2005-01-03_HYDROLOGY - M1977493 - Laserfiche WebLink

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11 1. Introduction The purpose of this site - specific probable maximum precipitation (PMP) study is to provide rainfall values based on analyses of the most significant rainfall storm events that have occurred over regions that are meteorologically and geographically similar to the Climax Mine tailings ponds drainage basin. 1.1 Background of Previous PMP Studies The definition of PMP used in the most recently published Hydrometeorological Report (HMR), published by the National Weather Service (NWS), is "theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given area at a particular geographical location at a certain time of the year." (HMR 59, p.5). Since the mid- 1940s, several government agencies have been involved in developing methods to calculate PMP for various regions of the United States. The NWS (formerly the U.S. Weather Bureau) and the Bureau of Reclamation have been the primary agencies involved in this activity. The PMP estimates derived in the HMRs are used to calculate the probable maximum flood (PMF), which in turn is used to evaluate the adequacy of hydraulic structures (Tomlinson 2002). In very basic terms, the PMP is the largest rainfall that can occur over the drainage basin under current climate conditions. Depending on basin and spillway characteristics, the "greatest depth of precipitation" may either be a very intense storm of relatively short duration or a multi -day storm event with smaller rain rates but a large volume of rainfall (Tomlinson 2001). In the case of the Climax Mine tailings ponds drainage basin, the critical storm is one of relatively short duration, which produces very heavy rainfall over several hours. The generalized PMP studies currently in use in the conterminous United States include HMR 49 (1977) for the Colorado River and Great Basin drainage; HMRs 51 (1978), 52 (1982), and 53 (1980) for all of the U.S. east of the 105 meridian; HMR 55A (1988) for the region between the Continental Divide and the 103` meridian; HMR 57 (1994) for the Columbia River Drainage in the northwestern U.S.; and HMR 58 (1998) and 59 (1999) for the state of California (Tomlinson 2002). The region surrounding the Climax Mine tailings ponds drainage basin is covered by the generalized precipitation analysis of HMR 49. These NWS HMRs provide PMP values for both short duration storms with heavy, intense and centralized rainfall periods and for longer duration events with continuous, widespread regional rainfall over several days. These values for PMP are generally referred to as "estimates" since they are theoretical values that depend on judgment decisions by hydrometeorologists. However, each analysis should follow accepted procedures as defined by the NWS and the World Meteorological Organization (WMO), be objective as possible, and be reproducible (Tomlinson 2001).