face and abutments as well as the reservoir rim can be inspected. ⢠Reservoir full or preferably spilling. This permits checking for leakage or piezometer pressure under maximum head conditions. It also allows the inspector to assess hydraulic conditions of the spillway and its energy dissi
THE SAFETY OF DAMS 21 pator. Operations of gates and valves can be checked, and downstream flow conditions can be assessed. Spillway approach conditions and potential debris problems can be reviewed. As the inspectors become more familiar with the dam, and adequate data have been compiled, frequency of inspections may be reduced to perhaps once per year, then in some cases to once in 2 or more years, depending on the hazard rating. It is recommended that these "expert" inspections never be extended beyond 5 years even under the best of dam conditions. Special inspections should always be conducted following any major problems or unusual event, such as earthquake, flood, vandalism, or sabotage. Inspection Staff The inspection staff should be multidisciplined and need not include each member of the team each time. Most critical to the inspection is a civil engineer with significant (10 or more years) experience in the design and evaluation of dams. An engineer whose sole experience has been in earth dams would obviously not be the best-qualified engineer to assess a concrete or masonry dam and vice versa. There is considerable value in having an independent (not a member of the owner's staff) civil engineer on the inspection team. This provides a peer review considered to be extremely valuable in obviating bias. This should not preclude the owner from having a staff civil engineer accompany the independent engineer. It is extremely important to have the operating staff member, preferably the normal caretaker, assist in the inspection. This gives him an opportunity to learn what to look for in his frequent visits to the dam and permits the "experts" to gain firsthand information from him. A geologist should be a member of the team on its initial inspection and at about 5-year intervals on others, particularly where problems relating to geology are suspected or known to exist. Some continuity of inspection personnel from year to year is important. The first inspection should, if at all possible, include interviews with the original designer, the owner, the constructor, and current as well as previous caretakers/operators. Inspection Scope The field examination must be both systematic and comprehensive because very subtle changes or visual indicators can often be important in the evaluation of an existing or potential safety problem. Probably the greatest value of such an inspection is the direct and early disclosure of obvious, develop
THE SAFETY OF DAMS 22 ing, or incipient conditions that threaten the integrity of the dam. Often, on the basis of this visual examination alone, an experienced engineer can judge the severity of any problems and determine the rapidity with which remedial measures should be taken to prevent a failure or a serious accident that might lead to a failure. Liberal notetaking and photographs of items of interest are important factors in documenting the conditions noted and establishing the basis for determining and evaluating subsequent changes that may occur. The geology and topography of the surrounding area, including the reservoir, should be inspected in order to assess general features and the quality of the foundation and the reservoir. Inspectors should look for discontinuities, slides, artificial cuts and fills, and signs of erosion, particularly in the vicinity of the spillway and the dam/foundation contact. Existing records, such as preceding inspection reports and notes, water levels, spill and leakage records, movement survey results, photographs, and piezometric and other instrumentation records, should all be reviewed. The adequacy of the existing records and their maintenance also should be reviewed. A review of all past records should be made. These should, whenever available, include preconstruction investigation records, design criteria and design analysis records, and available construction records. Photos taken during initial construction, or subsequent photos, are often valuable. The entire downstream face and, whenever feasible, the upstream face should be inspected for overall quality of materials, leaks, offsets, cracks, erosion, moisture, crazing, vegetation, and surficial deposits. Parapets, walls, spillway channels, galleries, and bridges also should be inspected for these items. The spillway channel should be examined for erosion, condition of log booms, and susceptibility to blockage. The condition of gates and operating equipment, including motors, cables, chains and controls, should be noted and the gates operated if feasible. Outlets, including conduits, gates, and machinery, should be inspected. Galleries should be checked for signs of seepage, leakage, internal pressures, and condition of drains or signs of blockage. The evaluation of safety, which is a principal component of a technical inspection, is discussed in detail later in this chapter. Techniques and procedures for accomplishing technical dam inspections and making the associated safety evaluations are described in detail in Recommended Guidelines for Safety Inspection of Dams (Chief of Engineers 1975), Safety Evaluation of Existing Dams (U.S. Bureau of Reclamation 1980), and Guide for Safety Evaluation and Periodic Inspection of Existing Dams (Forest Service and Soft Conservation Service 1980).
THE SAFETY OF DAMS 23 Checklists and Inspection Forms It is extremely important that checklists and inspection report forms be used and completed for all inspections. These forms can be formal or informal. They should be completed during and immediately after the inspection, not the next day or later. The checklist should be developed prior to the inspection and should reflect the features peculiar to that particular project. Photographs There is considerable value in aerial and close-up photographs, especially of areas of deterioration or those suspected of deterioration. Stereo-paired aerial photos are particularly valuable for reviewing possible progression of slides or of erosion of spillway dissipators or flow channels. They can also help in assessing downstream growth or habitation conditions. Infrared photos could be useful in locating wet areas that might otherwise not be obvious or detectable and that might indicate seepage. Maintenance Inspections Formal maintenance inspections should be conducted on a semiannual-to- annual basis to monitor the behavior and condition of the structure and of all operating equipment. The inspection should be performed by an engineer or experienced supervisor of dam operations, who should note any adverse changes in physical conditions, such as erosion, corrosion, blockages of drains, blockages of spillway channels and other water passages, and subsidence. The condition and adequacy of all monitoring equipment and instruments also should be reviewed. All gates and emergency power sources to operating equipment, including motors, cables, chains, and controls, should be inspected and operated if feasible. A principal objective of this inspection, besides offering an opportunity to check on aspects pertinent to the safety of project features, is to promote an efficient and effective maintenance program. It is desirable that such inspections be made by persons not directly involved in or responsible for the day-to-day operation and maintenance of the project. Informal Observations Dam tenders and maintenance personnel should be charged with the responsibility of helping to monitor the behavior and safety of dams. If alert, such personnel could discover existing defects during routine operational
THE SAFETY OF DAMS 24 and maintenance activities. For example, a mowing-machine operator cutting grass along the toe of an embankment may come across an upward bulging mass of freshly disturbed ground, which might indicate incipient slope instability. Or a gate tender approaching the gate controls might observe a small sinkhole on the embankment crest that could be developing from the piping of fines into a deteriorating outlet conduit. Rodent holes might also be discovered in this manner. Personnel such as operators, maintenance crew members, and all others, including the owners of smaller dams, who are at a dam in the course of their normal duties should be watchful for any unusual events or strange conditions and should report them at once to those in authority. Alertness and inquisitiveness on the part of such individuals could afford an important surveillance program for the project. MAINTENANCE Normal maintenance activities include the surveillance of the project's physical conditions and the timely correction of any deficiencies that might develop, as well as the preservation of the operating capability of the project. The inspections described in the section Maintenance Inspections should be an integral part of the maintenance program. Maintenance activities should include the surveillance of all aspects of the structure pertinent to safety. For example, seepage or leakage through the foundation or abutment areas should be closely monitored. (This can take on greater importance, depending on the integrity of the material.) Also, uplift pressures are critical to stability. If instruments are not available to monitor this pressure, they should, if possible, be installed. Collecting, processing, and evaluating surveillance instrumentation data are ways to detect the development of defects in a dam and are helpful in the investigation of a specific or suspected defect. Often, records are collected, but processing and evaluation are delayed or long neglected. All data relating to dam safety should be promptly evaluated. Installations for the collection of precautionary types of surveillance data are usually made as a matter of course during construction but may also be installed after construction. Installations for investigating specific or suspected defects are usually made upon the appearance of new or changing conditions and events. Instrumentation for dams is discussed in Chapter 10. Another important maintenance objective is to preserve the water-passing capability of the project. Heavy growth or landslides upstream or downstream of a spillway could reduce its ability to pass its design flow. Also, the electrical and mechanical operating machinery and the spillway
THE SAFETY OF DAMS 25 gates should all be maintained to ensure their operability under all conditions. The failure of any one of these could lead to failure of the dam. Maintenance is an ongoing process that should never be neglected and that should continue throughout the operating life of the dam. To provide proper maintenance services, all material regarding the design, construction, and operation of the dam should be available in a location where it is readily accessible for the inspection and maintenance programs. RECORDS Complete records on each dam, including initial site investigations; pre- construction and final geologic reports; design assumptions and criteria; contract plans and specifications; construction history; descriptions of repairs or modifications; and documentation of conditions and performance after the dam is in operation, including history of major floods and instrumentation records, should be maintained. Construction photographs are extremely valuable. Experience has proven that many questions and concerns arise in the operating life of a dam for which thorough records are vital to assess such situations properly. This documentation should, of course, be continuously supplemented throughout the life of a dam by periodic inspection reports. At present, some dams have adequate records, while many have little or none at all. An important objective of a periodic inspection program is to collect and develop such data. The inspection reports should eventually provide most of the information relating to safety of the dam, as they should usually contain a summary of major preoperational information and a documentation of all observations, assessments, damage, and repairs during operation. The importance of keeping such information well organized and readily available cannot be overemphasized. In extracting and assimilating record data, the quality and accuracy of the records must be carefully assessed. If certain types of existing information, such as exploration and materials testing reports, are overlooked or are questionable, exploration and testing may have to be repeated, at considerable cost. It is imperative that all of these records be made available, not simply filed, to those involved in the evaluation of a dam's safety. Comprehensive descriptions of all types of records and their utilization are contained in Safety Evaluation of Existing Dams (U.S. Bureau of Reclamation 1980), Guide for Safety Evaluation and Periodic Inspection of Existing Dams (Forest Service and Soil Conservation Service 1980), and Feasibility Studies for Small Scale Hydropower Additions, Vol. IV, Existing Facility Integrity (U.S. Army Corps of Engineers 1979).
THE SAFETY OF DAMS 26 EVALUATION OF SAFETY An evaluation of the stability and safety of an existing dam is a principal component of all technical inspections. Such an evaluation must be carefully and thoroughly performed by experienced personnel. It should consider all data of record, including design, construction, and operating history, and the results of a field inspection and any analyses necessary to determine the safety of project structures and the adequacy of maintenance and operating procedures. A safety evaluation is generally amenable to a staged approach. The basic idea behind such an approach is to attempt to establish the integrity of a structure or to resolve a problem associated with it at the least possible cost. For example, if an adequate determination can be made from review and analysis of existing data and field observations, then that is all that should be done. If the review or observations indicate that additional special investigations are required to determine the condition of a facility or to evaluate and correct a problem, then these investigations can most effectively be planned on the basis of what the data review or observations show to be required. For the foregoing reasons the first stage of the evaluation should include, as a minimum, three basic steps: (1) review of existing data, (2) site inspection, and (3) evaluation of data and formulation of conclusions. Review of Existing Data A thorough knowledge must first be gained on the basis of a dam's original design and its performance history and records, to provide a basis for judgments that will be made later. Whatever data are available for review can normally be obtained from the owner's files or from the files of the state agency regulating the safety of dams, if such an agency exists and takes an active role in the particular state in which the facility is located. Design Data The review should reveal whether the original design criteria and assumptions are satisfactory based on the current state of the art and, if not, whether they are acceptable. Original design assumptions may have been inconsistent with construction conditions or with subsequent events and conditions. The review should include hydrology and spillway capacity, materials investigations and specifications, criteria for outlets and other appurtenances, all geological and seismological reports, and all design analyses.
THE SAFETY OF DAMS 27 Original design methodologies and techniques can be very meaningful. Equally important are data on any analyses or reviews subsequent to original design. The same applies to any modifications or alterations in design. Construction Data Construction data (relating to original construction or alterations) are as important, if not more so, than design data. Inspections and engineers' reports relating to foundation cleanup; grouting; concreting operations, such as strengths, placing methods, cleanup, cement, water-cement ratio; and aggregate source are important. For example, in one case knowledge of the method of placing concrete aided in determining the cause of deterioration of the dam. Confirmation of compliance with specifications or information on changes to suit field conditions are important. Often, in the past, field changes were made to save money without the designers' knowledge or to mitigate damages to the construction resulting from unexpected events, such as flooding or accidents. Very often, in the case of very old dams, calculations or records cannot be found. In these cases construction records are all the more important. They are often documented in old publications or photos. All of the above can be invaluable in reviewing the safety of a dam as well as in establishing investigational programs. Operating Records and Maintenance Records of operation and maintenance activities are often more readily available to the reviewer than are design or construction records. Any dam owner should make a concerted effort to compile such records regardless of whether past records exist. In other words, a late start is better than no start. These records should be compiled whether the dam is presently considered to be in excellent condition, in good or fair condition, or a hazard or a high risk. The reviewer (evaluator) should consider the frequency and quality of inspections. Infrequent inspections and casual maintenance should alert the evaluator to potential problems. At the very least the reviewer should undertake a more intensive evaluation unless inspections and maintenance have been frequent and of good quality. The housekeeping level at a dam is often a good indication of the level or quality of care given to the dam. In the absence of suitable records the evaluator should establish a program of frequent inspections and should initiate survey and data collection procedures in order to establish baseline data. The level of this program depends on the hazards and risks presented by the dam.
THE SAFETY OF DAMS 28 Often an owner will have a lot of scattered, irregular, incomplete, or unreliable data. Data often need to be plotted. There is then a need for data analysis, in addition to collection. All data should be reviewed and challenged for reliability and completeness. Upon first evaluation the evaluator should gather information from all feasible sources. Information provided by a regular dam caretaker, for example, is probably better and more reliable than that provided by a nearby powerhouse operator. A water superintendent may be a reliable source. Of particular value are data on leakage, water levels, deflections, flood levels at the dam and downstream thereof, oral and written comments about repairs, and reports of operating problems with equipment. All of the above should be plotted on a time scale to permit evaluation of interrelationships of the data. Adam's age, quality of maintenance, presence of operating records, and apparent deterioration all have a bearing on its safety. It is important to acknowledge that often the worst problem at a dam may not be the dam itself but a lack of knowledge thereof. It is impossible to evaluate a dam's safety without knowledge of the structure. The greater the knowledge, the better the evaluation. Instrument data (from the dam) should be programmed for computer storage and analysis. This method of data handling will permit development of a computer analysis program that will 'red-flag' critical data points. For example, piezometer data can be programmed to indicate with a special symbol when the readings approach a critical level of uplift or pore pressure. Thus, when the evaluator scans the computer listing his attention will be drawn to potentially dangerous conditions. Site Inspection Once the evaluator has thoroughly reviewed the existing data, a site inspection should be performed to observe pertinent visual evidence. This inspection should conform to all the requirements of a formal technical inspection and should provide the evaluator with intimate knowledge of project conditions and problems. It should also provide the evaluator the opportunity to resolve any discrepancy or question that may exist concerning record data such as drawings, instrumentation data, or operating procedures. Hazard Potential A safety evaluation should include a review of the dam's hazard potential and should determine whether new developments in the downstream area substantiate a change in the hazard level. (See section on Classification of Inundation Areas.) In this connection the emergency action plan (discussed
THE SAFETY OF DAMS 29 later) should be reviewed to ensure that it reflects the current hazard potential status. Evaluation and Conclusions After all available data have been reviewed and the site has been examined in detail, the evaluator should analyze all pertinent information revealed by the record, all conditions observed at the site, and the results of any engineering calculations. One of the most useful techniques to apply in the process of evaluating the safety and stability of an existing dam and its appurtenant works is to compare performance, as indicated by field observations, instrumentation measurements, and the results of any required special investigations performed to evaluate a specific problem, with the assumptions and calculations made in the original design of the facility. In doing this the engineer will often be made aware of criteria that were in vogue at the time the design was originally accomplished. In many cases such criteria will still be appropriate. However, it is important to bear in mind that the state of the art is not static. It changes as engineering knowledge and technology advance and as natural events occur that deviate from prior experience. As a result the reliability of designs based on the state of the art that existed when the dam was designed and constructed must always be compared with existing practices. Following the analyses of all the data the evaluator should prepare a report detailing his findings. If sufficient information is available to make a judgment regarding the project's stability and safety, the report should in-elude such a conclusion with any associated recommendations. If not, the report should detail the additional information needed and recommend the investigations required to develop the needed data. It is a well-known fact that in the initial assessment or reassessment of an existing dam, particularly an old one, the unknown is the principal determinant for an investigation or exploratory program. Lack of knowledge makes it virtually impossible to determine the potential hazard of a dam and makes a viable risk assessment impossible. It is also axiomatic that the more dams that are investigated, or the more a single dam is investigated, the more likely that a potential or real problem will be discovered. Only then can the problem be assessed and corrected if necessary. Additional Investigations The need and nature of additional investigations, exploratory programs, or monitoring programs will depend on the potential for problems as determined by the inspection and on the availability of good information and
THE SAFETY OF DAMS 30 records. They will also depend on the evaluator's assessments of risks and consequences of failure. Such investigations may involve theoretical studies as well as field investigations. Additional studies are sometimes needed to better define the stress and stability conditions and to evaluate alternative remedial measures. The investigative program can consist of a wide variety of tasks, depending on the nature of the known or suspected problem. The type of supplemental information and numerical data needed will concern structural, geologic, and performance features unobtainable by direct visual examination. Some kind of exploration may be required for sample extraction; for providing access for direct observation; and for instrumental measurements of deformation, hydrostatic pressures, seepage, etc. Data may also be obtained by nondestructive testing. Laboratory tests may be required to determine engineering properties of the materials of the dam and appurtenances and of the foundation for use in analyses and to assess their general condition. Performance instrumentation may be required. Applicable techniques of subsurface exploration, geologic mapping, laboratory testing, and instrumentation are described in numerous excellent references, such as the Handbook of Dam Engineering and various other references listed in Chapters 5 through 10 of this volume. After additional data have been obtained, the engineering analyses and methods employed are generally similar to those that would have been conducted in the initial evaluation had the data been available. Particular care should be taken to study suspicious or questionable features and conditions. The engineering data and information to be used in the analyses are those specifically obtained for that purpose during the investigations. For example, unless available data on spillway design indicate conclusively that the spillway meets present-day design standards, a new flood estimate should be made, and the existing spillway should be analytically tested for its ability to safely handle the updated flood. Or, as another example, if the stability of an embankment dam appears marginal for any reason (such as apparently over-steep slopes, unusual saturation patterns, low-strength soils, or indications of high foundation pore pressures), a stability analysis and companion seepage analysis should be made using soil strengths and permeability rates obtained by sampling and testing for use in those specific analyses. As valuable as they are, numerical analyses cannot provide total and absolute answers upon which to base the evaluation. Many physical conditions and reactive mechanisms cannot be mathematically analyzed. Therefore, after all the objective factors that may influence the evaluation have been gathered, interpreted, analyzed, and discussed, the investigator must still exercise judgment as to whether the dam is adequate in its present condition or requires remedial or other measures. There are no clear-cut rules
THE SAFETY OF DAMS 31 by which the decisions can be made. Instead, the investigator may need to employ empirical reasoning and objective assessments, compare the case with successfully performing similar dams, and apply criteria in common use by the profession. When the perceived problems involve areas of specialized engineering practice and there would be significant losses from failure of the structure, experts in the pertinent specialties should be brought in as consultants. EMERGENCY ACTION PLANNING Current Policies and Practices While the intent of dam design, construction, operation, maintenance, and inspection of dams is to minimize the risk of clam failures, it is recognized that the possibility of dam failures still exists. Even though the probability of such failures is usually small, preplanning is required to (1) identify conditions that could lead to failure, in order to initiate emergency measures to prevent such failures as a first priority, and (2) if this is not possible, to minimize the extent and effects of such failures. The operating and mobilizing procedures to be followed upon indication of an impending or postulate clam failure or a major flood should be carefully predetermined. Following the failure of Teton Dam in 1976, President Carter directed the appropriate federal agencies to develop guidelines for clam safety. Subsequently, in June 1979, Federal Guidelines for Dam Safety was published by the Federal Coordinating Council for Science, Engineering and Technology. While these guidelines were developed to encourage high safety standards in organizational and technical management activities and procedures of federal agencies, they are also considered applicable to state dam safety agencies and private and nonfederal dam owners. A basic tenet of these guidelines is that an emergency action plan, commensurate with the dam size and location (i.e., hazard classification), should be formulated for each dam. The guidelines require an evaluation of the emergency potential created from a postulated dam failure by use of flood inundation maps; development of an emergency action plan, coordinated with local civil preparedness officials; and a formal procedure to detect, evaluate, and mitigate any potential safety problem. Owners of private dams should evaluate the possible modes of failure of each dam, be aware of indicators or precursors of failure for each mode, and consider the possible emergency actions appropriate for each mode and the effects on downstream areas of failure by each mode. Evaluation should recognize the possibility of failure during flood events as well as during normal operating conditions and should provide a basis for emergency planning actions
THE SAFETY OF DAMS 32 in terms of notification and evacuation procedures where failure would pose a significant danger to human life and property. Plans should then be prepared in a degree of detail commensurate with the hazard, and instructions should be provided to operators and attendants regarding the actions to be taken in an emergency. Planning should be coordinated with local officials, as necessary, to enable those officials to draw up a workable plan for notifying and evacuating local communities when conditions threatening dam failure arise. Some states and several federal agencies have already developed their own emergency action planning guidelines and have implemented plans at dams consistent with the major elements contained in the Federal Guidelines. Among the federal agencies, the U.S. Army Corps of Engineers (1980) has published Flood Emergency Plans, Guidelines for Corps Dams. The Corps publication merits consideration by private dam owners for its detailed procedure and case study examples. Additionally, the Corps (1982a) has recently published a manual, Emergency Planning for Dams, Bibliography and Abstracts of Selected Publications, for assisting planners with relevant materials and references to emergency planning for dams and preparation of flood evacuation plans. Finally, the user is directed to technical guidelines and recommendations on emergency action planning for federal agencies that have been published by a subcommittee of representatives from federal agencies having responsibilities for dam safety for the Interagency Committee on Dam Safety (ICODS) (FEMA 1982). Evaluation of Emergency Potential Prior to development of an emergency action plan, consideration must be given to the extent of land areas and the types of development within the areas that would be inundated as a result of dam failure and to the probable time available for emergency response. Determination of Mode of Dam Failure There are many potential causes and modes of dam failure, depending on the type of structure and its foundation characteristics. Similarly, there are degrees of failure (partial vs. complete) and, often, progressive stages of failure (gradual vs. sudden). Many dam failures can be prevented from reaching a final catastrophic stage by recognition of early indicators or precursor conditions and by prompt, effective emergency actions. While emergency planning should emphasize preventive actions, recognition
THE SAFETY OF DAMS 33 must be given to the catastrophic condition, and the hazard potential must be evaluated in that light. Analyses should be made to determine the most likely mode of dam failure under the most adverse condition and the resulting peak water outflow following the failure. Where there is a series of dams on a stream, analyses should include consideration of the potential for progressive ''domino effect'' failure of the dams. Appendix A of Chapter 4 provides an example of guidelines on estimating modes of dam failure for formulating emergency action plans by an investigator-owned utility. Inundation Maps To evaluate the effects of dam failure, maps should be prepared that delineate the area that would be inundated in the event of failure. Inundation maps should account for multiple dam failures where such failures are possible. Land uses and significant development or improvements within the area of inundation should be indicated. The maps should be equivalent to or more detailed than the United States Geological Survey (USGS) 1:24,000-scale quadrangle maps, 7.5-minute series, or of sufficient scale and detail to identify clearly the area that should be evacuated if there is evident danger of failure of the dam. Copies of the maps should be distributed to local government officials for use in the development of an evacuation plan. Figure 2-6 is a sample inundation map. A 1980 dam break flood study of 50 dams located in Gwinnett County, Georgia (prepared by cooperating state and federal agencies for the county) reported flood inundation study results on 1:12,000 (1 inch = 1,000 feet) maps, which were scaled from the USGS maps (Georgia Environmental Protection Division 1980). Classification of Inundation Areas To assist in the evaluation of hazard potential, areas delineated on inundation maps should be classified in accordance with the degree of occupancy and hazard potential. The potential for loss of life is affected by many factors, including but not limited to the capacity and number of exit roads to higher ground and available transportation. Hazard potential is greatest in urban areas. Since the extent of inundation is usually difficult to delineate precisely because of topographic map limitations, the evaluation of hazard potential should be conservative. The hazard potential for affected recreation areas varies greatly, depending on the type of recreation offered, intensity of use, communication facilities, and available transportation. The potential for loss of life may be increased
THE SAFETY OF DAMS 34 Figure 2-6 Sample inundation mapping.
THE SAFETY OF DAMS 35 where recreationists are widely scattered over the area of potential inundation, since they would be difficult to locate on short notice. Many industries and utilities requiring substantial quantities of water for one or more stages in the manufacture of products or generation of power are located on or near rivers or streams. Flooding of these areas and industries can (in addition to causing the potential for loss of life and damage to machinery, manufactured products, raw materials, and materials in process of manufacture) interrupt essential community services. Rural areas usually have the least hazard potential. However, the potential for loss of life exists, and damage to large areas of intensely cultivated agricultural land can cause high economic loss. Time Available for Response Analyses should be made to evaluate the structural, foundation, and other characteristics of the dam and to determine those conditions that could be expected to result in slow, rapid, or practically instantaneous dam failure. Wave travel times, as discussed in Chapter 4, should also be established to help determine the time available for response. Actions to Be Taken to Prevent Failure or to Minimize Effects of Failure Development of an Emergency Action Plan An emergency action plan should be developed for each dam that constitutes a hazard to life and property, incorporating preplanned emergency measures to be taken prior to and following assumed dam failure. The plan should be coordinated with local governmental and other authorities involved in public safety and should be approved by the appropriate top-level agency or owner management. To the extent possible, the emergency action plan should include notification plans, which are discussed in the section Notification Plans. Emergency scenarios should be prepared for possible modes of failure of each dam. These scenarios should be used periodically to test the readiness capabilities of project staff and logistics. A procedure should be established for review and revision, as necessary, of the emergency action plan, including notification plans and evacuation plans, at least once every 2 years. Such reviews should be coordinated among all organizations responsible for preparation and execution of the plans.
THE SAFETY OF DAMS 36 Notification Plans Plans for notification of key personnel and the public are an integral part of the emergency action plan and should be prepared for slowly developing, rapidly developing, and instantaneous dam failure conditions. Notification plans should include a list of names and position titles, addresses, office and home telephone numbers, and radio communication frequencies and call signals, if available, for agency or owner personnel, public officials, and other personnel and alternates who should be notified as soon as emergency situations develop. A procedure should be developed to keep the list current. Each type of notification plan should contain the order in which key owner supervisory personnel or alternates should be notified. At least one key supervisory level or job position should be designated to be manned or the responsible person should be immediately available by telephone or radio 24 hours a day. A copy of each notification plan should be posted in a prominent place near a telephone and/or radio transmitter. All selected personnel should be familiar with the plans and the procedures each is to follow in the event of an emergency. Copies of the notification plans should be readily available at the home and the office of each person involved. Where dams located upstream from the dam for which the plan is being prepared could be operated to reduce inflow or where the operation of downstream dams would be affected by failure of the dam, owners and operators of those dams should be kept informed of the current and expected conditions of the dam as the information becomes available. Civil defense officials having jurisdiction over the area subject to inundation should receive early notification. Local law enforcement officials and, when possible, local government officials and public safety officials should receive early notification. (In some areas such notification will be accomplished by civil defense authorities.) The capabilities of the Defense Civil Preparedness Agency's National Warning System (NAWAS) should be determined for the project and utilized as appropriate. Information can be obtained from state or local civil defense organizations. Potentially affected industries downstream should be kept informed so that actions to reduce risk of life and economic loss can be taken. Coordination with local government and civil defense officials would determine responsibility for the notification. Normally, this would be a local government responsibility. When it is determined that a dam may be in danger of failing, the public officials responsible for the decision to implement the evacuation plan should be kept informed of the developing emergency conditions.
THE SAFETY OF DAMS 37 The news media, including radio, television, and newspapers, should be utilized to the extent available and appropriate. Notification plans should define emergency situations for which each medium will be utilized and should include an example of a news release that would be the most effective for each possible emergency. Use of news media should be preplanned insofar as is possible by agency and owner personnel and the state and/or local government. Information should be written in clear, concise language. Releases to news media should not be relied on as the primary means of notification. Notification of recreation users is frequently difficult because the individuals are often alone and away from any means of ready communication. Consideration should be given to the use of standard emergency warning devices, such as sirens, at the dam site. Consideration should also be given to the use of helicopters with bullhorns for areas farther downstream. Vehicles equipped with public address systems and helicopters with bullhorns are capable of covering large areas effectively. Telephone communication should not be solely relied on in critical situations. A backup radio communication system should be provided and tested at least once every 3 months. Consideration should be given to the establishment of a radio communication system prior to the beginning of construction and to the maintenance of the system throughout the life of the project. Evacuation Plans Evacuation plans should be prepared and implemented by the local jurisdiction controlling inundation areas. This would normally not be the dam agency or owner. Evacuation plans should conform to local needs and vary in complexity in accordance with the type and degree of occupancy of the potentially affected area. The plans may include delineation of the area to be evacuated; routes to be used; traffic control measures; shelter; methods of providing emergency transportation; special procedures for the evacuation and care of people from such institutions as hospitals, nursing homes, and prisons; procedures for securing the perimeter and for interior security of the area; procedures for the lifting of the evacuation order and reentry to the area; and details indicating which organizations are responsible for specific functions and for furnishing the materials, equipment, and personnel resources required. The assistance of local civil defense personnel, if available, should be requested in preparation of the evacuation plan. State and local law enforcement agencies usually will be responsible for the execution of much of the plan and should be represented in the planning effort. State and local laws
THE SAFETY OF DAMS 38 and ordinances may require that other state, county, and local government agencies have a role in the preparation, review, approval, or execution of the plan. Before finalization, a copy of the plan should be furnished to the dam agency or owner for information and comment. Stockpiling Repair Materials Where feasible, suitable construction materials should be stockpiled for emergency use to prevent failure of a dam. The amounts and types of construction materials needed for emergency repairs should be determined based on the structural, foundation, and other characteristics of the dam; design and construction history; and history of prior problems. Locating Local Repair Forces Arrangements should be made with, and a current list maintained of, local entities, including contractors, and federal, state, and local construction departments for possible emergency use of equipment and labor. Training Operating Personnel Owners of large impoundments should have technically qualified project personnel who are trained in problem detection, evaluation, and appropriate remedial (emergency and nonemergency) measures. These personnel should be thoroughly familiar with the project's operating manual. This is essential for proper evaluation of developing situations at all levels of responsibility that, initially, must be based on at-site observations. A sufficient number of personnel should be trained to assure adequate coverage at all times. If a dam is operated by remote control, arrangements must be made for dispatching trained personnel to the project at any indication of distress. Increasing Inspection Frequency Frequency of appropriate surveillance activities should be increased when the reservoir level exceeds a predetermined elevation. Piezometers, water-level gauges, and other instruments should be read frequently and on schedule. The project structures should be inspected as often as necessary to monitor conditions related to known problems and to detect indications of change or new problems that could arise. Hourly or continuous surveillance may be mandated in some instances. Any change in conditions should be reported promptly to the supervisor for further evaluation.
THE SAFETY OF DAMS 39 The owner or his supervisor should issue additional instructions, as necessary, and alert repair crews and contractors for necessary repair work if developing conditions indicate that emergency repairs or other remedial measures may be required. Actions to Be Taken Upon Discovery of a Potentially Unsafe Condition Notification of Supervisory Personnel It is essential, if time permits, to notify the proper supervisory personnel since development of failure could vary in some or many respects from previous forecasts or assumptions and advice may be needed. Initiation of Predetermined Remedial Action At least one technically qualified individual, previously trained in problem detection, evaluation, and remedial action, should be at the project or on call at all times. Depending on the nature and seriousness of the problem and the time available, emergency actions can be initiated, such as lowering the reservoir and holding water in upstream reservoirs. Other actions to be taken include notifying appropriate highway and traffic control officials promptly of any rim slides or other reservoir embankment failures that may endanger public highways. Determination of Need for Public Notification To the extent possible, emergency situations that will require immediate notification of public officials in time to allow evacuation of the potentially affected areas should be predefined for the use of management and project personnel. If sufficient time is available the decision to notify public officials that the dam can be expected to fail will be made at a predetermined supervisory level within the agency or owner organization. If failure is imminent or has already occurred, project personnel at the dam site would be responsible for direct notification of the public officials. The urgency of the situation should be made clear so that public officials will take positive action immediately. REFERENCES ASCE/USCOLD (1975) Lessons from Dam Incidents, USA, American Society of Civil Engineers, New York. Chief of Engineers (1975) Recommended Guidelines for Safety Inspection of Dams, National
THE SAFETY OF DAMS 40 Program of Inspection of Dams, Vol. I, Appendix D, Department of the Army, Washington, D.C. Federal Coordinating Council for Science, Engineering and Technology (1979) Federal Guidelines for Dam Safety, Federal Emergency Management Agency, Washington, D.C. Federal Emergency Management Agency (1982) Interagency Committee on Dam Safety (ICODS), Subcommittee on Emergency Action Planning, Dam Safety, Emergency Action Planning. Forest Service and Soft Conservation Service, U.S. Department of Agriculture (1980) Guide for Safety Evaluation and Periodic Inspection of Existing Dams. Georgia Environmental Protection Division (1980) Georgia Soil and Water Conservation Committee, et al., Dam Breach Flood Maps for Gwinnett Co., Georgia. Golze, A. R., ed. (1977) Handbook of Dam Engineering, Van Nostrand Reinhold Co., New York. ICOLD (1973) Lessons from Dam Incidents, Abridged Edition, USCOLD, Boston, Massachusetts. ICOLD (1979) Transactions of New Delhi Congress. Independent Panel to Review the Cause of Teton Dam Failure (1976) Failure of Teton Dam, Report to U.S. Department of Interior and State of Idaho. Jansen, R. B. (1980) Dams and Public Safety, U.S. Bureau of Reclamation, Government Printing Office, Washington, D.C. (reprinted in 1983). Jansen, R. B., Carlson, R. W., and Wilson, E. L. (1973) Diagnosis and Treatment of Dams, Transactions of 1973 Congress, ICOLD, Madrid, Spain. Seed, H. B., Lee, K. L., Idriss, I. M., and Makdisi, F. (1973) Analysis of the Slides in the San Fernando Dams During the Earthquake of February 9, 1971, Earthquake Engineering Research Center, University of California, Berkeley. Sowers, G. F. (1961) "The Use and Misuse of Earth Dams," Consulting Engineering, July. U.S. Army Corps of Engineers (1979) Feasibility Studies for Small Scale Hydropower Additions, Vol. IV, Existing Facility Integrity. U.S. Army Corps of Engineers (1980) Flood Emergency Plans, Guidelines for Corps Dams, Hydrologic Engineering Center, Davis, Calif., June, 47 pp. U.S. Army Corps of Engineers (1982a) Emergency Planning for Dams, Bibliography and Abstracts of Selected Publications, Hydrologic Engineering Center, Davis, Calif. U.S. Army Corps of Engineers (1982b) National Program for Inspection of Non-Federal Damsâ Final Report to Congress. U.S. Bureau of Reclamation (1980) Safety Evaluation of Existing Dams , Government Printing Office, Washington, D.C. U.S. Department of Interior Teton Dam Failure Review Group (1980) Failure of Teton Dam, Final Report.
