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Lower Rum River Watershed Management Organization Andover -Anoka -Ramsey 2015 First Avenue; Anoka, MN 55303 To: Lower Rum River Water 11 aua ement Organization From: Barr Engineering Company Date: March 10, 2016 Re: Permit #2016-02: Riverplace Counseling Center: Anoka We have received plans and a LRRWMO permit application has been submitted for a new facility for Riverplace Counseling to be located south of T.H. 10 approximately 1/2 mile east of Sunfish lake Boulevard in Anoka. The site is approximately 4.1 acres and recently consisted of a 35 unit mobile park and one single family home. The project proposes the construction of a 10,000 square foot counseling center along with 10 townhomes that will house current patients. There are also future plans for 4 additional townhome units and an easterly extension of the interior roadway. Two storm water basins, east and west of the driveway onto the site, are to be constructed for volume retention, rate control and water quality treatment to meet the LRRWMO storm water requirements. The tributary area to the eastern basin and design includes the proposed future plans for the site. The following table summarizes the existing and proposed discharges leaving the site for the 2, 10, and 100-year frequency storm events: Existing Discharge Proposed Discharge Frequency c.f.s. c.f.s 2-Year 4.0 <1.0 10-Year 8.3 2.2 100-Year 19.4 16.9 Volume retention of 4,392 cubic -feet is required from the 1-inch of runoff from the proposed 52,708 square feet of new on -site impervious area. Soil borings taken in the area of the proposed storm water basins indicate the on -site soil as poorly graded sand (SP). An infiltration rate of 0.8 inches/hour has been assumed using the Minnesota Storm Water Manual. An area of 1,366 square feet is required for volume retention. The proposed basins provide a volume of 11,434 cubic -feet acre-feet (4,392 cubic -feet required) and an area of 5,678 square feet (1,366 square feet required). The soil boring Logs indicate groundwater was not encountered to a depth of 11 feet, elevation 846 +/- M.S.L — the bottom of the boring. The eastern basin has the lowest bottom elevation, 850 M.S.L., of the two basins. A 4 foot P:\Mpls\23 MN102L23020471WorkFiles\LRRWMO PermitMemos12016`, r1602.docx