Antecedent Moisture Conditions for Utah Local Storm Probable Maximum Floods

Introduction: The critical inflow design flood for most dams in Utah is the probable maximum flood (PMF) resulting from the local storm probable maximum precipitation (PMP) event. Commonly, the Soil Conservation Service (SCS) curve number method is used to determine the PMF from the local storm PMP. An important factor in this determination is the assumption of antecedent moisture conditions (AMC) existing immediately prior to the onset of the PMP event. At one northern Utah dam site the use of AMC III increased the PMF peak flowrate by 50 percent over the peak obtained when AMC II was used (Win 1993). In this study we explore the occurrence of AMC II (average) and III (saturated) conditions at locations throughout Utah. The occurrence of AMC II or III, which is defined by the magnitude of rainfall over the previous five days, is shown to be independent of the magnitude of precipitation on the sixth day. Also, the probability of occurrence of AMC II and III during the critical months for local storm PMP is shown to be low. While these conclusions do not rule out the possibility of the joint occurrence of a PMP event and AMC III, they do demonstrate that it is an unlikely event. If AMC II is accepted for use in local storm PMF determinations in Utah, a significant reduction in Utah PMF peak flowrates can be expected. In any event, this study should be an important contribution to the evaluation of dam safety in Utah through providing a better basis for the selection of AMC conditions in PMF determinations. Throughout the course of this research, we have chosen to take the conservative approach to the study. It is the intent of this research to evaluate the use of AMC II or III in semi-arid and arid Utah. Trends were evaluated using upper limits instead of averages and snowmelt was included as a contributor to soil saturation. It is our belief that if one can disprove a theory or practice by being conservative, it is a much stronger case than if a more liberal approach were taken