PL PACKET 08172021 - Laserfiche WebLink

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Civil Site Group, PC • 4931 West 35th Street • Suite 200 • St. Louis Park • Minnesota • 55416 www.CivilSiteGroup.com • (612) 615-0060 2.0 Summary Analysis / Narrative: 2.1 Introduction: This stormwater management report accompanies the Civil Engineering Plans prepared by Civil Site Group for the subject project dated 07/20/2021. This report includes a summary of the existing and proposed site conditions, the stormwater requirements of relevant regulatory agencies, and proposed design calculations and data to meet the requirements. 2.2 Existing Site Conditions : Site Description: The existing site is an office building with associated landscaping and parking. Area [SF]CN Value Area [SF]CN Value Area [SF]CN Value EX1 (DRAINS TO STINSON STORM SEWER)66759 98 8198 80 74957 96 EX2 (DRAINS OFFSITE OVERLAND)700 98 6455 80 7155 82 Total AreaImpervious AreaDrainage Area Pervious Area Existing Soils: A geotechnical exploration has not been completed for the site as of July 2021. Soils are known to be contaminated due to past site uses such as dry cleaning and fuel storage. Groundwater: Groundwater observations have not been taken. 2.3 Proposed Site Conditions: Site Description: The proposed site is a multi-family residential building with associated parking, landscaping, utility and stormwater improvements. The project proposes to reduce the total amount of impervious surface by approximately 13,300 sf, a reduction of 16 percentage points of the total site area. The proposed site surface coverage areas are shown in the table below: Area [SF]CN Value Area [SF]CN Value Area [SF]CN Value PR1 (TO 1P)51989 98 7539 80 59528 96 PR2 (DRAINS OFFSITE)2107 98 20477 80 22584 82 Pervious Area Total AreaDrainage Area Impervious Area 2.4 Stormwater Requirements City (St. Anthony): Requirement threshold - Projects over 1 acre in size. Rate Control – Peak runoff rate during the 1-year, 2-year, 10-year, and 100-year Atlas-14 storm events may not exceed the existing conditions. Water Quality – Provide 60% Total Phosphorus and 90% Total Suspended Solids removal ef ficiencies. Volume Control – Providing 1.1” volume abstraction from the site’s impervious surfaces is sufficient to meet the Water Quality requirement. 2.5 Stormwater Requirements Watershed District – (Mississippi Watershed Management Organization): MWMO defers to the city of St. Anthony. 2.6 Stormwater Requirements - Minnesota Pollution Control Agency – NPPDES permit (MPCA): Requirement threshold - A permit is required for projects with a disturbed area over 1 acre in size, Stormwater management is required for a project adding 1-acre of more of NEW impervious surface (reconstructed impervious is not included). Rate Control – No specific regulation, may not degrade downstream facilities.