08/12/2020 P&Z Packet - Laserfiche WebLink

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Ms. Katie Larsen 8/6/2020 Page 2 The surface water will flow to a new stormwater basin. Storm sewer is proposed to collect water from the parking lot and private road and then direct it via overland flow to the proposed Basin 1 P. Basin 1 P is designed to discharge southwest to the existing wetland, Wetland A. Basin 1 P is designed to meet City requirements for water quality, volume control and rate control. After the project is completed, the site will include approximately 1.3 acres of new impervious surfaces including the streets, paths, parking lots, and rooftops. Groundwater was encountered at depths ranging from 3 to 9 feet below the ground surface. Approximately 1" to 1.5" of topsoil was encountered. Below the topsoil, soils consist of native glacial till with lean clay, sandy lean clay, clayey sand, silty sand, and poorly graded sand with silt. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer conveyance system and basin. The site is designed to reduce site runoff rates for the 2-, 10-, and 100-year rainfall events. Pre- and Post- Development Discharge Rates (cfs) 2-Year 10-Year 100-Year Condition North South North South North South Pre- 0.32 2.72 0.70 6.62 1.65 12.18 Development Post- 0.27 2.38 0.40 6.00 0.69 11.86 Development Post -development stormwater discharge rates are less than the pre -development rates. Comments: 1. The City does not allow 12-inch diameter storm sewer as part of the public system due to maintenance concerns. Applicant shall update pipes within Outlot D to a minimum 15-inch diameter pipe. 2. Basin 1 P is assumed to be temporary based on the proposed location and concept plans for this area. What is the applicant's ultimate stormwater management plan for this site? • Water Supply There is existing 16-inch diameter ductile iron pipe (DIP) watermain along Otter Lake Road east of the site. There is an existing 8-inch diameter DIP watermain stub at the intersection of Otter Lake Road and the existing private road. The City of Lino Lakes Comprehensive Plan does not designate any trunk watermain going through this site. The applicant is proposing an 8-inch diameter PVC watermain to serve the development. The watermain will extend from the east (Otter Lake Road) along Street A (the private road) and dead end at the end of the proposed private road. A 6-inch diameter PVC watermain service is proposed to service the building. Comments: Otter Crossing Engineering Review