PL PACKET 04152025 - Laserfiche WebLink

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STORM WATER SUMMARY The proposed project is designed so that the vast majority of storm water run-off from the site drains to a proposed filtration basin via storm sewer and surface drainage. The proposed filtration basin and associated outlet structure have been designed such that overall post-developed site runoff rates do not exceed overall pre- developed site runoff rates. Site drainage has been modeled for the 24-hour – 2, 10, and 100-year storm events. Our calculations assume that the 2-year storm event in this area of St. Anthony Village would be approximately 2.83 inches, the 10-year event would be 4.24 inches, and the 100-year event would be 7.38 inches, based on current NOAA Atlas 14 rainfall data. We have utilized HydroCAD software to generate storm water calculations, as shown on the following pages. Existing Soils Based on a geotechnical report completed by Chosen Valley Testing, dated 02/10/2025, the soils at the site predominantly consist of lean and sandy clays (CL). These soils are considered Hydrologic Soil Group (HSG) “D” as per the MN Storm Water Manual. Based on the above-mentioned geotechnical report, the surficial soils at the site were assumed to consist predominantly of Hydrologic Soil Group (HSG) “C” soils for the purposes of calculating subcatchment (drainage area) runoff, for the existing conditions. The surficial soils at the site were assumed to consist predominantly of Hydrologic Soil Group (HSG) “D” soils for the purposes of calculating subcatchment (drainage area) runoff, for the proposed conditions. Ground Water Based on the soil borings by CVT referenced above, ground water is assumed to be below 925.0 as no water or redoximorphic soils were observed in the borings. WATER QUALITY Required Treatment Volume Proposed impervious surfacing = 19,870-SF Base Filtration Volumes: 19,870 SF x 1.1-in = 1,825 cf Adjusted Filtration Volume: 1,825 cf ÷ 0.50 = 3,650 cf (TP Removal Factor)