05-11-2020 Council Packet - Laserfiche WebLink

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8 A Storm Water Management Plan has been prepared by a licensed, professional engineer and reviewed for compliance by licensed, professional engineers from both WSB & Associates (City) and Houston Engineering (RCWD). The revised plan increases the size of Pond 100 and includes a plunge pool energy dissipator for additional stormwater management. Per the revised March 20, 2020 Stormwater Management Plan prepared by Carlson McCain: “The stormwater on the site flows to three different locations from the site. The majority of the site drains to an existing private ditch/wetland area to the north of the site. A small portion at the south of the site drains through an existing culvert along Birch St (CSAH 34). The western portion of the site drains to the northwest to an existing private ditch. The surface water will flow through three new ponds. Pond 200 located in the middle of the site will pick up surface water from the south portion of the site and direct it towards Pond 100. Pond 100 will pick up surface water from the majority of the site and discharge to the west to an existing private ditch. Pond 400 will receive the remaining surface water and discharge to an existing ditch to the north. After the project is completed, the site will include approximately 8.8 acres of new impervious surfaces including the streets, sidewalks, paths, parking lots, and rooftops. Due to poor soils in the south portion of the site and a high groundwater table in the north portion of the site, infiltration is not feasible. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer conveyance system and ponds. The site is designed to reduce site runoff rates for the 2-, 10-, and 100-year rainfall events.” Per the April 27, 2020 WSB City Engineer’s Review Memo: “During the public hearing held at the Planning and Zoning Board meeting on February 12, 2020 there were a number of questions regarding the proposed developments’ stormwater management plan. Below is a summary of findings and actions to address the questions: • The site outlets to two private ditches, one in the northwest, and one to the north. There is also a small area of the site that drains to the south. • The proposed project is required to mitigate surface water and address water quality through City, Watershed District and State Rules and Statues. This includes maintaining the existing rate or reducing the rate of runoff from the site, and providing water quality treatment for the proposed development. The project meets the City of Lino Lakes’ stormwater management requirements and RCWD rules. RCWD approved a CAPROC on April 8, 2020. o This project is proposing to reduce rates as detailed in the Stormwater Report, and will utilize stormwater ponding for water quality treatment. The stormwater management plan incorporates existing soils and time of concentrations to calculate the needed ponding systems to mitigate the new impervious surfaces. o The surface water from the new impervious surfaces will be accommodated through three new ponds. Pond 200, located in the middle of the site, will pick up surface water from the south portion of the site and direct it towards Pond