2012-04-10_REVISION - M1981185 (43)

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The following text generally describes basin conveyance to design points. Hydrologic calculations for each sub -basin are presented in the rational method SF -3 forms in Appendix B of this report. Sub -basin A, 4.78 acres, is the largest basin and is located along the north and east boundaries of the area east of the La Plata River. This basin will be largely undisturbed excepting the proposed gravel road that lies along the eastern boundary. This open space flow is directed to a roadside swale along the east side of the proposed gravel road that outfalls to the proposed detention pond, Design Point 1. Sub -basin B1, 1.48 acres, is located along the eastern boundary of the basin and on the north side of the proposed road adjacent to the proposed base of operations site. This basin will be largely undisturbed. This open space flow will follow the historic path for the area on the upper east side. The southeast portion of the basin will flow to the proposed roadside swale along the proposed gravel road outfalling to the proposed detention pond, Design Point 1. Sub -basin B2, is 0.82 acres, is the area designated for the proposed base of operations. The proposed base will consist of gravel parking areas, minimal sidewalk and a building. The proposed area is graded to a swale that will be directed through a culvert (Design Point 3) under the proposed gravel road outfalling into the proposed roadside swale, Design Point 2. Flows are then routed to the proposed detention pond, Design Point 1. Sub -basin P, 0.95 acres, is the basin that historically outfalls to the proposed detention pond and is located at southeast portion of the boundary. The proposed gravel road delineates the eastern basin boundary. The open space flow is directed towards the pond or the proposed roadside swale that outfalls at the proposed detention pond, Design Point 1. Sub -basin OS, 0.58 acres, is located at the southwest corner of the site and consists of the helicopter pad and a small portion of the base of operations. This portion of the site will maintain the native landscaping. The runoff from this site will follow the historic path, sheet flowing to the south. Detention Runoff from the proposed site will be detained by the proposed water quality /detention pond located in Basin P, along the eastern side of the site. The pond will provide water quality and detention for the 100 -year storm. The water quality volume is calculated based on the total basin area which is considerably larger than basing it on the amount of disturbed area which is standard practice. The required water quality based upon 8.6% imperviousness was determined to be 2205 cubic -feet (0.05 acre -ft). The 100 -year pond volume was determined using the V =KA method resulting in a 100 - year volume of 4828 cubic -feet. The overall pond volume is comprised of the 100 -year volume plus the water quality volume. The total detention is 7033 cubic -feet. One foot of freeboard indicates the proposed top of pond at an elevation of 8631.0 feet. A three stage outlet structure (C -DOT Type C inlet) has been designed and is included in the construction documents. The water quality elevation was determined to be 8628.05 feet. A water quality plate will be fitted to the outlet structure with drilled holes in a single column to restrict the stormwater release to 40- hours. A weir will be notched into the water quality plate to restrict the 10 -year release rate (2.58 cfs). The top of structure will be set at the 100 -year water surface elevation, 8630.0 feet. A restrictor plate will be placed over the outlet pipe to restrict the release rates as determined by the STANDARDS which restricts flow to 1.0 cfs /acre. The pond will have a 100 -year storm release rate of 8.60 cfs. The calculations are included in Appendix C. CLC ASSOCIATES Page 3