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Long Hollow Reservoir <br />Feasibility Study <br />10. Unit Runoff VoIume (V~. Volume of runoff generated by 1 inch of excess precipitation <br />expressed in 1-day cubic feet per second. <br />11. Discharge for Subbasin (Qs). The discharge for the subbasin is calculated as follows: <br />Qs = (V' / (Lg+D/2))*q <br />Where q is the corresponding ordinate from the dimensionless unit hydrograph for the <br />Southwest Desert, Great Basin, and Colorado Plateau region. (See Table 4-13 from the <br />Flood Hydrology Manual.) The SEO recommended that the Southwest Desert, Great <br />Basin, and Colorado Plateau unit hydrograph be used for this study rather than the Rocky <br />Mountains thunderstorm unit hydrograph. <br />Once the unit hydrograph was developed, it was input into the HEC-1 model to develop the IDF. <br />The resulting unit hydrograph of the tributary basin is included in Appendix C. <br />3.4.4 HEC-1 Rainfall Runoff Simulations <br />Precipitation data and unit hydrograph data were entered into HEC-1 using Visual HEC-1. The <br />loss rate methods empZoyed is the Green and Ampt method. There is no routing method <br />associated with this study because the tributary area was modeled as one basin draining directly <br />to the reservoir. Appendix D contains reference documents for model parameters. <br />Values selected for Green and Ampt parameters are discussed below. <br />1. Initial Loss (~. The initial loss was assumed to be zero in conformance with the <br />recommendation in the Dam Safety Project Review Guide that studies should assume no <br />initial abstractions because antecedent moisture conditions may prevail. This is <br />consistent with USBR's experience, which indicates that a storm of sufficient magnitude <br />has preceded the PMP to satisfy initial infiltration losses. <br />2. The Volumetric Moisture Deficit. The volumetric moisture deficit (the soil porosity <br />minus the initial water content) was assumed to be 0.083 feet per foot (1 inch per foot). <br />991-077.120 Wright Water Engineers, Inc. Page 19 <br />April 2005 <br />