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® MONTGOMERY WATSON <br /> May 13, 1997 <br /> Mr. Dave Nyberg <br /> MSA Consulting Engineers <br /> 1326 Energy Park Drive <br /> St. Paul, MN 55108 <br /> SUBJECT: City of Centerville Surface Water Management Plan <br /> Dear Dave: <br /> We have completed our review of your draft surface water management plan for the city of <br /> Centerville and have the following comments. To facilitate - understanding, we have <br /> organized our comments according to management categories. <br /> FLOODPLAIN MANAGEMENT. e�ie 5 2 Lowest Floor Elevations. The city plan currently allows only 1 foot of freeboard <br /> between lowest floors and the 100 -year floodplain for commercial and industrial <br /> developments. The RCWD requires a minimum freeboard of 2 feet above the 100 -year flood <br /> elevation (or 1 foot above an emergency overflow) for all new developments and new <br /> additions to existing developments. <br /> See en 5 -2- Floodplain Fill. There is no reference in the text of the plan requiring compensatory <br /> floodplain storage for any 100 -year floodplain fill. The RCWD requires a minimum 1:1 ratio <br /> of compensatory storage for all floodplain fill. <br /> Ton S z Easements. The RCWD requires easements over all areas of 100 -year floodplain. The <br /> city plan needs to be amended to include this requirement also. <br /> �ce�ion S:3 Rate Control Policies. To prevent flooding associated with increased runoff rates <br /> occurring with development, the RCWD requires that new developments provide rate control <br /> to existing conditions for the 1- and 100 -year critical storm events. For those areas of the <br /> city which drain to Clearwater Creek, ate control is of concern to the watershed. With this in <br /> mind, the city should require rate control policies for subwatersheds NE and CC. <br /> WATER QUALITY <br /> SeC �i Ss rJaier Quaiiry Ponding. To protect the water quality of receiving waters, the RCVVD <br /> requires that all new developments meeting the requirements for District review provide for <br /> treatment of their stormwater discharges. This is most commonly provided through use of an <br /> on -site water quality treatment pond. Regional ponds serving several developments are also <br /> acceptable and in most cases, are preferable because of Iower construction and maintenance <br /> costs and increased pond and land use efficiency. <br /> �d�iort S -S The pond design required by the RCWD conforms to the Nationwide Urban Runoff Program <br /> (NURP) . In general, NURP ponds have a permanent pool volume equal to the runoff form a <br /> 2.5 -inch rainfall event and have an average depth of at least 4 feet. Also of importance in <br /> pond design is the inclusion of an outlet skimming device which retains oils, trash, and other <br /> fl otable materials within the stormwater pond. <br /> Waterford Park Tel: 6125939000 <br /> 505 U.S Hwy. 169 Fax 671593 9975 Sevin_ rbe World's Fn vironmar:;a! Ncads <br /> . <br /> Suite 555 <br /> Minneapolis, Minnesota 55441 <br />