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minutes would result in a 100-year rainfall intensity of 1.32 inches per hour. Applying the rational <br />method produced a peak runoff of 24.7 cfs. Subbasin H6 returns to the Cache la Poudre River as <br />overland flow and a borrow ditch along WCR 25. <br />Subbasin H7 is entirely grassland with an area of 22 acres and an imperviousness of 0 percent. The <br />runoff coefficient was calculated to be 0.254 for the 100-year storm. A time of concentration of 93 <br />minutes would result in a 100-year rainfall intensity of 2.07 inches per hour. Applying the rational <br />method produced a peak runoff of 11.4 cfs. Subbasin H7 returns to the Cache la Poudre River as <br />overland flow. <br />Subbasin H8 is entirely cropland and grassland and has an area of 85 acres and an imperviousness <br />of 0 percent. The runoff coefficient was calculated to be 0.365 for the 100-year storm. A time of <br />concentration of 114 minutes would result in a 100-year rainfall intensity of 1.79 inches per hour. <br />Applying the rational method produced a peak runoff of 55.4 cfs. Subbasin H8 returns to the Cache <br />la Poudre River as overland flow. <br />Subbasin H9 is entirely cropland and grassland with an area of 25 acres and an imperviousness of 0 <br />percent. The runoff coefficient was calculated to be 0.416 for the 100-year storm. A time of <br />concentration of 51 minutes would result in a 100-year rainfall intensity of 3.13 inches per hour. <br />Applying the rational method produced a peak runoff of 32.5 cfs. Subbasin H9 passes overland <br />flow drainage to the neighboring property to the east. <br />Phase 1 Mining Conditions <br />Phase 1 of the mining plan represents the initial phase of five planned phases. The processing plant <br />and the silt storage pond created in Phase 1 will remain for all later phases of mining. The use of <br />temporary topsoil and overburden stockpiles around the perimeter of the mining area in Phase 1 is <br />typical of the procedure used in later phases. Mined regions are reclaimed using the temporary <br />topsoil and overburden stockpiles at the end of each phase. Conveyors across WCR 25 and the <br />Cache la Poudre River will be used to move mined materials to the processing plant. Lafarge <br />anticipates mining and reclaiming the site in approximately 20 years; however, the rate of mining <br />and overall life of the mine is dependent upon demand and market conditions. <br />Subbasin P1-1 has an area of 11 acres. The imperviousness of the subbasin is 15 percent since the <br />subbasin will be under mining operations. Based on the runoff equation in the USDCM, the runoff <br />coefficient was calculated to be 0.435 for the 100-year storm. A time of concentration of 212 <br />minutes would result in a 100-year rainfall intensity of 1.13 inches per hour. Applying the rational <br />method produced a peak runoff of 6.2 cfs. Subbasin P1-1 collects in the borrow ditch along WCR <br />25 and discharges to the Cache la Poudre River via the drainage channel on the southern end of the <br />property and contributes flow to DP8. <br />Subbasin P1-2 has an area of 2 acres. The imperviousness of the subbasin is 15 percent since the <br />subbasin will be under mining operations. The runoff coefficient was calculated to be 0.435 for the <br />100-year storm. A time of concentration of 51 minutes would result in a 100-year rainfall intensity <br />of 3.13 inches per hour. Applying the rational method produced a peak runoff of 2.3 cfs. Subbasin <br />P1-2 collects in a dewatering trench and is routed east to discharge off the property near where <br />historical basin H9 had overland flow discharges and contributes flow to DP9. <br />-7- Revised Pw1imi=y Dminage Repott.docx