Agenda Packets - 2021/07/06 - Laserfiche WebLink

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s A. Tributary Draina ae Area The tributary drainage area to the Oak Lake Park ponding area includes the Kraus Anderson development, a major portion of the Scottland Green Apartments complex, and lands to the west of Silver Lake Road and north of County Road I that are presently tributary to the wetlands. Not included are the ONiell property to the east, the Mounds View City Hall area, and the shopping center at County Road I and Highway 8. The attached figure shows the limits of the drainage area that is considered as tributary to the ponding area. The total area involved is 203 acres. In terms of land use, the breakdown of the tributary drainage area is as follows: Multiple Residential (Apt.) 18.0 A Single Family Residential 19.8 Park (includes pond) 53.7 Multiple Residential + Comm. 45.5 Single Family and Commercial 66.0 Total 203.0 A. At final development, the amount of impervious surface can be calculated as follows: Acres % Impervious Acres Imn. Acres Perv. Multiple Apt. 18.0 60 10.8 7.2 Single Family 19.8 35 • 6.9 12.9 Park 36.7 10 3.7 33.0 Lake 17.0 (100) 17.0 0.0 Mult. Res. and Comm. 45.5 60 27.3 18.2 SFR and Comm. 66.0 50 33.0 33.0 Totals 98.7 104.3 Total Percentage 49% 51% B. Derivation of the Unit Hydroaraph Method used in calculating the unit hydrograph is from the "Denver Method" L = 5400/5280 = 1.03 Lca = 0.57 Ct = 0.45 Cp = 0.40 tp = 0.45(1.03x0.57) = 0.38 hrs. = 23 min. qp = 0.4(640)/0.38 = 674 cfs. Qp = 674(203)/640 = 214 cfs Time to peak = 23 + 15 = 38 min. Vol for 1" runoff = 203/12 = 16.91 AF. Hydrograph width @ 75 percent = 23 min, @ 50 percent = 40 min.