02-22-2011 Planning Commission Packet

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� } � � Memorandum � �Bonestroo 2335 Highway 36 W To: Darren Amundsen Project: 875 Wayzata BLVD Date: 02/09/11 St.Paul,MN 55113 From: Jesse Carlson Client: City of Orono Tel 651-636-4600 Fax 651-636-1311 Re: Stormwater Review File No:139-11000-1 www.bonestroo.com Remarks: This memo summarizes review of the revised stormwater calculations submitted for 875 Wayzata BLVD, Orono Senior Housing. The submittal was reviewed for compliance with the policies of the City of Orono SWMP, October 2010. The foilowing revised materials were reviewed as a part of the submittal received February 7, 2011: • Revised Stormwater Narrative • Revised Site Plans • Revised HydroCAD Calculations for Existing and Proposed Conditions • P8 Modeling for Existing and Proposed Conditions 1. RunofF rates shail not exceed existing runoff rates for the 1-yr or 2-yr, 10-yr, and 100-yr, 24- hr storm events. The applicant has demonstrated that the runoff rates are equal to or less than existing runofF rates. However, in their modeling they use a CN of 65 for the proposed conditions in the areas not designated as turf grass. The applicant shall use a consistent CN of 68 for all pervious areas not designated as turf grass in the proposed � ' conditions and resubmit their calculations for review,unless the applicant can provide adequate reasoning why a CN of 65 should be allowed. Drainage area maps shall be included with the revised submittal. 2. The SWMP requires that the applicant reduce the phosphorous load by 20%from existing conditions and also meet the wetland protection ordinance requirement, where for manage 1 wetlands the phosphorus load shall be limited to 0.28 Ibs/acre/yr. The applicant provided a P8 model to demonstrate that they have met this requirement. The applicant states"Overall TP watershed inflows of 0.90 Ibs/yr were calculated with 0.58 Ibs/yr trapped and 0.32 Ibs/yr of outflow. The allowable TP discharge is 80% of existing or 0.37 Ibs/yr(0.47 Ibs/yr* 0.80), which we are below, and therefore the site meets the criteria."In reviewing the output data the above statement appears to be incorrect. It appears as though the overall TP watershed inflow is 0.90 Ibs/yr with 0.52 Ibs/yr trapped and 0.38 Ibs/yr of outFlow. In addition, the proposed conditions model identifies the impervious as indirectly connected. The applicant shall revise the proposed conditions model to include the impervious areas as directly connected and resubmit their calculations for review.The stormwater narrative shall be revised to be consistent with the model results and any modifications to the biofiltration design made, as necessary.