Laserfiche WebLink
Subbasin RB18 will be predominately grassland with an area of 1 acre. The imperviousness of the <br />subbasin is 0 percent. The runoff coefficient was calculated to be 0.500 for the 100-year storm. A <br />time of concentration of 26 minutes would result in a 100-year rainfall intensity of 4.74 inches per <br />hour. Applying the rational method produced a peak runoff of 2.6 cfs. Subbasin RB18 would be <br />captured by Pond 4C and would no longer contribute runoff to DP6. <br />Subbasin RB10 will be predominately grassland with an area of 30 acres. The imperviousness of <br />the subbasin is 0 percent. The runoff coefficient was calculated to be 0.283 for the 100-year storm. <br />A time of concentration of 205 minutes would result in a 100-year rainfall intensity of 1.16 inches <br />per hour. Applying the rational method produced a peak runoff of 9.7 cfs. Subbasin RB 10 returns <br />to the Cache la Poudre River as overland flow and via the borrow ditch along WCR 25 at DP6. <br />Elimination of Subbasin RB8, Subbasin RB9, and Subbasin RB18 runoff from contributing to DP6 <br />is the major cause of the reduction from the historical peak flow rate of 24.7 cfs down to the <br />reclaimed peak flow rate of 9.7 cfs. <br />Subbasin RB11 will be entirely grassland with an area of 22 acres. The imperviousness of the <br />subbasin is 0 percent. The runoff coefficient was calculated to be 0.254 for the 100-year storm. A <br />time of concentration of 93 minutes would result in a 100-year rainfall intensity of 2.07 inches per <br />hour. Applying the rational method produced a peak runoff of 11.4 cfs. Subbasin RB11 returns to <br />the Cache la Poudre River as overland flow at DP7. <br />Subbasin RB12 is predominately open grassland and cropland with an area of 97 acres. The <br />imperviousness of the subbasin is 2 percent. The runoff coefficient was calculated to be 0.459 for <br />the 100-year storm. A time of concentration of 62 minutes would result in a 100-year rainfall <br />intensity of 2.75 inches per hour. Applying the rational method produces a peak runoff of 122.1 cfs. <br />Subbasin RB12 collects in a series of gullies which open up to overland flow which returns to the <br />Cache la Poudre River as such at DP4. <br />Subbasin RB13 is predominately open grassland with an area of 131 acres. The imperviousness of <br />the subbasin is 2 percent. The runoff coefficient was calculated to be 0.394 for the 100-year storm. <br />A time of concentration of 74 minutes would result in a 100-year rainfall intensity of 2.43 inches <br />per hour. Applying the rational method produced a peak runoff of 125.8 cfs. Subbasin RB 13 <br />collects in a series of gullies which open up into a lake or overland flow which eventually returns to <br />the Cache la Poudre River via an ancient meander of the river which now serves as a slough at DP3. <br />Subbasin RB14 will be predominately grassland with an area of 63 acres. The imperviousness of <br />the subbasin is 0 percent. The runoff coefficient was calculated to be 0.364 for the 100-year storm. <br />A time of concentration of 41 minutes would result in a 100-year rainfall intensity of 3.60 inches <br />per hour. Applying the rational method produced a peak runoff of 82.0 cfs. Subbasin RB 14 would <br />be captured by Pond 5C and would no longer contribute runoff to DP3. Elimination of Subbasin <br />RB14 runoff from contributing to DP3 is the major cause of the reduction from the historical peak <br />flow rate of 283.0 cfs down to the reclaimed peak flow rate of 125.8 cfs. <br />Subbasin RB15 will be predominately grassland with an area of 3 acres. The imperviousness of the <br />subbasin is 0 percent. The runoff coefficient was calculated to be 0.480 for the 100-yr storm. A <br />time of concentration of 70 minutes would result in a 100-year rainfall intensity of 2.53 inches per <br />- 11 - Rcviwd Preliminary Drainage Report.docx