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.r ~ The U.S. Geological Survey published an open-file report (84-137) of stream statistics that shows a stream gaging station on Surface Creek at Cedaredge with 64 years of data, The station (number 01943500) is listed as having a basin area of 39.5 square miles, mean basin elevation of 9930 feet and mean annual precipitation of 30A-inches, Land use for this basin is shown at 59-percent forest, 14-percent agricultural, 26-percent range and I-percent urban use. The study area is above this gage but very little tributary area contributes to stream flows through the study area due to the topographic conditions. 5.0 100-YEAR FLOOD FLOW The 100-year flood flow given in the USGS Report for the gaging station on Surface Creek at Cedaredge is 1140-cfs based on 64 years of stream data. This number is consistent with flows used in the 1983 study and is used for the purpose of this study, No hydrologic analysis was performed. As measured stream gage data was used for establishment ofthe 100-year flow, no adjustments are required to account for irrigation diversions. 6.0 METHOD The hydraulic analysis utilized the U.S. Army Corps of Engineers HEC-2 model to determine water surface profile and flow characteristics. A field survey was used to determine geometric conditions for the creek, overbanks and bridges for input to the HEC-2 model. Roughness factors (Manning's "n" values) were determined based on visual observation and were estimated at 020 for overbanks and 0,045 for the channel. The flow regime for this section of the creek is supercritical and the model was run from upstream to downstream to calculate supercritical water surface elevations. The slope area method was used to determine starting water surface elevation at the upstream cross- section. The model interpolated cross-sections between measured cross-sections where necessary due to changes in channel conditions, Bridges were modeled several different ways with no significant impact to calculated water surface elevation (CWSE). Manning's "n" for overbanks was increased to OA5 to check sensitivity and no significant change to CWSE was noted, The model was also checked using flow records and measured water surface elevations for the date of survey. The hydraulic analysis was made based on unobstructed flow. Visual observations of high water marks at bridges were used to correlate results with field data. All elevations are referenced to the NGVD Datum 1929. The HEC-2 model results are attached to this report along with stream profiles and associated mapping. A summary of results in given in Table 1. 2