07/08/2020 P&Z Packet - Laserfiche WebLink

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Ms. Katie Larsen 7/012020 Page 3 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. Groundwater was encountered at depths ranging from 4 to 19 feet. Topsoil was encountered at to 1 foot. Below the topsoil, soils consist of glacial outwash sands and native clayey glacial till and are classified as hydrologic groups HSG D in the southern portion and HSG C in the north. 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. Pre- and Post- Development Discharge Rates (cfs) Condition 2-Year 100-Year Pre -Development 189 19.2 46.446.7 103.4103.3 Post -Development 12 1 9.8 41 732.2 91.Q 87.0 Post -development stormwater discharge rates are less than the pre -development rates. 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 100. Pond 100 will pick up surface water from the northwest portion 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. o The design includes a standard outlet control structure with pipe outlet. An outlet control structure is a device that holds water back in the proposed pond and then discharges it slowly into the ditch so that the flow to the downstream ditch is less than the existing condition flow into the ditch. Adjacent property owners expressed concerns over the stormwater runoff from the site and the downstream private ditches. Staff had multiple conversations and Zoom meetings with the property owners to better understand their concerns and explain the existing and proposed conditions. This included an onsite evaluation, reviewing topographic data, record plans, survey, and incorporation of the NorthPointe development drainage plan. The downstream systems appear to be working correctly and the NorthPointe development has reduced the surface water flow to the subwatershed in question. We will continue to work the property owners and provide information and findings. 0 2019 was a record year for rainwater in Minneapolis -St. Paul area according to the Minnesota DNR Year to Date Precipitation Chart. Soil saturation in the fall of 2019 was similar to a spring snow melt condition where typically the fall is a dryer condition. These conditions have led to standing water due to soil saturation and higher flows in conveyance systems. 0 We are working with one of the downstream property owner to complete culvert maintenance on their property. In addition, we are having the developer verify the downstream conditions to confirm their assumptions. Nadeau Acres Engineering Review