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�_ :.:�. � <br /> Memorandum �Bonestroa <br /> To: TomKellogg Project: OronoOfficeParkPlat# Date: 7/3/2007 Z335Highway36W <br /> 139-3303 St.Paul,MN 55113 <br /> From: Dan Murphy, Lance Client: City of Orono <br /> HOff Tel 651-636-4600 <br /> Fax 651-636-1311 <br /> Re: Stormwater Review File No: 139Gen wvuw.bonestroo.com <br /> Summary of Recommendations . <br /> A review of the submitted grading plan(dated 6/20/07)and stormwater calculations(dated <br /> 6/20/07)for the Orono Office Park development has been completed. Comments and <br /> recommendations related to this review are as follows: <br /> 1. The proposed pond outlet structure does not meet City standards. Skimming should <br /> be provided up to at least the 10-yr event. A skimmer structure with a concrete baffle <br /> wall and small diameter orifice should be used. The City standard detail is attached. <br /> 2. The pond has sufficient flood storage to control flow rates for the 10-and 100-yr, 24- <br /> hr event. The proposed site does not meet existing flow rates for the 2-yr, 24-hr event <br /> per the developer's submitted stormwater calculations. Additional rate control should <br /> be provided for the 2-yr event using the outlet described above in Comment 1. <br /> 3. The pond contains sufficient water quality volume to treat the volume of runoff <br /> generated from the site for the 2.5", 24-hr event(NURP requirement). <br /> 4. The average depth of the proposed pond is less than 4 ft(water quality volume/ <br /> surface area at NWL).According to NURP water quality standards,this average depth <br /> must be increased to at least 4 ft. At a minimum,the mean depth should be increased <br /> to 3'. <br /> 5. No maintenance access is provided for the proposed outlet structure. A number of <br /> alternatives exist for providing access. These include widening the berm, providing a <br /> maintenance bench with a 10:1 slope above the NWL, and/or shifting the outlet east. <br /> 6, A 10:1 aquatic bench must be provided for the first 1 foot of depth below the normal <br /> water elevation. <br /> 7. A riprapped emergency overflow(EOF)should be provided at or above the 100-yr <br /> HWL. <br /> 8. EOF elevations with associated flow arrows should be noted for all catch basin low ' <br /> points within the proposed parking lot. 1' of freeboard must be provided between the <br /> EOF elevations and adjacent structure elevations. <br />