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<br />. VII. ENGINEERING ANALYSES <br /> <br />Hydrolol!Y <br />Design drawings and technical backup data for the existing storm drain are unavailable, <br />Therefore, the peak flows used for this study were estimated using the Rational Method. <br />The 10-year and 100-year peak flows for the existing storm drain outfall at Holita ditch <br />near Russell A venue were computed for the purposes of this study, The Rational Method <br />is an acceptable methodology for estimating runoff from small basins in developed areas, <br />The equation for the Rational Method is: <br /> <br />Q=CIA <br /> <br />Where: <br /> <br />Q = peak discharge (cfs) <br />C = runoff coefficient (varies with land use) <br />I = rainfall intensity (inches per hour) for the specific frequency of interest for a <br />specific time duration which is equal to the time of concentration (Ie) of the <br />watershed <br />A = area of watershed (acres) <br /> <br />. <br /> <br />The watershed area for the existing storm drain was planimetered to be 105 acres. The <br />runoff coefficients for the 10-year and 100-year storms were estimated to be 0.70 and <br />0,80 based on mixed residential and commercial development of the watershed. The time <br />of concentration for the watershed was estimated using an equation published in the <br />Urban Drainage and Flood Control District's storm drainage criteria manual. The time of <br />concentration was computed using tc = (UI80) + 10, where L is the length of the flow <br />path (in feet) from the hydrologic point of interest to the most distant point in the <br />watershed, For this study, L was measured to be 3,000 feet, and thus tc was computed to <br />be approximately 27 minutes, Finally, the rainfall intensity was estimated to be 2,8 inlhr <br />for the 10-year event and 4,5 inlhr for the 100-year event. The hydrologic parameters for <br />the IO-year and 100-year events are shown in the table below, <br /> <br />Watershed Parameter to.year Eyent tOO. year Event <br />Peak Flow (cfs) 205 380 <br />Area (acres) 105 105 <br />Runoff Coefficient 0,70 0,80 <br />Rainfall Intensity (inlhr) 2,8 4,5 <br />Time of Concentration (min) 27 27 <br /> <br />. <br /> <br />In addition to the flows contributed by the storm drain, the NRCS estimated additional <br />flow amounts that would enter the Holita Ditch between Russell Avenue and Interstate <br />25, The additional flows were estimated to be 230 cfs and 430 cfs for the IO-year and <br />lOO-year events, respectively. <br /> <br />6 <br />