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Long Hollow Reservoir <br />Feasibility Study <br />3.4 Hydrology <br />3.4.1 Background <br />, The purpose of this part of the study is to predict the IDF generated by the PMP for LHR. The <br />approximate latitude and longitude of LHR is 37° 3' 30" and 108° 10' 30", respectively. The <br />dam will be located in the SW'/4 of Section 32, Township 33 North, Range 12 West, and will be <br />classified as a large, high-hazard dam. The IDF will be generated for 90 percent of the PMP, as <br />required by the SEO's proposed Rules and Regulations for Dam Safety and Dam Construction. <br />The PMP was developed according to the procedure outlined in Hydrometeorological Report No. <br />49 (National Oceanic and Atmospheric Administration [NOAA]). A unit hydrograph was <br />developed for the basin using Excel spreadsheets in accordance with the guidance provided in <br />Chapter 4 of the Flood Hydrology Manual, a USBR publication. The tributary area was modeled <br />as one basin, per USBR guidance and as recommended by the SEO. The unit hydrograph and <br />precipitation data were entered into the HEC-1 program to carry out runoff simuIations. <br />3.4.2 Probable Maximum Precipitation <br />The PMP estimate was developed using Hydrometeorological Report No. 49, Probable <br />Maximum Precipitation Estimates, Colorado River and Great Basin Drainages, a NOAA <br />publication, hereafter referred to as HMR No. 49. The HMR No. 49 procedure gives PMP <br />estimates for both the general storm and the local storm. In the case of the general storm, PMP <br />estimates are developed for several durations, specifically 6, 12, 18, 24, 48, and 72 hours. For <br />the local storm, a duration of 6 hours is implicit. It is left to the engineer to determine which <br />storm is the most critical based on the problem at hand. For the purpose of determining the <br />feasibility of constructing the LHR, the local storm is the controlling precipitation event because <br />of the intensity. This is consistent with the USBR's experience in this region. Table 3.4.2-1 <br />below summarizes the results of the PMP estimates accounting for a 10 percent reduction of the <br />PMP for design. <br />991-077.120 Wright Water Engineers, Inc. Page 16 <br />April 2005 <br />