Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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History of Development
History of Development
Pages 35-43
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From page 35... ...
As noted by Myers, the evolution in the United States of techniques for determining spillway capacity requirements can be divided into four stages, outlined below. Early Period Before 1900 the designer of a dam in the United States usually had little hydrologic data on which to base estimates of spillway requirements.
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From page 36... ...
Often the designer of a dam had little information except highwater marks on the stream he was damming or in adjacent watersheds to indicate the flood potentials at his dam site. The designer might estimate peak discharge rates of past floods based on such meager data and base his spillway design on such estimates with, perhaps, some added capacity provided as a safety factor.
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From page 37... ...
Storm Transposition Period In the 1920s engineers in the Miami Conservancy District in Ohio undertook a program of studying past major flood-producing storms to develop rainfall duration-area-depth relationships for use in planning and design of the comprehensive flood control project for the Miami Valley. It was recognized that measured flood peaks are dependent on topography and size of individual watersheds and chance placement of storm centers over the watershed.
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From page 38... ...
on frequency analysis of either rainfalls or streamflows are in use for determining sizes of spillways. Some agencies specify various percentages of the PMP or the probable maximum flood (PMF)
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From page 39... ...
Other risk-base`] analysis methods similar to the method advocated by the 1973 ASCE committee report, but which do not attempt to place a value on human life, are being advocated in those agencies attempting to apply risk analysis techniques to decisions regarding dam safety.
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From page 40... ...
There is every indication that many design agencies were at that time making some analytical studies to evaluate seismic safety, or incorporating simple defensive measures into design of projects to increase the safety of dams against earthquake-shaking effects. In the period 1930-1970, design practice usually considered earthquake effects by simply incorporating in the stability or stress analysis for a dam a static lateral force intended to represent the inertia force induced by the earthquake.
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From page 41... ...
2. The Division of Safety of Dams of the California Department of Water Resources decided to require owners of many earth dams to reevaluate the seismic stability of the dams using dynamic analysis methods, regardless of the results indicated by pseudostatic methods of analyses.
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From page 42... ...
Earth Dams etc.~; · defensive design measures (ample freeboard, wide transition zones, · for reasonably well-built dams on stable foundation soils, no analysis required if peak ground accelerations are less than about 0.2 g; · a dynamic deformation analysis for clams constructed of or on soils which do not lose strength as a result of earthquake shaking and located in areas where peak ground accelerations may exceed about 0.2g; and · a dynamic analysis for liquefaction potential or strain potential for dams involving embankment or foundation soils that may lose a significant portion of their strengths under the effects of earthquake shaking. Private engineering companies follow generally similar practice.
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From page 43... ...
History of Present Practices mination of the earthquake motions (either in the form of a response spectrum or in the form of time histories of accelerations) for which the safety of the dam must be evaluated and for which the safety of the dam must be assured.
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