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Red Mesa Ward Site Hydrology <br /> Qs=(V'/(Lg+D/2))*q <br /> Where q is the corresponding ordinate from the dimensionless unit hydrograph for a Rocky <br /> Mountain thunderstorm. (See Table 4-11 from the Flood Hydrology Manual.) <br /> Once the unit hydrographs were developed, they were input into HEC-1 to model develop the <br /> IDF. The resulting unit hydrographs for the eight subbasins are included in Appendix C. <br /> HEC-1 Rainfall Runoff Simulations <br /> Precipitation data and unit hydrograph data were entered into HEC-1 using Graphical HEC-1. <br /> The loss rate and routing methods employed are the Green and Ampt and Muskingum-Cunge <br /> methods, respectively. Appendix D contains reference documents for model parameters. <br /> Values selected for Green and Ampt parameters are discussed below. <br /> 1. Initial Loss (I). The initial loss was assumed to be zero in accordance with USBR <br /> experience, which indicates that a storm of sufficient magnitude has preceded the PMP to <br /> satisfy initial infiltration losses. <br /> 2. The Volumetric Moisture Deficit. The volumetric moisture deficit (the soil porosity minus <br /> the initial water content)was assumed to be zero. According to the USBR, it is reasonable to <br /> assume that any antecedent precipitation has satisfied any soil moisture deficiencies. <br /> 3. Wetting Front Suction. The Soils Survey for La Plata County indicates that the soil unit in <br /> the area of interest is comprised mainly of loam (to relevant depths). The wetting front <br /> suction was estimated to be 12 inches. This value was taken from Table 5-2 of the <br /> Hydrologic Modeling System (HEC-HMS) Technical Reference Manual. <br /> 4. Hydraulic Conductivity. The hydraulic conductivity was estimated to be 0.52 inch per hour. <br /> This value was taken from Table 5-2 of the Hydrologic Modeling System (HEC-HMS) <br /> Technical Reference Manual. <br /> 991-077.070 Wright Water Engineers, Inc. Page 4 <br /> September 2002 <br />