Laserfiche WebLink
1 <br /> <br />t <br />1 <br /> <br />1 <br /> <br /> <br />r <br /> <br />1 <br />Historically the majority of the storm drainage generated on the Site would likely flow in to the <br />existing drainage channel used to transport off-site flows from north of HWY 392 to the Poudre <br />River. All other storm drainage is likely to sheet flow to shallow undefined drainage paths and <br />in to the Poudre River. The peak runoff for this site is 7.6 cubic feet per second (cfs) and 64.1 <br />cfs for the 2-yr and 100-yr storms, respectively. <br />PROPOSED DRAINAGE <br />Criteria <br />Because there will be no increase in imperviousness and all flows within the mining area will be <br />retained within the pit, no proposed basin calculations were completed. Runoff calculations for <br />the historic drainage basins were done using the Rational Method in accordance with The Town <br />of Windsor drainage guidelines for basins smaller than 160 acres. <br />Open Channel Flow <br />A typical drainage channel section was calculated to carry the estimated storm flow entering the <br />north side of the Site through the 36" RCP under HWY 392. Required capacity of the channel <br />was estimated by calculating the full flow capacity of a 36" RCP culvert at a 2% slope. One half <br />foot of freeboard for the channel was used as a factor of safety because of the lack of design <br />information for the existing 36" RCP. Calculations have been included in the appendix of this <br />report. The channel will be routed along the western property line to the Poudre River. In order <br />to reduce the chances of erosion within the channel it has been recommended that the channel be <br />planted with the grass mixture below, which was selected to be long lasting and regenerating. <br />The seeding and fertilizers will be applied according to recommendations from the Natural <br />Resources Conservation Service. Based on the soils in the area, the following upland seed <br />mixture is recommended: <br />Upland Grass Seed AreaCover <br />(/o) Application Rate <br />(#PLS/acre) <br />Vaughn Side Oats Grama 30% 1.35#PLS/ac <br />Grenville Switchgrass 20% 0.5 #PLS/ac <br />Lovington Blue Grama 20% 0.3 #PLS/ac <br />Barton Western Wheatgrass 30% 2.4 #PLS/ac <br />Application rate is for drilling the seed. If seed is to be broadcast, the <br />application rate will be doubled. <br />CONCLUSIONS <br />It is the intent of this report to provide a stormwater drainage analysis for the Site in accordance <br />with the Town of Windsor criteria and to minimize any negative impacts on downstream <br />properties. If the Site is developed in the manner specified in this report, the mining and <br />reclamation activities will not alter the historic flow patterns and will not increase the historic <br />flow quantities, and in fact will reduce historic flow patterns for downstream properties. The <br />proposed improvements and stormwater management methods that will be used will insure that <br />historic runoff patterns and quantities will be maintained. <br />Final Drainage Plan and Report - SerFer Pit Applegate <br />Windsor, CO 4 r Group, Inc. <br />January 2008