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IMO CONSULTING <br />GROUP <br />Lake Minnetonka North Arm Boat Launch - Stormwater Management Memo <br />(Minnehaha Creek Watershed District Permit Application) - September 5, 2025 <br />PVC) Hennepin County intends to reconstruct the existing North Arm Boat Launch to create a much <br />more aesthetically pleasing and accessible place for all. Fishing piers, docks, a new boat ramp, <br />boat cleaning facilities and site lighting are all part of the proposed project. In addition, the <br />project will include a sustainable system of stormwater management that not only meets but <br />exceeds the Minnehaha Creek Watershed District stormwater management criteria. Through a <br />combination of permeable paving, sand filters, underground storage, swirl concentrators and <br />natural vegetation we have developed an integrated system that captures the runoff from the <br />site and detains/treats all the water prior to discharge to Lake Minnetonka. <br />This memo is an updated Stormwater Management Memo that incorporates the hydrologic and <br />hydraulic (H&H) analysis of the North Arm Public Lake Access based on the final Storm Drainage <br />Concept that is intended to be constructed in 2026-2027. We developed a "Preliminary Design <br />Report" which conceptualized the proposed improvements in January of 2022 and completed a <br />Final Report in April of 2023. <br />The emphasis of this study is the evaluation of stormwater volume, rate, and pollutant load <br />being discharged from the site to Lake Minnetonka. The proposed Best Management Practices <br />(BMPs) in and around the site will help achieve a reduction in each of these areas from the <br />current conditions. With the current design, approximately 92% of the entire drainage area <br />(which includes a portion of CSAH 51— North Shore Drive) is being collected and treated. <br />A. EXISTING CONDITIONS <br />Hydrology: <br />1. A site visit was conducted on August 1, 2021. Preliminary details such as the <br />observable drainage patterns, existing landscaping, and boat decontamination <br />practices were recorded. Historic aerial photos were also reviewed. <br />2. Hennepin County performed a topographic survey and a draft was provided to IMO. <br />This information was used to verify existing drainage patterns and <br />pervious/impervious areas. <br />3. The entire contributable drainage area is approximately 2.3 acres. <br />4. Drainage from the site consists of runoff from North Shore Drive, the bituminous <br />parking lot, and the vegetated portions of the site. Runoff ultimately outflows <br />directly into Lake Minnetonka from all parts of the site. <br />5. There are no existing water quality treatment or other stormwater management <br />practices present on the site. There are two sediment traps that have reached the <br />end of their service life. <br />6. A Web Soil Survey was completed, and the results suggest that this site is unsuitable <br />for infiltration without removal and replacement of the existing soils. <br />12900 Whitewater Drive - Suite #100 - Minnetonka, MN 55343 <br />P: 952.446.7898 — imoconsultinggroup.com <br />112 <br />