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HYDROLOGIC RESULTS <br />Existina Conditions <br />The CUHP2005 computer program was used to model subbasins Hl and H3. These basins exceed <br />the 160-acre limitation of the rational method. Design storm 1-hour depths of 1.04, 1.49, 1.76, 2.5 1, <br />and 2.78 inches were entered for the 2, 5, 10, 50 and 100-year storm events, respectively. <br />Maximum depression storage depths and infiltration rates were based on SCS soil type and land <br />use. These values are available in the City of Greeley Storm Drainage Design Criteria and <br />Construction Specification document. Peak runoff values from all subbasins with drainage areas <br />less than 160 acres were calculated using the rational method. All CUHP and rational method <br />calculations are provided in Appendix B. <br />Subbain Hl has an area of 194 acres. The imperviousness of the subbasin is 2 percent per the <br />historical flow analysis entry in Table RO-3 of the Urban Storm Drainage Criteria Manual <br />(USDCM) since the subbasin is predominately cropland. CUHP calculated a 100-year peak runoff <br />of 203 cfs occurring at a time of concentration of 24.3 minutes. Subbasin H1 returns to the Cache la <br />Poudre River through overland flow. <br />Subbasin H2 is predominately cropland with an area of 159 acres and an imperviousness of 2 <br />percent. Based on the runoff equation in the USDCM, the runoff coefficient was calculated to be <br />0.400 for the 100-year storm. A time of concentration of 162 minutes would result in a 100-year <br />rainfall intensity of 1.39 inches per hour. Applying the rational method produced a peak runoff of <br />87.9 cfs. Subbasin H2 collects in a slough and returns to the Cache la Poudre River via a deep cut <br />channel. <br />Subbasin H3 is predominately open grassland with an area of 196 acres and an imperviousness of 2 <br />percent. CUHP calculated a 100-year peak runoff of 283 cfs occurring at a time of concentration of <br />18.1 minutes. Subbasin H3 collects in a series of gullies which open up into a lake or overland flow <br />which eventually returns to the Cache la Poudre River via an ancient meander of the river which <br />now serves as a slough. <br />Subbasin H4 is predominately open grassland and cropland with an area of 97 acres and an <br />imperviousness of 2 percent. The runoff coefficient was calculated to be 0.459 for the 100-year <br />storm. A time of concentration of 62 minutes would result in a 100-year rainfall intensity of 2.75 <br />inches per hour. Applying the rational method produced a peak runoff of 122.1 cfs. Subbasin H4 <br />collects in a series of gullies which open up to overland flow which returns to the Cache la Poudre <br />River as such. <br />Subbasin H5 is entirely cropland with an area of 28 acres and an imperviousness of 0 percent. The <br />runoff coefficient was calculated to be 0.463 for the 100-year storm. A time of concentration of 139 <br />minutes would result in a 100-year rainfall intensity of 1.55 inches per hour. Applying the rational <br />method produced a peak runoff of 20.2 cfs. Subbasin H5 returns to the Cache la Poudre River as <br />overland flow. <br />Subbasin H6 is entirely grassland with an area of 57 acres and an imperviousness of 0 percent. The <br />runoff coefficient was calculated to be 0.329 for the 100-year storm. A time of concentration of 173 <br />-6- Revised Preliminary Dminage Report.docx