3 Chapter 3: CHAPTER Hazards, Risks, Failures

3.0 General Natural hazards such as floods, some idea of the major flood prone areas. earthquakes, and landslides are also Flash floods can happen anywhere--even Dam failures are severe threats to life important contributors to risk. These and property and are now being recorded on small drainage areas but especially in natural phenomena existed long before and documented much more thoroughly the west. Floods are the most frequent and humanity established patterns of settle- than in the past. Recorded losses have been costly natural events that lead to disaster in ment and are considered hazards because high. Statistics on losses of life and the U.S. Therefore, flood potentials must development has placed people and property fully justify the need for dam be included in risk analyses for dam owners to better understand the risks to property in their way. Failure to adjust to failure. Hurricanes and tropical storms can the public posed by dams, the kinds of these events has been costly both to dam stall over an area, creating a significant hazards that promote those risks and owners and to the public in general. precipitation event that can result in owner liabilities associated with them, and, Human behavior is another element flooding. See Table 3.1 for extreme generally, the reasons that dams fail. of dam failure risk; simple mistakes, precipitation events in Texas. operational mismanagement, negligence, Improving a dam owner’s understanding Texas has design flood criteria derived unnecessary oversights, or destructive of realistic risks and possible reasons for from a percentage of the probable intent can interact with other hazards to failure is an essential first step in any maximum flood (PMF) based on the dam’s compound the possibility of failure. Thus, overall effort to improve dam safety and hazard potential and size classification. A a broad range of natural and human preserve the benefits of dam ownership. PMF is the flood that may be expected hazards, taken separately or in combina- from the most severe combination of tion, increase the probability of dam 3.1 Hazards as critical meteorologic and hydrologic failure and injury to people and property. conditions that are reasonably possible in Sources of Risk The following discussion of some of the region. This assumed event becomes The dam structure itself can be a the most significant hazards that lead to the basis for the design of structural and source of risk due to possible construction public risk illustrates the interrelationships hydraulic elements of the dam. flaws and weaknesses that develop because among events that can lead to dam failure. Flooding from dam failure. When a of aging. The site immediately surrounding dam fails as a result of a flood, more the structure may also increase the struc- 3.1.1 Natural Hazards people and property are generally placed in tural risk if the dam is not positioned or That Threaten Dams jeopardy than during natural floods. The anchored properly or if excessive reservoir The most important natural hazards Rapid City, South Dakota, flood of 1970, seepage erodes the foundation or abutments. threatening dams include: The physical hazards that can cause which killed 242 people, caused a dam ■ flooding from high precipitation dam failure are translated into high risks failure which added significantly to the ■ flooding from dam failure when people or property are threatened, loss of life. When a natural flood occurs ■ and where the high risks to which earthquakes near a dam, the probability of failure and Americans are exposed are exacerbated by ■ landslides loss of life almost always increases. a number of important factors. For Flooding from high precipitation. Of The sudden surge of water generated instance, in most states, people are allowed the natural events that can impact dams, by a dam failure usually exceeds the to settle below dams in potential inunda- floods are the most significant. A flood- maximum flood expected naturally; dam- tion zones, thereby compounding risk. plain map of the U.S. (Figure 3.1) gives failure inundation zones and 100-year

13 Guidelines for Operation and Maintenance of Dams in Texas

Table 3.1 floodplains are seldom congruent. The Extreme Precipitation in Texas upper portion of an inundation zone almost always exceeds the 100-year floodplain Location Dates Inches Duration (hr) Comments considerably; therefore, residences and Thrall Sept. 9–10, 1921 38.2 24 36.4" in 18 hr businesses that would escape natural D’Hannis May 31, 1935 22 3 flooding can be at extreme risk from dam New Braunfels May 11, 1972 16 4 failure flooding. Hence, it is important to Taylor Ranch July 3, 1976 17.83 24 inform residents and business personnel of (San Saba Co.) the full risk to which they are exposed so Albany August 4, 1978 29.05 24 23" in 8 hr that they can respond accordingly. Medina August 4, 1978 48 52 When one dam fails, the sudden surge Alvin July 26, 1979 25.75 24 NWS reported of water may well be powerful enough to 42" in 19 hr destroy another dam downstream, Odem Oct. 19, 1984 26 4 compounding the disaster. The potential Comanche May 31, 1988 18 5 for such a snowball effect is great, but the Pearland Oct. 17, 1994 28.2 48 problem may seem remote to a dam owner Lake Conroe Oct. 16–19, 1994 27.76 96 who has not studied the potential impacts of upstream dams on his or her own Sources: Bomar (1983); National Weather Service, San Antonio; NWS, Houston-Galveston; U.S. Geological Survey (2003). structure. Upstream dams may seem too

Figure 3.1 Estimated Proportion of Land in Floodplain

Source: Thompson and White (1985: 417).

14 TEXAS COMMISSION ON ENVIRONMENTAL QUALITY Guidelines for Operation and Maintenance of Dams in Texas far away to be a real threat, but inundation history of seismic activity in their locality 3.1.2 Hazards From zones and surge crests can extend many and should develop their emergency Human Activity miles downstream, especially if the procedures accordingly. Human activity must also be consid- reservoir behind the collapsed dam held a Landslides. Rock slides and landslides ered when analyzing the risks posed by large quantity of water. may affect dams directly by blocking a dams. In Texas, the hazard classification of Earthquakes. Earthquakes are also spillway or by eroding and weakening dams is based on the potential for loss of significant threats to dam safety. Both abutments. Indirectly, a large landslide life and economic loss in the area down- earthen and concrete dams can be into a reservoir behind a dam can cause an stream of the dam, not on its structural damaged by ground motions caused by overflow wave that will exceed the capacity safety. Thus, dams that may be of very seismic activity. Cracks or seepage can of the spillway and lead to failure. A sound construction are labeled “high develop, leading to immediate or delayed landslide (or mudslide) can form a natural hazard” if failure could result in cata- failure. Dams such as those in Califor- dam across a stream which can then be strophic loss of life—in other words, if nia—located near relatively young, active overtopped and fail. In turn, failure of people have settled in the potential faults—are of particular concern, but dams such a natural dam could then cause the inundation zone. The “high hazard” (especially older concrete and earthen overtopping of a downstream dam or by structures) located where relatively low- designation does not imply structural itself cause damage equivalent to the scale seismic events may occur are also at weakness or an unsafe dam. See 30 Texas failure of a human-built dam. In addition, risk. Areas of the U.S. where significant Administrative Code Chapter 299 for the large increases in sediment caused by such seismic risks exist are indicated in Figure Texas criteria for classifying dams in the 3.2. However, recent detailed seismic events can materially reduce storage three hazard potential categories. analyses have indicated a much broader capacity in reservoirs and thus increase a Risk may well increase through time area of seismic activity sufficient to downstream dam’s vulnerability to because few governmental entities have damage dams than previously considered; flooding. Sedimentation can also damage found the means to limit settlement below the seismic risk is essentially nationwide. low-level gates and water outlets; damage dams. The hazard level of more dams is Dam owners should be aware of the to gates and outlets can lead to failure. rising to “high” or “significant” as develop-

Figure 3.2 Seismic Map of the United States ZONE 0 No damage. ZONE 1 Minor damage, distant earthquakes may cause damage to structures with fundamental periods greater than 10 seconds, corresponds to intensities V and VI of the M.M.* Scale. ZONE 2 Moderate damage, corresponds to intensity VII of the M.M.* Scale. ZONE 3 Major damage, corre- sponds to intensity VIII and higher of the MM* Scale. ZONE 4 Those areas within Zone 3 determined by the proximity to certain fault systems

*Modified Mercatal Intensity Scale of 1931 Source: U.S. Army Corps of Engineers (1985).

TEXAS COMMISSION ON ENVIRONMENTAL QUALITY 15 Guidelines for Operation and Maintenance of Dams in Texas ment occurs in potential inundation zones Mechanical equipment and associated also be important and useful. Still, exact below dams previously rated “low hazard.” control mechanisms should be protected quantitative and probabilistic tools are not Many other complex aspects of from tampering, whether purposeful or yet applicable in many situations and do settlement and development must be inadvertent. Buildings housing mechanical not fully supplement or replace qualitative considered in assessing dam risks. Because equipment should be sturdy, have pro- analyses—informed perception and of short-term revenue needs or other tected windows, and heavy-duty doors, judgment of the risks. Judgment and pressures, governments often permit and be secured with padlocks. Detachable engineering experience should play an development in hazardous areas despite controls, such as handles and wheels, important role in reaching useful conclu- long-term danger and the risk of high should be removed when not in use and sions in any site-specific analysis of future disaster costs. Diversion of develop- stored inside the padlocked building. structural risk. ment away from potential inundation Other controls should be secured with As mentioned in Chapter 2, structural zones is a sure means of reducing risk, but locks and heavy chains where possible. risks tend to result from design and is not always a policy suitable to the Manhole covers are often removed and construction problems related to the dam immediate needs of local government. sometimes thrown into reservoirs or materials, construction practice, and Perhaps the ultimate irony for a dam spillways by vandals. hydrology. The complexity of the hazard is owner is to have developed and imple- Rock used as riprap around dams is such that structural design and causes of mented a safety program and then to have sometimes thrown into the reservoirs, dam failure are significant areas of research development permitted in the potential spillways, stilling basins, pipe-spillway in engineering. Indeed, better design inundation zone so that the hazard rating risers, and elsewhere. Riprap is often criteria have been developed and safer and owner’s liability increase. displaced by fishermen to form benches. dams are being built, but there is no basis Two extremes of human purpose, the The best way to prevent this abuse is to for complacency. Dams continue to age, will to destroy through war or terrorism use rock too large and heavy to move people continue to move into inundation and the urge to develop and to build, can easily, or to slush-grout the riprap. zones, and enough hazards exist that the both result in public risks. Dams have Otherwise, the rock must be regularly net risk to the public will remain high. proven to be attractive wartime targets, replenished and other damages repaired. and they may be tempting to terrorists. Regular visual inspection can easily detect 3.3 Sources On the other hand, a terrorist’s advantage such human impacts. from holding the public at risk may well Owners should be aware of their of Dam Failure be illusory; the deliberate destruction of a responsibility for the safety of people using There are many complex reasons— dam is not at all easy to bring about. Yet their facility even though their entry may both structural and non-structural—for the possibility exists that such an act could not be authorized. “No Trespassing” signs dam failure. Many sources of failure can be take place, and it should not be discounted should be posted, and fences and warning traced to decisions made during the design by the dam owner. signs erected around dangerous areas. As and construction process and to inad- All sorts of other human behavior discussed in Chapter 10, liability insurance equate maintenance or operational should be included in risk analyses; can be purchased for protection in the mismanagement. Failures have also vandalism, for example, cannot be event of accidents. resulted from the natural hazards already excluded and is in fact a problem faced by mentioned—large-scale flooding and many dam owners. Vegetated surfaces of a 3.2 Site-Specific earthquake movement. However, from dam embankment, mechanical equipment, your perspective as owner, the structure of manhole covers and rock riprap are Structural Risk a dam is the starting point for thorough particularly susceptible to damage by Developing site-specific risk analyses understanding of the potentials for failure. people. Every precaution should be taken involves consideration of a number of to limit access to a dam by unauthorized hazards. Such analyses are helpful in 3.3.1 Three Categories persons and vehicles. Dirt bikes (motor- stimulating better awareness, planning, cycles) and off-road vehicles, in particular, and design. In some cases dam-structure of Structural Failure can severely degrade the vegetation on analyses are quantitative and precise Three categories of structural failure embankments. Worn areas lead to erosion conclusions about engineering and design alluded to in Chapter 2 are: and more serious problems. can be made. Probabilistic analyses can ■ overtopping by flood

16 TEXAS COMMISSION ON ENVIRONMENTAL QUALITY Guidelines for Operation and Maintenance of Dams in Texas

■ foundation defects Box 3.1 ■ piping and seepage Examples of Earthen-Dam Failures Overtopping may develop from many SOUTHFORK, PENNSYLVANIA sources, but often evolves from inadequate The famous Johnstown disaster, caused by the failure of the South Fork Dam in spillway design. Alternatively, even an 1889, in which 2,209 people were killed, is an example of the overtopping of an earthen adequate spillway may become clogged dam. Heavy rainfall in the upper drainage basin of the dam filled the reservoir and with debris. In either situation, water caused overtopping. It was later calculated that, if a spillway had been built according pours over other parts of the dam, such as to specifications and if the original outlet pipes had been available for full capacity abutments or the toe, and erosion and discharge, there would have been no overtopping. failure follow. Concrete dams are more susceptible TETON DAM, IDAHO to foundation failure than overtopping, The Teton Dam failure in 1976 was attributed to (1) internal erosion (piping) of whereas earthen dams suffer from seepage 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 and piping. highly pervious abutment rock and talus to points at the right groin of the dam; (2) Overall, these three events have about destruction of the exit avenues and their removal by the outrush of reservoir water, (3) the same incidence. A more specific the existence of openings through inadequately sealed rock joints which may have analysis of the potential sources of failure developed through cracks in the core zone in the key trench; (4) the development of has to take into account types of dams. piping through the main body of the dam that quickly led to complete failure; and (5) Similarly, the characteristics of the type of the design of the dam did not adequately take into account the foundation conditions dam being monitored will point to problems and the characteristics of the soil used for filling the key trench. requiring more careful attention by the owner when developing a safety program. BALDWIN HILLS AND ST. FRANCIS DAMS, CALIFORNIA The Baldwin Hills Dam failed in 1963 following displacement of its foundation. 3.3.2 Failures Foundation problems were ultimately traced to seismic activity along nearby faults. by Dam Type The failure of the large St. Francis Dam (part of the water supply system for Los Ange- Embankment or Earthen Dams. The les) in 1928 was also attributed to a variety of problems related to foundation pres- major reason for failure of fill or embank- sures, seepage around the foundation, and faulty operation. ment dams is piping or seepage, though Source: Jansen, 1980. other hydrologic failures are significant, including overtopping and erosion from water flows. All earthen dams exhibit some Box 3.2 seepage; however, as discussed earlier, this Examples of Concrete-Dam Failures seepage can and must be controlled in AUSTIN, PENNSYLVANIA velocity and amount. Seepage occurs An example of a foundation problem can be found in the failure of the Austin, through the structure and, if uncontrolled, Pennsylvania Dam in September, 1911. Evidently, the reservoir was filled before the can erode material from the downstream concrete had set sufficiently. Eventual failure near the base occurred because of weak- slope or foundation backward toward the ness in the foundation or in the bond between the foundation and the concrete. upstream slope. This “piping” phenom- enon can lead to a complete failure of the WALNUT GROVE, ARIZONA structure. Piping action can be recognized In 1890, the Walnut Grove dam on the Hassayompa River failed due to overtop- by an increased seepage flow rate, the ping, killing about 150 people. The failure was blamed on inadequate capacity of the discharge of muddy or discolored water spillway and poor construction and workmanship. A spillway 6 x 26 feet had been below the dam, sinkholes on or near the blasted out of rock on one abutment, but, with a drainage area above the dam site of embankment, and a whirlpool in the reservoir. about 500 square miles, the spillway did not have nearly enough discharge capacity. Earthen dams are particularly Source: Jansen, 1980. susceptible to hydrologic failure since most

TEXAS COMMISSION ON ENVIRONMENTAL QUALITY 17 Guidelines for Operation and Maintenance of Dams in Texas

sediments erode at relatively low waterflow As dams age, maintenance becomes has developed an approach to predicting velocities. Hydrologic failures result from more critical. Lack of maintenance will the condition of metal conduits in the uncontrolled flow of water over the result in deterioration and eventually, failure. embankment dams. Utilizing the results of dam, around it, and adjacent to it, and the Texas dams are aging as shown in dam safety inspections from New Jersey, erosive action of water on the dam’s Table 3.2, and problems as described Washington, Virginia, Ohio, Kansas, and foundation. Once erosion has begun above are slowly becoming apparent. Oklahoma, a rating system was used to during overtopping, it is almost impossible Table 3.2 characterize the condition of metal to stop. In a very special case, a well- Ages of Dams in Texas conduits. Using these data, along with the vegetated earthen embankment may age of the dam, a statistical model was withstand limited overtopping if water Percentage Dates developed to predict the condition of flows over the top and down the face as an of Dams metal conduits as a function of age. The evenly distributed sheet and does not Prior to 1950 15.3 results of this assessment, combined with become concentrated in a single channel. 1950–59 15.3 the recommendations of the dam inspec- Box 3.1 lists examples of earthen-dam 1960–70 42.2 tors, allow us to predict, as a function of failures caused by some of these conditions. 1971–80 18.7 age, the likelihood that a conduit will Concrete Dams. Failure of concrete 1981–present 8.5 dams is primarily associated with founda- require repair or replacement (Figure 3.3). tion problems. Overtopping is also a Knowledge of the hazards, risks, and significant cause again primarily when 3.3.4 Condition failures associated with dams is critical for spillways are built with inadequate Rating of Dam Conduits owners. Consider each aspect of a safety capacity. Other causes include failure to let As part of research work supported by program in relation to the most probable concrete set properly and earthquakes. The the National Dam Safety Program, the sources of failure for your dam in examples summarized in Box 3.2 illustrate National Performance of Dams Program particular. typical foundation problems leading to dam failure. Figure 3.3 3.3.3 Age and Its Prediction of the Likelihood of Metal Conduits Requiring Relation to Failure Repair or Replacement as a Function of Age 0.5 Foundation failures occur relatively early in the life of a dam, whereas other 0.45 causes generally take much longer to 0.4 materialize. Thus, it is not surprising that a very large percentage of all dam failures 0.35 occur during initial filling, since that is 0.3 when design or construction flaws, or latent site defects, appear. 0.25

0.2

0.15

0.1 Probability of Repair/Replacement 0.05

0 0 5 10 15 20 25 30 35 40 45 50 Age (Years) Source: National Performance of Dams Program, Stanford University (e-mail Communication, March 26, 2004).

18 TEXAS COMMISSION ON ENVIRONMENTAL QUALITY