| Popis: |
The probable maximum flood (PMF) is used for the xxi assessment of maximum flood potentials in spillway sizing of new dams and in evaluating the adequacy of existing hydrologic structures. Determination of the PMF begins with the estimation of the probable maximum precipitation (PMP) for a particular dam site. Selecting hydrologic methods and assumptions for converting PMP to PMF, which are most appropriate for Utah conditions, is important to meet current inflow design flood (IDF) safety standards. The objectives of this study were (a) to demonstrate a maximization approach to PMF determination, (b) to evaluate effects of basin characteristics and isohyetal or uniform rainfall pattern on PMP and hence PMF, (c) to evaluate uncertainties due to hydrologic procedures through sensitivity studies, (d) to analyze historical events for indications of actual loss rates, (e) to analyze historical snowpack and melt conditions for critical snowmelt conditions, (f) to assess implications of this study for a degree of conservatism index, and (g) to evaluate the implications of the study for the selection of procedures for PMF determinations. A sensitivity study was conducted to evaluate the hydrologic methods and assumptions (e.g., loss rates, unit hydrographs, basin subdivision, temporal storm distribution, flood routing, and antecedent events) which may be used in northern Utah PMF studies. The maximization of local storm PMF produced a higher peak reservoir stage than using basin average PMP only. Evaluation of effects of basin characteristics on PMF showed that local storm PMP is critical for basins up to about 2,000 square miles in northern Utah. Estimation of maximum probable rain-on-snow flood conditions for Porcupine Basin for April, May, and June revealed that there are significant increases in peak flows and flood volumes due to the contribution of snowmelt in the months of May and June. The results from the degree of conservatism assessment performed on local storm PMF estimates for Porcupine Basin showed that the overall effect of assumptions made and procedures followed in a typical PMF determination is almost certainly a conservative estimate. |
