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01/02/96 17: 16 ^p612 937 5522 1;ESTVOOD PRO SN"S X013 Page 11 DRAFT—Silver Lake Center EAW January 2, 1996 Existing Conditions The existing site is a commerciaVretali center that Is predominantly Impervious paved surfaces draining directly Into the urban storm sewer system. The 50.6 acre center discharges runoff via three privately operated storm sewer lines that connect to the public trunk storm sewer under Silver Lake Road. This stormwater ultimately discharges to Silver Lake. The water quality In Sliver Lake has been an Issue of local concern. In 1989,the Ramsey County Department of Public Works presented a Report to the Rice Creek Watershed District on the Water ouallty of Silver Lake in Columbia Heights and St.Anthony. The report characterized Silver Lake as having relatively high concentration of plant nutrients such as phosphorus and nitrogen and that urban stormwater runoff was a significant contributor to the nutrient loading for this water body. The report states that runoff from Apache Plaza and adjacent land contributes 20 percent of the phosphorus load to Silver Lake. in a memorandum dated November 28,1994, Barr Engineering concluded that the actual shopping center property contributes approximately 14 percent of the total phosphorus loading to Silver Lake. Effects of the Project In preparation of this EAW,pre-and post-project modeling of water quality and quantity was performed to evaluate anticlpated effects..The results of these efforts are summarized below. Table 5 summarizes the stormwater runoff rates from the project area under existing,Initial(Phase 1),and ultimate project conditions. The volume of proposed on-site stormwater detention is also indicated In this table. The results Indicate that the project will reduce peak runoff rates from the existing conditions for both the Interim and ultimate redevelopment proposal. Table 5: Estimated stormwater runoff rates from the project area under existing, Initial(Phase 1),and ultimate project conditions Flow Rates Pond Volume Watershed Area 10•yr Peak 100-yr Peak 10yr pond 100yr pond ad: :,cis cfa acft acft MOM � .. DA1 .12.9 . 30 . 50 na na DA2 3.8 11 17 na na DA3 5.1 19 30 na na DA4(roof) 10.2. 50 72 na na DA5 21.4 63 100 na na u DA1 10.1 z 20 w_ 35 0.6 1.0 DA2 6.4- 17. 28 0.4 0.6 DA3 8.3. - 21 34 0.2 0.6 DA4(roof) 7.84 38 55 na na DA5 21.4 63 100 na na DA1 M , 7.9. 16_ ~22 0.4 0.8 DA2 6.7 17 30 0.4 0.6 DA3 8.2` 21 1 34 0.2 0.6 DA4 31.2. 60 85 1.6 2.9 Phase 1 of the project Incorporates three water quality basins and would treat 24.8 acres of the total 54-acre drainage area.these stormwater basins would Include both permanent pool and"live" Westwood Professional Services,Inc. W