2009-10-16_PERMIT FILE - M2009082 (13)

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Subbasin R133 will be predominately grassland with an area of 107 acres. The imperviousness of the subbasin is 2 percent. The runoff coefficient was calculated to be 0.418 for the 100-year storm. A time of concentration of 156 minutes would result in a 100-year rainfall intensity of 1.43 inches per hour. Applying the rational method produced a peak runoff of 63.5 cfs. Subbasin R133 collects in a natural slough and returns to the Cache la Poudre River via a deep cut channel at DP2. Subbasin RB4 will be predominately grassland with an area of 38 acres. The imperviousness of the subbasin is 2 percent. The runoff coefficient was calculated to be 0.362 for the 100-year storm. A time of concentration of 105 minutes would result in a 100-year rainfall intensity of 1.90 inches per hour. Applying the rational method produced a peak runoff of 26.1 cfs. Subbasin RB4 would be captured by Pond 2A and would no longer contribute runoff to DP2. Elimination of Subbasin RB4 runoff from contributing to DP2 is the major cause of the reduction from the historical peak flow rate of 87.9 cfs down to the reclaimed peak flow rate of 63.5 cfs. Subbasin RB5 will be predominately grassland with an area of 52 acres. The imperviousness of the subbasin is 0 percent. The runoff coefficient was calculated to be 0.362 for the 100-year storm. A time of concentration of 89 minutes would result in a 100-year rainfall intensity of 2.14 inches per hour. Applying the rational method produced a peak runoff of 40.5 cfs. Subbasin R135 would be captured by Pond 1 B and would no longer contribute runoff to DP8. Subbasin RB6 will be predominately grassland with an area of 28 acres. The imperviousness of the subbasin is 0 percent. The runoff coefficient was calculated to be 0.369 for the 100-year storm. A time of concentration of 85 minutes would result in a 100-year rainfall intensity of 2.21 inches per hour. Applying the rational method produced a peak runoff of 22.8 cfs. Subbasin R136 returns to the Cache la Poudre River as overland flow at DP8. Elimination of Subbasin RB5 runoff from contributing to DP8 is the major cause of the reduction from the historical peak flow rate of 55.4 cfs down to the reclaimed peak flow rate of 22.8 cfs. Subbasin RB7 will be predominately grassland with an area of 25 acres. The imperviousness of the subbasin is 0 percent. The runoff coefficient was calculated to be 0.416 for the 100-year storm. A time of concentration of 51 minutes would result in a 100-year rainfall intensity of 3.13 inches per hour. Applying the rational method produced a peak runoff of 32.5 cfs. Subbasin R137 discharges to a neighboring property as overland flow at DP9. Subbasin RB8 will be predominately grassland with an area of 3 acres. The imperviousness of the subbasin is 0 percent. The runoff coefficient was calculated to be 0.407 for the 100-year storm. A time of concentration of 50 minutes would result in a 100-year rainfall intensity of 3.17 inches per hour. Applying the rational method produced a peak runoff of 3.4 cfs. Subbasin RB8 would be captured by Pond 4A and would no longer contribute runoff to DP6. Subbasin RB9 will be predominately grassland with an area of 9 acres. The imperviousness of the subbasin is 0 percent. The runoff coefficient was calculated to be 0.353 for the 100-year storm. A time of concentration of 56 minutes would result in a 100-year rainfall intensity of 2.94 inches per hour. Applying the rational method produced a peak runoff of 9.7 cfs. Subbasin RB9 would be captured by Pond 4B and would no longer contribute runoff to DP6. -10- Revised Pmliminary Drainage Report.docx