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11 Continuing Development of Hydrologic and Earthquake Engineering Technologies OVERVIEW As noted in Chapter 3, the methods in use for estimating the hazards to dams that may result from extreme floods and earthquakes are based on relatively new and still developing branches of technology. Because so many agencies and individual scientists and engineers are involved in problems related to extreme floods and earthquakes, further development is confi- dently expected in our understanding of the natural phenomena involved and in the ability to analyze the effects of such events on dams and to design for such effects. Occasior~al failures can be expected that will focus public attention on dam safety problems and the need for further improvements. Such developments in many areas of science and engineering improve the ability to design and construct dams that can meet the tests of extreme floods and earthquakes. Thus, the safety criteria developed today may not be appropriate in the future. They should be reviewed and updated periodi- cally by a committee consisting of designers, hydrologists, meteorologists, seismologists, engineers, economists, and representatives of the general pub- lic. In the remainder of this chapter, some areas of research and development are pointed out that are most closely related to engineering for the effects of extreme floods and earthquakes. With present knowledge of meteorological, hydrological, and seismologi- cal phenomena, one can only regard the occurrence of a large earthquake or an extreme flood at a given site as a random event. It can never be certain just how big an earthquake or flood a dam will experience during its useful life. 110
Hydrologic and Earthquake Engineering Technologies 111 Thus, a dam designer or an official responsible for balancing the interests of all segments of the public usually cannot prove conclusively that the provi- sions in a dam design for extreme floods and earthquakes are adequate and have resulted in efficient use of resources. As discussed in earlier chapters, this lack of certainty in project needs has been met in two ways: (1) by trying to establish probabilities for these extreme natural events and then selecting a probability considered adequate as a design basis or (2) by attempting to define the maximum flood or earthquake at the site that conforms to our present knowledge. As has been noted, each approach offers certain advan- tages and disadvantages, but neither may provide the basis for complete confidence in a dam design by all concerned. The broad aim of research and development efforts related to dam safety should be the improvement of this confidence factor. At present the methods available for acceptably thorough analyses of a dam's safety from extreme floods and earthquakes require substantial com- mitments of financial and technical resources. To many owners of dams and to many agencies responsible for safety of many dams, these costs in techni- cal manpower and in dollars are so high that they have delayed or prevented action to protect the public from the dangers of unsafe dams. Thus, appro- priate research and development efforts should focus on development of simplified evaluation methods that could be applied with confidence with defined limits. HYDROMETEOROLOGICAL RESEARCH For the past several decades, most areas of meteorology, hydrology, and hydrologic engineering related to dam design have received attention in research programs. Thus, the field has not been neglected, but it is felt that attention to the research needs in these areas should continue. Research in methods of assessing probabilities of extreme floods has not received much attention. The ability to assign probabilities with confidence to such rare events would greatly enhance the capability to make rational decisions on allocation of resources in dam design and construction. Past experience has shown the severe limitations of methods of estimating proba- bilities of future extreme floods based on stream discharge records at specific sites. However, it appears that the considerable collection of data on major storm rainfalls developed by the National Weather Service (in cooperation with other agencies) for studies and estimates of probable maximum precipi- tation could provide a basis for generalized estimates of probabilities of extreme rainfalls, which could then be the bases for estimates of probabilities of extreme floods in specific drainage basins. Research in this area, possibly by the Hydrometeorological Branch of the National Weather Service, should
112 SAFETY OF DAMS be considered. It appears that use of paleobydrological methods could offer methods of assessing magnitude of prehistoric great floods in some geo- graphic areas. Further development of this approach should be considered. Finally, the committee suggests that the Corps of Engineers and the Bu- reau of Reclamation and other agencies in the water resources research field pursue a program of research and development designed to upgrade our ability to implement risk-based analyses. This research can and should range from the appraisal of issues relating to the probability distribution of very rare events to the analysis of procedures for implementing reliable flood- warning and flood-fighting techniques. EARTHQUAKE RESEARCH Research relevant to earthquake effects on dams has made rapid strides in the past 20 years. Knowledge of where and why earthquakes occur has been much expanded, and knowledge of earthquake ground shaking and the consequent vibrations of structures has been markedly advanced. Continua- tion of this ongoing research will contribute to improved hazard assessment and to improved methods of seismic design of dams. However, earthquake research, in general, has not been particularly directed at seismic problems of dams, though much of it does have a bearing on these problems, for example, compilation of earthquake statistics, study of source mechanisms, recording of strong earthquake ground shaking, and development of power- ful methods of computing the dynamic response of structures. It is felt that much more could be gained through research concentrating on the seismic problems of dams. Such studies include research on the nature of ground shaking at dam sites as affected by topographic and geologic features pecu- liar to dams and reservoirs; improved methods of identifying active faults and estimating the frequency with which they generate earthquakes; the dynamic response of dam-foundation-abutment-reservoir systems; genera- tion of surface waves in the reservoir and behavior of pressure waves and their interaction with the dam and reservoir bottom; dynamic performance of concrete dams and their ability to withstand high tension stresses; capabil- ity of three-dimensional, dynamic analysis of embankment dams to predict permanent seismic deformations; earthquake sensors on selected dams to record the actual behavior during earthquakes; and additional strong mo- tion instrumentation networks to further develop attenuation relationships for the various seismotectonic regimes, especially in the central and eastern United States.
APPENDIXES .
