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2 0 U U` Z W m U) 0 00 z �i J 0 a a W Z Z 2 0 0 M W U) T W Z) z W a a z W x 0 Memorandum To: Diane Hankee, PE From: Madison Rogers Date: April 1, 2020 Re: Preliminary Plat amendment Lyngblomsten Stormwater Review WSB Project No. 012365-000 WSB has reviewed the preliminary plat amendment for the Lyngblomtsten Senior Communities development for compliance with the City's stormwater management ordinance. There are four main stormwater management systems proposed with the project: • Storm sewer system for surface water to drain into the proposed BMPs • Groundwater lift station to address high groundwater on the site • Stormwater lift station to address surface flow into the underground parking garage area • Draintile gravity system to alleviate groundwater around the proposed main building Storm Sewer System The storm sewer system will drain surface water from storm events and drains into two BMPs, a dry basin (4P) and a stormwater pond (3P). The project proposes approximately 12.5 acres of impervious surface, and the proposed stormwater pond dead pool volume exceeds the required treatment volume for the site. Additionally, the hydroCAD models submitted show the runoff rates for the site meet existing rates, which meets the City requirements. Groundwater Lift Station The proposed lift station plans will be developed by Braun for the final plat plans and report. The flow values from the proposed lift station will not be regulated by the City's stormwater management rate control regulations. However, the final plat must show there are no adverse impacts to the proposed site as a result of the proposed groundwater levels and discharge rate from the pumped system into the stormwater system. Stormwater Lift Station and Holding Tank There are approximately 0.6 acres of area proposed to be drained into the underground parking garage. The drainage will be captured by a trench drain at a low point in the garage entrance, and then the water will be stored in a holding tank and pumped into the proposed storm sewer system and ultimately into the proposed stormwater pond. The HydroCAD model submitted by Andesron Engineering shows the 5 proposed holding tanks that have a capacity of 2,000 gallons each. Total they provide the volume capacity to hold the 100-year storm event in the case of pump failure. Draintile Gravity System The draintile gravity system is proposed to encompass the entire building perimeter as shown on C1.42. This system is proposed to outlet into the existing ditch system,and will not be routed through the storm sewer. Our understanding is that this discharge location is to prevent water levels in the pond from backing up into the draintile system. KA012365-000\WR\Stormwater Plan Review\012365-000 MEMO PrelimPlat Amend StormwaterReview 04012020.docx