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WSB — 701 Xenia Avenue South I Suite 300 1 Minneapolis, MN 55416 1 (763) 541-4800 Memorandum To: Rachel Juba, City of Hugo Cc: Mark Erichson, WSB & Associates From: Stephanie Hatten, WSB & Associates Date: August 17, 2018 Re: Stormwater Management Requirements — Gregoire Solar Site WSB Project No. 010469-000 The following memo summarizes the City of Hugo/RCWD stormwater management requirements for the Gregoire Solar Site. These requirements will need to be met prior to issuing a Stormwater and Erosion Control Permit. A Permit is needed before construction will begin. Volume Control/Water Quality The Solar Site will need to meet the City's volume reduction requirement of 1.1 inches off new impervious surfaces. The MPCA Solar Panel Calculator spreadsheet must be used to calculate the volume reduction needed for the site. Since the entire solar array is not a continuous impervious surface, the MPCA has chosen to modify the calculations to allow for stormwater credit for a portion of the runoff that will infiltrate into the ground below the solar panels. Any additional stormwater runoff from access roads (gravel or paved) or other impervious surfaces must also be added to the volume reduction requirement from the MPCA spreadsheet for the solar panels. Infiltration or water reuse is the preferred method for volume reduction. Soil borings or other documentation must be submitted. Some site restrictions for infiltration may include HSG C and D soils, soil contamination or high groundwater elevations. If infiltration or reuse is not feasible onsite, any BMP may be used. The TP removal factor from Table C1 of RCWD rules must be applied for the chosen BMP. TP and TSS removal will be considered met once the volume control requirement has been met. Area not treated for phosphorus may not exceed 15% of all new or reconstructed impervious surface. For all untreated surface, TSS must be removed to the maximum extent practicable. Rate Control Peak stormwater runoff rates for the site must not exceed existing rates for the 2 -year and 10 -year 24- hour rainfall events using Atlas 14 rainfall depths. Peak stormwater runoff rates for the 100 -year 24-hour rainfall event is limited to 0.1 cfs/acre. Submittal Requirements • Property lines and delineation of lands under ownership of the applicant • Delineation of the subwatershed contributing runoff from off-site, proposed and existing subwatersheds onsite, emergency overflows, and drainageways. • Geotechnical analysis including soil borings at all proposed stormwater management facility locations utilizing ASTM D5921 and D5879, as amended. Building a legacy —your legacy. Equal Opportunity Employer I wsbeng.com