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2.0 DRAINAGE BASIN DATA 2.1 FLOODPLAIN Technical Memorandum The Sandy Acres Lake project lies within a FEMA designated Zone X, indicating an area of minimal flood hazard. Refer to the HhMA hirmette attached. 2.2 MAJOR DRAINAGE BASIN DESCRIPTION In August of 2004. Kio%%a Engineering prepared the .Second Creek (Downstream of DL4) and DFA 0053 Ougbll Systems Planning Study Update. The study identifies Sandy Acres Lake as lying within the Direct Flow Area (DFA) #0053 watershed (see attached Basin Map from the OSP). The watershed is an cast - bank tributaq- to the South Platte River. The watershed vegetation consists mostly of native rangeland grasses and is fairly flat with slopes ranging from 0.1 to 1.5 percent. Due to the flatness of the area. there are very few defined drainage paths within the watershed. One of the main outfalls to the South Platte River identified in the studv is Fox Run. The Outfall Svstems Plan shows a proposed flow path for Fox Run that enters the project site on the northeast via a 36 -inch diameter outfall pipe from a proposed detention pond to the east. Flows would pass directly through the site and exit from the northwest comer via a 36- diameter pipe ultimately discharging flows into the South Platte River. See attached Master Plan from the OSP. 2.3 EXISTING SUB -BASIN DESCRIPTION Currently. the project site is a lake bisected to the north and south by an access road. The lake contains all direct site runoff in addition to small amounts of piped flow from small diameter culverts that drain the roadside ditches and in -field areas immediately adjacent to the site. There is no outlet from the lake at this time. The overflow path from the site is love point in the northwest comer at approximately elevation 5020.0 feet. 3.0 STORM DRAINAGE DESIGN A proposed 118 -foot long. 6'x4' box cuh crt is to be installed for hauling fill material into the site and is not intended to act as a drainage facility. The fill material will be placed to an elevation of 5006.5 feet and the box culvert will be set with an invert elevation of 5022.9 fed on the cast (lake) side of the culvert. The culvert will drain to the west with a slope of 1.75 percent for positive drainage from the facility should water ever reach it. There is no proposed pavement as part of the project that will increase the percent imperviousness of the site. Since there are no major drainage flows entering or exiting the site. the infill activities will have little to no effect on the existing drainage patterns. Runoff will pond on the surface and will either infiltrate into the fill soils or evaporate. 4.0 CONCLUSION In general. the proposed improvements will match the existing conditions and there are no anticipated major changes to the existing flows and drainage patterns. Additionally. these improvements will not preclude the future impletion of the Outfall Systems Plan described above. Enclosures cc: Neil Thomson. E -470 Craig Friesen. URS Page2