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Memorandum <br />To:Tom Kellogg Project:1500 Bracketts Point Road <br />Plat # 139-3293 Date:7/3/2007 <br />From:Lance Hoff, Dan <br />Murphy Client:City of Orono <br />Re: Drainage Review File No: 139GEN <br />Remarks: <br />Existing and proposed grading plans (dated 11-20-06) and a HydroCAD model (dated 6/22/07) <br />were reviewed for the site at 1500 Brackett’s Point Road in Orono. We offer the following <br />comments: <br />1.The proposed rain gardens provide sufficient rate control such that proposed runoff <br />rates meet existing runoff rates for the 2-, 10-, and 100-yr, 24 hr storm events. <br />2.The two rain gardens provide sufficient water quality volume to treat the volume of <br />runoff generated by the 2.5”, 24 hr storm over the entire contributing drainage area <br />to each (NURP requirement). <br />3.No water quality treatment is provided for the northwest section of the proposed <br />private drive. An option would be to construct another rain garden on the south side <br />of the drive to receive this drainage. This could outlet to the west to prevent runoff <br />from the proposed north rain garden from flowing over southern driveway stub. <br />4.The EOF for the south rain garden is noted on the plans at 940.20. The riprapped <br />overflow is noted at an elevation of 940.00. This discrepancy should be resolved. In <br />general, the EOFs for the proposed rain gardens should be set at or above the HWLs. <br />5.The HWLs for the two proposed raingardens were modeled assuming the rain gardens <br />are dry. When calculating outflow rates and high water levels the rain gardens <br />should be assumed to be filled to the NWL. We calculate HWLs of 934.4 and 940.3 <br />for the north and south rain gardens, respectively. <br />6.Riprap or another type of permanent erosion control measure should be provided at <br />the rain garden outlet pipes. <br />7.How is runoff routed to existing Bracketts Point Road proposed to be collected and <br />conveyed? <br />8.The annualized percentage of runoff infiltrated is approximately 86% and 84% for <br />the north and south rain gardens, respectively, assuming the rain gardens are <br />maintained in good working condition. <br />2335 Highway 36 W <br />St. Paul, MN 55113 <br />Tel 651-636-4600 <br />Fax 651-636-1311 <br />www.bonestroo.com