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<br />Gateway Business Center - Arden Hills, MN <br /> <br />EAW <br /> <br />September 2001 <br /> <br />a. Compare the quantity and quality of site runoff before and after the project. Describe e <br />permanent controls to manage or treat runoff. Describe any stormwater pollution prevention <br />plans. <br /> <br />An existing regional stormwater management pond is located in the northwest comer <br />of the site. The pond was originally designed and built for the Gateway Business <br />Center development across Gateway Boulevard to the north of the proposed <br />development site. This pond will be expanded and surfaee water runoff from the <br />proposed development will be piped to the pond, The pond will provide measures for <br />rate control and water quality treatment. The existing regional stormwater pond was <br />designed to provide 2.95 ae-ft of permanent pool volume for the runoff from the <br />properties to the north, The permanent pool volume of the proposed pond is designed <br />to incorporate the permanent pool volume in the existing pond plus the EPA <br />Nationwide Urban Runoff Program (NURP) requirements of the proposed <br />Chesapeake development. The NURP requirements are calculated using a pervious <br />portion curve number of 61 and a design storm of 2.5," The existing stormwater pond, <br />on Pareel 2 was designed for the treatment of stormwater runoff for the existing site. <br />The proposed stormwater management pond will provide treatment for the runoff of <br />the proposed site; therefore, there will no longer be a need for the existing pond on <br />Parcel 2 and it will be filled. <br /> <br />Total required NURP permanent pool volume <br />Permanent pool volume of existing pond <br />Total permanent pool volume required <br /> <br />= 3,2149 ac-ft <br />= 2.95 ac-ft <br />= 6.1649 ac-ft <br /> <br />e <br /> <br />Total permanent pool volume provided in the proposed regional stormwater pond <br />=6.1685 ae-ft (proposed) > 6.1649 ac-ft (required); OK. <br /> <br />This proposed pond is designed to control post-development peak discharge rates to <br />the pre-development discharge rates of the same events, The overflow elevation of <br />the proposed pond is 890,50, <br /> <br />24-hr Storm Event <br />100-yr <br /> <br />Before (cfs) <br />89 <br /> <br />After (cfs) <br />42 <br /> <br />toO-V' HWL -Pond Wetland <br />889.81 884,61 <br /> <br />b. Identify routes and receiving water bodies for runoff from the site; include major downstream <br />water bodies as well as the immediate receiving waters. Estimate impact of runoff on the <br />quality of receiving waters. <br /> <br />The site drains to the regional storm water pond on site, This pond diseharges into the <br />adjacent wetland and eventually drains into Rice Creek. The proposed pond is <br />designed with the criteria derived from the EPA's Nationwide Urban Runoff Program <br />(NURP) , Based upon NURP and other monitoring studies, ponds designed according <br />to these guidelines should have pollutant removal efficiencies similar to those shown <br />in the figure below, These calculations focus on sizing and shaping the permanent <br />pool, which is necessary for water quality control purposes. <br /> <br />. <br /> <br />Page 16 <br />