05/29/2001 Council Packet - Laserfiche WebLink

Home Search

17. Water quality: surface water runoff a. Compare the quantity and quality of site runoff before and after the project. Describe permanent controls to manage or treat runoff. Describe any stormwater pollution prevention plans. The existing site soil regime is of naturally high infiltration media. Drainage calculations for the predevelopment 100 -year flood event computed a total site runoff of only 1.54 cfs. Post - development runoff rates will be maintained at the pre - development rates by the addition of two ponds and a bioretention basin. Site grading will occur to facilitate construction of the SuperTarget Store and other retail shops, construction of parking lots, depressed islands, planting beds; one water quality treatment pond, creation of bio - swales and bio- retention basins. In order to maintain the post - development runoff rates at the predevelopment rates a storage pond with dry storage above the normal wet water level is planned on the site. Bio- retention basins are located in the northeast and southeast corners of the site. The storm water treatment pond is planned to have a wet storage volume of 13.5 acre -ft (National Urban Runoff Program (NURP) wet volume required 5.5 acre -ft) and a dry storage capacity of 9.9 acre -ft and discharge from the site through one each 2 -inch and 8 -inch diameter pipes. The smaller 2 -inch diameter is to minimize storm water discharge for the one -year event to 0.05 cfs and the two pipes combined limit the 100 -year event to 1.21 cfs. The storage depth will bounce for each of the various rainfall frequency events with maximum depth of 5.0 feet in the 100 -year event. b. Identify routes and receiving water bodies for runoff from the site; include major downstream water bodies as well as the immediate receiving waters. Estimate impact runoff on the quality of receiving waters. Runoff from the proposed development currently drains to the north side of the I -35W drainage ditch. A culvert under I -35W starts at the southeastern corner of the site and carries storm water runoff south to a shallow creek and ultimately discharges to George Watch Lake. After the site is developed, storm water will be routed through on -site storm water ponds designed to meet NURP (Nationwide Urban Runoff Program), city of Lino Lakes, and Rice Creek Watershed District criteria. Storm water rate control and treatment will occur prior to any water being discharged into the George Watch Lake. Converting from high infiltration soils to more than 28 acres of impervious surface will increase the volume of runoff. While runoff rates will be maintained at pre - development rates, it is not known what, if any, impact will result from the increased volume of runoff into the wetlands in the regional park reserve and George Watch Lake. The Metropolitan Council has prepared a list of "high priority lakes" for which a nutrient budget analysis is required when a project discharges into them. Downstream from the site are George Watch, Marshan, Rice, Baldwin, and Reshanau Lakes. Rondeau and Peltier Lakes are upstream from George Watch Lake. The project discharges into George Watch Lake, which is not on the priority list. However, George Watch Lake is included in a 1999 study of water quality, along with 113 other Metro lakes (Exhibit L). Though a nutrient budget analysis is not required, an analysis was performed using Walker Pondnet methods. Anticipated phosphorus runoff loads from the site are 22 Ibs per year in the existing (predevelopment) condition and 60 Ibs per year in the post development condition, incorporating the treatment and reduction effects of the proposed National Urban Runoff Program (NURP) ponds on site. Page 19 of 28 Draft EAW Lino Lakes Marketplace May 11, 2001