Regional Analysis of Extreme Precipitation in Wyoming

Safety of major hydraulic structures such as dams is a very important issue concerning public and governmental agencies. The occurrences of hydrologic extremes such as major floods pose great threat to the safety of those structures. Their failures not only cause significant economic losses but also could potentially inflict tremendous human tragedy due to loss of lives. In the evaluation of hydrologic adequacy of a dam, the probable maximum precipitation (PMP)/ probable maximum flood (PMF) are often used. A common practice to assess the hydrologic adequacy of a dam is to examine the spillway capacity as the percentage of PMF. It has been found that there is a great variation in return period corresponding to a specified percentage of PMP/PMF within a region (American Society of Civil Engineers, 1988). Such inconsistency creates difficulties for regulatory agencies to establish a uniform safety design policy for dams. In Wyoming, great majority of the existing dams are not high-hazardous structures. The adoption of PMP/PMF for their safety requirement could impose unnecessarily heavy financial burden to the dam owners and the public. The performance requirement for such non-high-hazard structures should be based on the proper traded between the anticipated failure probability, the consequence of failure, and the cost of structure, which can be cast in the framework of risk-based design (American Society of Civil Engineers, 1986). The risk-based design of hydraulic structures involves an evaluation of expected damage as the result of structural failure. This requires the knowledge of stochastic properties of extreme hydrologic events. Therefore, the systematic analysis of the characteristics of extreme storms and floods paves the way for risk-based design and evaluation of hydraulic structures. To evaluate safety protection of a dam or other hydraulic structures subject to hydrologic loadings, it is essential to have knowledge about the statistical characteristics of those hydrologic extremes. Records on precipitation and floods in Wyoming are not sufficient in time and in space. To improve reliability of statistical analysis for estimating rare storm and flood events, regionalization procedures can be utilized. Hence, Wyoming is currently conducting a study on regionalization of extreme precipitation based on the methodological framework developed by Schaefer (1993) for the State of Washington, using index flood type approach in conjunction with L-moments developed by Hosking (1986). Based on the elevation of rain gauges and sample L-moments of annual maximum precipitation for different durations, statistical cluster analysis, along with some adjustments for inconsistencies, was applied to obtain region delineation. Upon the completion of region delineation, regional equations that relate the essential statistical properties of extreme storms to the mean annual precipitation (MAP) were developed for purpose of establishing precipitation-frequency relations. In addition, methodology was developed to account for the uncertainties in regional equations. This paper briefly presents the procedures and results of the regional analysis.