02-10-1997 Council Packet - Laserfiche WebLink

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Cosiderable efforts have been made by the Minnehaha Creek Watershed District to improve the water quality of Long Lake, with over $2.2 million spent in an effort in recent years. During the design of the course, special attention has been, and wilt continue to be, given to improving runoff quality from the present condition. The nutrient budget analysis indicates significantly reduced levela from the existing condition (see Table 2). The stormwater treatment ponds are intended to be constructed to treat the runoff from the golf course. The runoff through the wetland complexes to Long Lake is routed through wet detention ponds prior to discharge to Long Lake. The irrigation pond in the center drainage will collect runoff for reuse as im'gation, thereby further reducing potential nutrient loading and reducing im'gation needs. Since there is little impervious area and the flow must pass across well vegetated grassy areas, it is anticipated that the golf course will not be a significant loading source. Water quality and quantity control will be provided by three NURP basins designed for both on-site and off-site drainage areas. The NURP basins are located along Spring Hill Road on the upstream end of the existing culverts. Outlet structures are placed at the existing culverts to control the flow rates. The ponds are sized to meet Minnehaha Creek Watershed District and City requirements for NURP basin design. The drainage was analyzed using stormwater modeling software based on Soil Conservation Service methodologies. The modeling was set up to route 100-year, 10- year and 1-year design storms through the existing wetlands, mitigation wetlands, NURP basins and road culverts. The proposed construction of the fairways, greens and tees will increase the volume of runoff by 120% to 175% over the existing conditions. By using the detention storage available in the wetlands, mitigation areas and NURP basins, the additional volume is controlled and the proposed runoff rates leaving the Site are reduced to below the existing rates. The existing and proposed runoff rates at the three major culvert crossings on Spring Hilt Road are presented in Table 1. TABLE 1 RUNOFF RATES TO LONG LAKE SPRING HILL RD CULVERT EXISTING CONDITIONS PROPOSED CONDITIONS 100 Yr.10 Yr.1 Yr.100 Yr.10 Yr.1 Yr. 15" CMP (NURP Basin 1)23 cfs 9 cfs 0 cfs 4 cfs 2 cfs 0.2 cfs 36" RCP (NURP Basin 2)23 cfs 11 cfs 1.4 cfs 16.5 cfs 11.5 cfs 0.4 cfs 42" RCP (NURP Basin 3)57 cfs 28 cfs 3 cfs 29 cfs 6.5 cfs 0.9 cfs The roadway culvert crossings on County Road 6 arjd Spring Hill Road were also included in the storm water flow rate modeling. Based on the results of the flow rating, these culverts are capable of handling up to 100-year design storms without overlopping the roads. The proposed golf course construction did not increase the flow rates to these culverts.