Agenda - Planning Commission - 12/01/2016

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Review File:M& G Trailer Sales Parking Lot Expansion Construction Plans for Site Grading and Parking Lot and Miscellaneous Construction Engineering Review November 22, 2016 Page 2 of 2 3. Add a note to the inspection section: "Daily (after rain) and weekly inspection logs must be emailed to the City. Grading Drainage and Erosion Control Plan 1. Spot elevations must be added to the plan. The area of coverage starts at the low area north of the project and extends to the existing pond west of the M & G Trailer Sales Building. This is to demonstrate that there is positive drainage from the new infiltration basin to the existing low area then west to the existing City pond. 2. The grades are extremely flat, 0.28% on the east line, 0.12% on the south line, 0.72% on the west line and 0.40 % from the south east corner to the northwest corner of the new gravel area. The grades along the north line range from 0.27% to 0.83%. Paved parking lot recommended minimum grade is 1.5%. This surface will be gravel and will not drain well at these flat grades. 3. Add note to protect existing piezometer. Stormwater Calculations A stormwater summary report was provided. The project is providing infiltration and rate control as required by the Lower Rum River Watershed Management Organization (LRRWMO) and City requirements. The report does not indicate that the project meets the water quality treatment goals of 90% removal of total suspended solids (TSS) and 60% removal of total phosphorus. Attainment of these goals must be demonstrated. The project is proposing to use a timber skimmer with an orifice to provide the detention for the infiltration volume. It has been our experience that timber skimmers deteriorate over time and will fail, negating the infiltration retention capacity of the basin. Timber is also easier to modify, changing the rate control provided by the pond. The outlet must be replaced with a concrete structure to preserve the integrity of the infiltration basin. Revised plans are required before the project can be approved for construction.