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<br /> <br /> <br />A number of additional curve-number based hydrologic methods have been used for <br />flood peak estimates within the study area. J.F. Sato (1996) reports an HEC-l analysis <br />done for North Clear Creek by Futura Engineering which used a Curve Number of 55. <br />An HEC-l analysis of Running Gulch, a small tributary to North Clear Creek within the <br />City of Black Hawk, used an average Curve Number of 75 (Taggart Engineering, 1995). <br />An HEC-l analysis ofFounnile and Silver Gulches used Curve Numbers between 73 and <br />76 (Carroll & Lange, 2000). <br /> <br />HYDROLOGIC ANALYSIS <br /> <br />The reconstituted hydrologic analysis of North Clear Creek, Gregory Gulch, and Chase <br />Gulch uses the TR20 hydrologic model, Version 2.04, developed by the SCS (SCS, <br />1992). The model uses information on precipitation, soils, land use, geometric <br />parameters of the watershed, and hydraulic structures to estimate flood hydrographs and <br />flood peak discharges, <br /> <br />Point precipitation estimates for the 24-hour storm were obtained from the NOAA Atlas <br />,fiMiller et aL, 1977). Point precipitation values were obtained using the May through <br />October maps, which represent non-snowfall events. Point precipitation values for the <br />24-hour 100-, 50-, and lO-year storms are 3,3, 3.0, and 2.2 inches, respectively. <br />Precipitation distribution used standard distributions available internally within the TR20 <br />modeL The precipitation distribution was selected using trial runs with the preliminary <br />modeL "Best fits" to the 1979 flood peaks were found using the SCS 24-hour "Type LA" <br />distribution for North Clear Creek and the SCS 24-hour "Type II" distribution for <br />Gregory and Chase Gulches. These distributions are presented in the Appendix to this <br />report. <br /> <br />Soils have only been mapped in the vicinity of the City in the study area, The soils map <br />in this area is presented on Figure 2. The map indicates the majority of the soils in the <br />mapped area are Hydrologic Soil Group "D". It was assumed for this study all soils in <br />the study area are Hydrologic Soil Group "D". <br /> <br />Land use in the study area is defined using land use information from 1979. A USGS <br />topographic map and aerial photography used for the 1979 analysis were obtained from <br />the eWCR The map delineated areas of high and low density forest based on the density <br />of shading on the aerial photographs. This shading density on the aerial photographs <br />closely matched the shading density on the USGS map. Land use was subdivided into <br />high density forest, low density forest, meadow, high density residential, low density <br />residential, and commerciaL This information was compiled and is shown on Figure 3, <br /> <br />The study area was divided into catclunents shown on Figure 4. Catclunents were <br />numbered following requirements in TR20, Design points are defined at various points <br />in the study area. Design points were placed at major confluences and at 'detention <br />storage areas. Time of concentrations were determined for each catchment for overland, <br />shallow, and channelized flows, following methods described in TR-55 (SCS, 1986). <br />Chromel flow velocities represent sub-critical to critical conditions following <br /> <br />-4- <br />