02-22-2011 Planning Commission Packet

Home Browse Search

, , � Memorandum � Bonestroo 2335 Highway 36 W To: Darren Amundsen Project: 875 Wayzata BLVD Date: 01/27/11 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 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. This site will need to comply with the flood prevention, nutrient and sediment loading, and volume control policies of the SWMP. The following materials were reviewed as a part of the submittal received]anuary 19, 2011: • Stormwater Narrative • Site Plans . • HydroCAD Calculations for Existing and Proposed Conditions • .�., � 1. Runoff rates shall 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 72 for the existing conditions and a CN of 65 and 74 for the proposed conditions. The applicant shall use a consistent CN of 68 for all pervious areas in both the existing and proposed conditions and resubmit their calculations for review. Drainage area maps shall be included with the revised submittal. 2. Wetland 1 is categorized as a��Manage 1"wetland, which requires that the bounce be maintained at or below existing conditions plus 0.5 feet.The applicant states that they have reduced runofF volumes when comparing the pre and post runofF volumes for the 100-year, 24-hour storm. In reviewing the modeling data it appears as though the runofF volumes actually increase from 0.73 ac-ft to 0.75 ac-ft. This is a more accurate representation of the actual conditions because the filtration basins are not reducing the volumes, but extending the peaks. The applicant shall provide documentation that they are meeting the bounce criteria for Wetland 1. 3. 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 shall provide water quality calculations using a suitable model such as PondNet,the simple method, or P8 to demonstrate that the filtration features are providing adequate treatment.The applicant shall use the Event Mean Concentrations (EMCs)from the Minnesota Stormwater Manual, Chapter 8,Table 8.7.The removal e�ciency of 65%for filtration media as provided in the stormwater narrative shall be considered acceptable as a part of this analysis.