03/08/2023 P & Z Combined Packet

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Ms. Katie Larsen 03/01/2023 Page 2 399 Elm St (Centennial Middle School) Engineering Review used to determine if further coordination with the USFWS is required. The new NLEB determination key is expected to be released in early 2023. • Grading The plans include grading for the proposed tennis courts, the proposed parking lot, and an infiltration basin. Comments: 1. (3/1/2023) Staff recommends that the swale grade on east side of proposed parking lot is increased to 2%. • Stormwater Management Stormwater from the existing tennis court area drains to the east and west of edges of the existing tennis courts. Stormwater from the existing parking lot area drains to storm sewer located in the northeast and southeast corners of the parking lot. The stormwater from both areas ultimately drains through the 399 Elm Street site through private storm sewer to the public pond. The project is proposed to rehabilitate approximately 1.89 acres of impervious surface and add new or reconstruct approximately 0.57 acres on the impervious area, according to the Stormwater Management Report. The surface water from proposed improvements will still ultimately flow to the existing pond, but the applicant is proposing to treat stormwater at the proposed parking lot site through an infiltration basin. The proposed tennis courts include a draintile system that outlets to existing private storm sewer. A geotechnical evaluation report prepared by Braun Intertec was submitted with the application. The report included soil borings which indicated that the native soils in the area consist of very-fine to medium-grained sand and is locally silt-rich. Groundwater was not observed in the borings. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer and infiltration basin. The proposed improvements are designed to reduce site runoff rates for the 2-, 10-, and 100-year rainfall events. Runoff rates for the 10-day snow melt event were not provided at this time and are not required as basin outlets are below the 100-year HWL. Pre- and Post- Development Discharge Rates (cfs) Condition 2-Year 10-Year 100-Year 10-Day Snowmelt Existing 9.49 15.82 32.09 Not provided Proposed 8.00 12.87 28.64 Not provided Comments: 1. (3/1/2023) Staff recommends adding cleanouts to the tennis court drain tile system. 2. (3/1/2023) For the pipe connection from FES-2 to the existing CB, applicant to verify if they can feasibly core drill another 12” RC pipe into the structure