06-12-2023 City Council Packet - Laserfiche WebLink

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Ms. Katie Larsen 06/01/2023 Page 2 Milestones Addition & Lil Explorers Childcare Center Engineering Review 2. (5/3/2023) Applicant to review grades near structures CB-7 to CBMH-5. Staff requires that swales are graded with at least 2% slopes. Current side slopes from the contours to the rim elevations exceed 16% and staff have concerns about erosion and maintenance of the structures. (5/19/23) Side slopes are 1:4 which is a standard roadway slope. There are no concentrated flow areas. No anticipated issues. 3.(6/1/23) The applicant shall show the correct grading contours at the driveway culvert to match the FES elevations. 4.(6/1/23) The applicant shall install a concrete commercial driveway entrance at Lilac Street. The City standard detail plate shall be added to plans. •Stormwater Management Stormwater from the existing 4.22 acre vacant lot area drains to storm sewer along Apollo Drive (CSAH 12), which eventually drains to a city maintained pond in the northwest quadrant of the Apollo Drive (CSAH 12) and Lilac Street intersection. The project is proposed to add approximately 0.98 acres of impervious surface for the childcare facility, parking lot, and entrance drive, according to the Stormwater Management Plan. The applicant is proposing two infiltration basins to meet the runoff rates and water quality standards. A geotechnical evaluation report prepared by Chosen Valley Testing, Inc. was submitted with the application. The report included soil borings which indicated that the soils in the area consist of poorly graded sand with silt. 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 basins. 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 0.64 2.73 9.34 Not provided Proposed 0.22 1.94 8.10 Not provided Comments: 1. (5/3/2023) See Comment 1 under Grading regarding infiltration basin location and grading. 2.(5/3/2023) Applicant to call out surface material being used in play areas. If an impervious surface is proposed, update the impervious surface calculations shown on sheet C1 and used in SWMP calculations and modeling. A map highlighting impervious surface would help clarify these areas. (5/19/23) Play areas are pervious artificial turf. A note will be added in the SWMP.