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Ms. Kate Dreu n- February 7, 1996 page 2 The existing mall building will continue to discharge roof runoff directly to a storm sewer along the ex,tension of 39th Avenue N.E. The balance of the site(approximately 25.5 acres)will continue with the existing drainage pattern towards the south where it is conveyed by storm sewer to Silver Lake Road. As outlined in the project EAW,the remaining portions of the site will be treated at the time of redevelopment of those areas. Site hydrology was evaluated using the Soil Conservation Service TR-55 computer model. Because the land use remains essentially unchanged,the proposed redevelopment will not appreciably affect peak rate or volumes of runoff. Modifications to the interior storm sewer routing and the development of on- site ponding will attenuate the actual 100-yr peak discharge rate from the site from an existing value of 106 cfs to 40 cfs. WATER QUALITY Treatment of runoff for suspended solids and nutrient removal will be accomplished by routing all internal storm sewer through wet detention basins. The"dead storage"volumes of these basins were designed using the methodologies of William Walker,Ph.D.and are predicated on the standards developed in the Nation-wide Urban Runoff Program(NURP). These ponds are equipped with outlet control structures consisting of baffled weirs to control discharge velocities and contain floatable debris. The total treated watershed area following this redevelopment project will be 25.3 acres. • MAINTENANCE Maintenance responsibility of all pond areas and connecting storm sewer outlets will be conveyed to the City of St.Anthony Village via protective easements or fee title as depicted on the preliminary plat. This will ensure public authority to maintain these facilities in the design condition. EROSION AND SEDUAENT CONTROL A construction Erosion Control and Sediment Control Plan has been developed for the construction of the proposed improvements. This plan calls for the installation of silt fences and construction of the detention basins prior to beginning site grading. Turf establishment and storm sewer inlet protection measures are also included. If Watershed staff or members of the Board have any questions or need additional information,please call me at 937-5150. I will also be in attendance at the February meeting to discuss the project at the Board's pleasure. Sincerely, wood Professional ' es,Inc. rice G.Miller,P.E. Principal Engineer attachments