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Red Mesa Ward Site Hydrology <br /> 5. Impervious Area. The impervious area for the watershed was determined to be negligible. A <br /> value of zero was entered into the model for this parameter. <br /> Values for the Muskingum-Cunge routing method are discussed below: <br /> 1. Parameters related to topographical characteristics were estimated from USGS Quadrangle <br /> Maps. These parameters include slope,watercourse length, channel width, and side slopes. <br /> 2. The Manning's Roughness for all reaches was estimated to be 0.07. The basis for this is <br /> Figure 2-2 from Design of Channels with Wetland Bottoms. Although Hay Gulch is not a <br /> wetland bottom channel, the agricultural crops (alfalfa and hay) should simulate the <br /> roughness to a reasonable degree. A nominal depth of four feet was assumed for flow <br /> through Hay Gulch. <br /> Appendix E contains the HEC-1 output for the model run. Table 2 below summarizes the results <br /> of the modeling run for each element in the model. Figure 2 presents the inflow hydrograph for <br /> Red Mesa Ward Reservoir, and Table 3 gives the hourly runoff volumes. <br /> Some flood hydrologists have opined that, if a frequency for a maximum probable flood were to <br /> be assigned, it would be about once in 10,000 years,the approximate time to the last Ice Age (Dr. <br /> Robert Clark, 1958). Using this approach, paleo-flood hydrologists examine on-site field <br /> evidence for estimating the magnitude of the maximum floods since the Ice Age. In 1996, <br /> Jonathan W. Pruess published the report entitled Paleoflood Reconstructions Within the Animas <br /> River Basin Upstream From Durango, Colorado (Pruess 1996). Pruess determined that the <br /> maximum unit discharges where evidence was found have not exceeded 100 cfs per square mile <br /> at elevations above 7,200 feet without relating the data to any specific time interval or frequency. <br /> Based upon this report (masters thesis), Wright Water Engineers opines that the recommended <br /> unit discharge for Red Mesa Ward Reservoir of about 1,800 cfs per square mile is adequately <br /> high. <br /> 991-077.070 Wright Water Engineers, Inc. Page 5 <br /> September 2002 <br />