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HomeMy WebLinkAbout2016-07-14 Stantec Drainage ReviewStantec Consulting Services Inc. 2335 Highway 36 West St. Paul MN 55113 Tel: (651) 636-4600 Fax: (651) 636-1311 July 14, 2016 Mr. Eric Johnson, Administrator City of Oak Park Heights 14168 Oak Park Blvd. N., P.O. Box 2007 Oak Park Heights, MN 55082-2007 Reference: Xcel Baytown Substation – Drainage Review Dear Eric, We have performed a review of stormwater calculations and drainage structures for the proposed Xcel Baytown Substation improvements. Following are our comments and recommendations: 1. A drainage map that includes drainage boundaries to verify the drainage areas should be submitted as part of the Stormwater Drainage Calculations. 2. Due to the site being located within the City’s DWSMA, and in particular, the 1-yr travel time emergency response area, infiltration is not allowed on this site based on requirements of the NPDES Construction Permit. 3. Rate Control: The site currently drains into an existing private storm sewer structure adjacent to the site. Because of this, proposed flow rates need to be maintained at or below existing rates in order to prevent the site negatively impacting the adjacent property. The proposed flow rates leaving the site are indicated as less than existing flow rates at the outfall of the stormwater pond and meet the rate control requirement. However, this is partially achieved by infiltrating part of the water in bioretention basins and sediment forebay, which is not allowed (see comment 2). 4. Water Quality: The site will be required to meet the MSCWMO standards for water quality. Filtration is allowed as long as the design prohibits infiltration. 5. Volume Control: The site is not required to achieve volume control due to no infiltration being allowed on site (see comment 2). 6. Emergency overflows should be noted on the plans. 7. Erosion control materials should be used in places where higher velocities are expected due to restricted flows or steep grades. Mr. Eric Johnson Page 2 of 2 Reference: Xcel Baytown Substation – Drainage Review If you have any questions or require further information, please do not hesitate to contact me. Regards, STANTEC CONSULTING SERVICES INC. Lee M. Mann, P.E. City Engineer Phone: 651-604-4850 lee.mann@stantec.com cc: Andy Kegley, Public Works Director Julie Hultman, Building Official Scott Richards, City Planner Mark Vierling, City Attorney Lisa Danielson, City Arborist