6.2.- 6.4. SR 11-18-2002 - Laserfiche WebLink

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Elk River Station EAW September 6, 2002 17. Water Quality: Surface Water Runoff Compare the quantiO, an,l qualio~ of site runoff before and after the project. Describe permanent controls to manage or treat r~.~noj~. Describe any stormwater pollution prevention plans. Surface water drainage generally flows to the wetland located at the north end of the site. The wetland allows the water to drain to the existing city ditch along the northern property boundary of the site. The quantity of surface water runoff will increase during storm events as additional impervious surface area is added with the construction of pavement and buildings. The increase in impervious surface will be mitigated by construction of onsite stormwater ponds, which will be constructed to protect water quality and limit peak discharge rates from the site. These discharge rates will be maintained at or below existing levels. The proposed ponding system will be designed to meet all requirements of the Minnesota Pollution Control Agency (MPCA) and the City of Elk River for water quality treatment, and the post development rate of runoff will not exceed existing rates. Stormwater ponds will be designed to remove suspended sediment and nutrients from stormwater runoff entering the city ditch. b) k'lent(~, routes and receiving water bodies for runoff fi'om the site; include major downstream water bodies as well as the immediate receiving waters. Estimate impact runoff on the quali~.' of receiving waters. According the DNR Division of Waters digital watershed mapping, the project site is located within the Mississippi River minor watershed (Exhibit 8). Stormwater generated from this development will be routed into a series of five newly created stormwater ponds, which will provide rate control and water quality treatment prior to discharging the wetland at the north end of the site. The stormwater ponds will be designed to protect water quality and limit runoff rates from the site, and the project is expected to have only minor effects on downstream wetlands and the Mississippi River. Construction of a total of approximately 2.6 acres of stormwater ponds will be required as development occurs. The ponds will be designed in accordance with NURP (Nationwide Urban Runoff Program) and be designed to handle a 100-year storm event. In a 100-year event, stormwater will be retained in the basins and discharged at or below existing levels. NURP guidelines indicate that ponds designed in accordance with Walker (Walker, W.W. 1987. Phosphorus Removal by Urban .Runoff Detention Basins. Lake and Reservoir Management: ~olume I11. North.American Lake Management Society) have: 1. A permanent pool (dead storage) volume greater than or equal to the runoff from a 2.5 inch st°rm over the entire contributing drainage area assuming full development. 2. A permanent pool average depth that is > 4 feet, with a maximum dePth of < 10 feet. 3. An emergency spillway adequate to control the one percent frequency/critical duration rainfall event. 4. Basin side slopes no steeper than 4:1, with a 10:1 bench for the first one foot above the normal water level. 5. Maximized distance between basin inlets and outlets. Page 10 W Westwood Professional Services, Inc.