11-17-2025 - Agenda Packet Planning Commission - Full Packet

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IMO CONSULTING GROUP 4. The site plan introduces approximately 0.68 acres of permeable pavement. The permeable pavement will filter out phosphorous, metals, and other solid waste from the stormwater and is intended to provide sufficient storage to attenuate the first inch of rainfall and will infiltrate back into the water table. a) As anticipated, the soil report confirms that infiltration is infeasible. Hence, the subgrade will need to be excavated and replaced with engineered fill and drain tile that will allow filtered and treated runoff to reach the lake. 5. The proposed grading of the site is designed to maximize the effectiveness of the permeable pavement by directing the majority of runoff from the site to these areas. Areas along the shoreline will continue to drain directly to the lake. 6. Water from the decontamination stations will be directed to swirl concentrators (FILTRATION DEVICE 1 in Figure C303) for pretreatment prior to discharge into the filtration system. 7. Other improvements unrelated to the site's H&H include: two new docks, a decontamination shed, several decontamination stations, ADA compliant sidewalks, a new accessible lift and lighted fishing piers. Hydraulic Modeling: 1. The proposed conditions split the property into nine distinct drainage areas (See Figure C303). 2. While the high traffic portions of the parking lot will remain bituminous, approximately 0.68 acres of permeable pavement will be added. These portions of the parking lot will have a lower CN value than other forms of impervious pavement. This means that water will move more slowly across these areas, percolate into the permeable pavement, and ultimately infiltrate into the groundwater table through a layer of sand. Excess runoff will be collected by a perforated pipe at the bottom of the filtration trench. Results — HydroCAD 1. In the 1-Year and 10-year events, 100% of the runoff and approximately 96% of the 100-year event from the contributing drainage areas is treated. 2. In the 1-Inch, 24-Hour rainfall event there will be no direct discharge from the site and 100% of the stormwater volume was either infiltrated or captured/stored via the voids in the permeable pavement and filtration trenches. 3. The peak flows and volumes to the lake through the site will be drastically reduced. In the 1-, 10-, and 100-Year rainfall events the runoff volumes discharging to the lake will be decreased by approximately 70%, 53%, and 43%, respectively. North Arm Stormwater Management 3 114