Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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Design Flood Estimates: Methods and Critique
Design Flood Estimates: Methods and Critique
Pages 44-60
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From page 44... ...
Details of the risk analysis approach, along with examples of its use, are presented in Appendix E DETERMINISTIC APPROACH Summary In estimating a PMF or any design flood by deterministic methods, several tools of meteorology, hydrology, and hydrologic engineering are employed 44
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From page 45... ...
Probable Maximum Precipitation (PMP) Evaluation Of the factors that have an influence on the magnitude of the probable maximum flood (PMF)
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From page 46... ...
(Adjust the storm rainfall by multiplying it by the ratio of an index of maximum atmospheric moisture in transposed location to that where the storm occurred.) TABLE 5-1 PMP Estimates (to Nearest 0.5 Inch)
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From page 47... ...
However, such future increases in estimates are likely to be incrementally less, in general, than those of the past during the period that the science of hydrometeorology was rapidly developing and the data base on past extreme events was being accumulated. Also, studies to redetermine probable maximum precipitation estimates for areas in the Tennessee Valley made in 1978 resulted in some lower estimates than those developed for the same areas in 1968; then more intensive studies sometimes can result in lower estimates even if more data became available.
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From page 48... ...
The antecedent conditions, such as expected reservoir levels, existing snowpack, and soil moisture, are considered in the context of the causative event of the primary flood. For example, if the extreme floods for moderate to large basins result
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From page 49... ...
These are additional items that must be weighed in importance, and from this analysis is obtained a reservoir level to be used as the initial point in the routing of the design flood and the antecedent storm through the storage and spillway facilities of the site. Usually to avoid increasing downstream flood damages, any assumed reservoir operation plan should include the requirement that no operating procedure will increase the peak reservoir outflow over the peak natural inflow unless specific flood easements to accommodate excess flow downstream have been obtained.
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From page 50... ...
Some regulatory bodies require a gate capacity such that a specific percentage of the PMF can be accommodated with one gate inoperable without the dam being overtopped. The routing of the PMF inflow hydrograph through the available reservoir storage and the spillway facilities of the project utilizes some variation of the volumetric conservation equation: I - 0 = AS where I = reservoir inflow 0 = the outflow or discharge AS = change of storage in reservoir The mechanics of applying and solving this basic equation are given in the standard hydrologic texts and will not be described herein.
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From page 51... ...
In contrast to practices in the United States, the criteria of the Institution of Civil Engineers call for use of estimates of floods with average return periods of 10,000 years in the British Isles. Past experience has indicated that estimates of magnitudes of very rare floods developed by probabilistic methods are even more likely to change as additional basic data become available than flood estimates developed by deterministic methods discussed earlier in this chapter.
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From page 52... ...
Appendix E discusses and gives an example of the latter approach. Through computerized modeling of floods and dam break inundation mapping, the safety evaluation flood can be selected at the flood peak level where downstream flood damages would not be increased by the overtoppingof the dam.
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From page 53... ...
Given their limited resources, as a practical matter, they must use a generalized system of assigning design standards according to generalized hazard and size classifications, at least as an interim step until more detailed site-specific studies can be made. However, the wide range of hazard versus size versus design standards among the
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From page 54... ...
In any case, a critique of present practices must point out that, though a generalized approach to selecting design standards is justified as a practical interim step, there is a need for more uniformity among the various federal and state agencies in establishing size and hazard definitions and correlative design standards. Dam Classification Systems Even if we recognize the need for generalized hazard versus size versus design criteria classifications, the almost universally used high-, intermediate-, and low-hazard classes are not well defined.
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From page 55... ...
Indicated overtopping of such a dam during the probable maximum flood may be permitted by some agencies, if the rock at the toe of dam is judged able to withstand the hydraulic forces imposed and the stability of the dam would not be compromised otherwise. For smaller dams and those with lower hazard ratings, there is much greater divergence in views concerning appropriate spillway capacity requirements.
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From page 56... ...
The newly established Association of State Dam Safety Officials may wish to consider action toward such a goal. Some differences among agencies have been noted in practices followed in developing probable maximum flood estimates from probable maximum precipitation values.
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From page 57... ...
to upgrade the system to meet changes in the area to be served.) · Even though some problems with current PMP estimates have been noted, such estimates still offer the bases on which the engineering profession has the most confidence for sizing spillways of new, large dams in the highhazard category.
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From page 58... ...
Additional problem areas noted in risk-cost analyses are as follows: · Future development below a dam is usually unpredictable and may invalidate the risk-cost determination and the safety evaluation flood selected for present (or inaccurately predicted future) conditions.
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From page 59... ...
the widespread generalized approach, relying largely on judgment to assess hazard and selecting design standards based on loosely defined categories; (2) using site-specific dam-break-routing studies to better define hazard and to select a spillway design flood without quantifying risks and costs; and (3)
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From page 60... ...
Exceptions exist for very large, very high-hazard dams for which there is a clear consensus that something like the probable maximum flood is the appropriate inflow design flood or is most likely the best of the available alternatives for such applications. However, this design flood is not necessarily appropriate for the thousands of existing smaller (yet highhazard)
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