06-05-2023 Council Work Session Packet

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Ms. Katie Larsen 05/03/2023 Page 2 Milestones Addition & Lil Explorers Childcare Center Engineering Review •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 1.02 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 0.67 2.82 9.56 Not provided Proposed 0.20 1.07 7.18 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. 3.(5/3/2023) Applicant shall revise location of STMH-8 so that it is within the property. Private structures shall not be placed in the right-of-way. 4. (5/3/2023) The City of Lino Lakes does not allow roof drains to be connected directly to storm sewer. Roof drains need to have a 6-inch air gap before draining to the storm sewer. Applicant to provide detail on plans. 5. (5/3/2023) Infiltration rate testing is needed to ensure sufficient infiltration capacity is present in site soils. Double ring infiltrometer or modified Phillip- Dunne testing is recommended to determine these rates. Testing must be completed prior to final plat approval. 6. (5/3/2023) Applicant to verify the constructability of CBMH-4. The pipes proposed to come in at an acute angle may require a large diameter drainage structure to ensure sufficient concrete is provided between pipe openings. 7. (5/3/2023) Applicant to provide riprap at the FES-5 inlet to the north infiltration basin.