R655-11-4. Hydrologic Design

In order to arrive at an Inflow Design Hydrograph or Inflow Design Flood (IDF) more representative of actual conditions in Utah, the State Engineer has commissioned, or has been involved in, numerous studies to supplement the National Oceanic and Atmospheric Administration's (NOAA) Report entitled "Hydrometeorological Report No. 49 (HMR49) - "Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages". The results of most of these studies are used to better identify soil conditions, discharge coefficients, and unit hydrograph parameters. The results of two of the studies are used directly to refine the calculation of the design rainfall values. Both studies were completed by Donald Jensen of the Utah Climate Center and are entitled, "2002 Update for Probable Maximum Precipitation, Utah 72 Hour Estimates to 5,000 sq. mi. - March 2003" (USUL) and "Probable Maximum Precipitation Estimates for Short Duration, Small Area Storms in Utah - October 1995 "(USUS). All of HMR49, Table 1, page 4 of USUL, and Table 15, pages 74-75 of USUS are hereby incorporated by reference. All High Hazard and Moderate Hazard dams in Utah must use the precipitation values obtained from the use of all three publications. To avoid confusion, precipitation values obtained from HMR49 exclusively will be referred to as the Probable Maximum Precipitation (PMP), while those obtained from using HMR49 in conjunction with USUL or USUS, will be referred to as the Spillway Evaluation Precipitation (SEP). The resulting hydrographs generated will be referred to as the Probable Maximum Flood (PMF) and the Spillway Evaluation Flood (SEF) respectively.