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Long Hoilow Reservoir <br />Feasibility Sfudy <br />Appendix A contains the detailed calculation sheets with relevant figures and tables taken from <br />HMR No. 49. <br />TABLE 3.4.2-1 <br />Summary of 90 Percent PMP Estimates <br />Storm Type Duration (Hours) Total Preci itation <br />General 6 4.5 <br />General 12 6.2 <br />General 18 7.4 <br />General 24 8.2 <br />General 48 10.6 <br />General 72 11.6 <br />Local 6 7.2 <br />The local storm PMP computation yields hourly incremental precipitation values throughout the <br />duration of the storm. The most intense precipitation occurs during the third hour of the storm. <br />For this hour, the procedure breaks down the rainfall into 15-minute increments. For input to the <br />hydrologic model (HEC-1), it was necessary to break up the entire event into 5-minute <br />increments. Appendix A contains the hyetograph showing how the data was entered into the <br />HEC-1 model.~ It is assumed that all precipitation occurs as rain (as opposed to snow or hail) <br />and is immediately available as runoff. <br />3.4.3 Unit Hydrographs <br />A spreadsheet was used to develop the dimensionless unit hydrograph for the tributary basin. <br />The unit hydrograph was developed in accordance with the guidance provided in Chapter 4 of the <br />Flood Hydrology Manual, a USBR publication. Following is a description of the variables and <br />calculations employed in determining the unit hydrograph for the tributary basin. Appendix B <br />contains pertinent excerpts from Chapter 4 from the Flood Hydrology Manual. Appendix C <br />contains values for the following parameters. <br />' WWE used a program called Visual HEC-l, which provides a Windows interface for HEC-1. <br />991-077.120 Wright Water Engineers, Inc. Page 17 <br />April 2005 <br />