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Ms. Katie Larsen 12/1 /2021 Page 2 After the proposed improvements, the surface water will flow to a proposed infiltration basin on the north side of the site. Storm sewer is proposed to collect water from the parking lot and then direct it to the proposed infiltration basin. The infiltration basin is designed to provide water quality treatment and rate control. The proposed basin includes an upstream rain guardian and forebay for pre-treatment. The infiltration basin's emergency overflow will discharge to the existing wetland north of the site. After the project is completed, the site will include approximately 10,873 square feet of new impervious surfaces including the sidewalks, patios, parking lot, and rooftop. Based on the draft soil boring information, the groundwater elevation in the area of the basin is 881.90. Soils consist of poorly graded sands and poorly graded sands with silt. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer conveyance system and basin. 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) Wetland Blue Heron Drive Condition 2-Year 10-Year 100-Year 2-Year 10-Year 100-Year Pre- 0.14 0.57 2.05 0.06 0.28 0.98 Development Post- 0.05 0.40 1.64 0.06 0.17 0.47 Development Post -development stormwater discharge rates are less than the pre -development rates. Comments: 1. Provide name of company that prepared the drainage report and calculations on those documents. 2. The proposed model uses a post -construction CN value of 61 for disturbed pervious areas. A post -construction CN value of 74 should be used for disturbed pervious areas in Type B soils per City Ordinance 1011.011 Table 1. 3. Applicant to confirm the low floor and low entry free board requirements for infiltration basins from City Ordinance 1011.011 Table 5 are met. 4. Provide hard armoring (e.g. riprap) or other permanent erosion control measures beyond established turf for the emergency overflow from the infiltration basin. 5. Applicant to provide information on the pretreatment for stormwater draining to the wetland. 6. Applicant to provide additional grading information (e.g. drainage arrows, spot elevations) to show that the entire parking lot drains to the Rain Guardian. 7. Applicant to provide calculations in the drainage report to show that a single Rain Guardian can convey flow from the entire parking lot with a 10-yr Rational Method analysis. • Water Supply There is existing 16-inch diameter ductile iron pipe (DIP) watermain located along Blue Heron Drive and Hodgson Road (CSAH 49) adjacent to the site. MTO Properties — Thrivent Financial Engineering Review