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. t . I >> j) t . I I . . . . . . . . >> t t . . . . . . ..' ;) V. Drainage / Ponding from chis subbasin prior to discharging into Langton Lake. Storm sewer Facilities on the proposed Twin Lakes Parkway extension east of Arthur Street will drain into the aisting twin ponds. Storm water runoff &om Parcels 14, 15, 16 and 17 would be directed to the existing storm water ponds for treatment (this is substantially true currendy). The ponds will require to be reconf1gured slightly to allow the construction of Twin Lakes Parkway, which could be accomplished in concert with the redevelopment of parcels 16 and 17. The western portion of the site includes approximately 56 acres of land which has been divided into the northwestern and southwestern drainage basins for storm sewer netWork purposes. Currc:ncly the runoff from this subbasin drains into a series of wetlands located west of Clc:vc:land Avenue through several culvert crossings on Cleveland Avenue. As a condition for redevelopment in this basin, provisions must be made to treat storm 'O"':1[er runoff prior to its d.isc.hargc into the nanual wetlands. The most efficient means of providing the required storm water trcaunent is through the common utilization of a regional pond for all the Parcds within the redevelopment. The alternative approach of constructing sc:vc:ral smaller ponds within the subbasin does produce: a savings in storm sewer system costs due to smaller pipe sizes. However. chis alternative would require a significantly larger total land area for ponding and would produce: an overall poorer quality of water. The proposed regional ponding site is located within MnDOT right of way betwccn Cleveland Avenue and I-35W on the northern edge of the existing wetland complex. An existing 1.0 acre drainage casement located in the southwest corner of the Cencrc Poime dc:vc:lopment is also proposed to be utilized for the regional pond. This site was chosen due: to itS proximity to receiving waters for the drainage basin and it requires no funds for acquisition. A pond site in the southcasr quadrant of the intersection of County Road C-2 and Cleveland Avenue was considered but rejeCted due to the loss of approximatdy 2.0 acres of dc:vc:lopable land. In order to utilize the existing 1.0 acre easement for the Twin Lakes westem regional pond, the pond design must incorporate treatment for the runoff from approximately 16.8 acres of land within the Cencrc Pointe development. The design parameters for the regional pond assume that approximately 80% of the rcdcvdoped sites will consist of impervious surfaces such as parking lors, roadways and buildings. Based on the requirements of the Rice Creek Watershed DistriCt and current Roseville design standards. the regional pond design must mcct the requirements of the Nationwide Urban Runoff Program (NURP) for storm water rrcaunent. These: requiccments indude me foHowing design criteria: Twin LaJm PlZrJrwIZY MiUm' PlAn 33