5.4. SR 06-05-2006 - Laserfiche WebLink

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Minnesota Pollution Control Agency April 13, 2006 Ms. Rebecca Haug Environmental Administrator City of Elk River 13065 Orono Parkway Elk River, 1v1N 55330 4p;r I -I 2008 Dear Ms Haug: The Environmental Assessment Worksheet (EA W) on the proposed Liberty Heights Estates residential development has been reviewed by :M:PCA staff. We have the following~.omment~. . Item lO-A rough calculation identifies that approximately 33 percent of the site is to be converted to impervious surface. There is no information provided as to the total size of the subwatershed this project is located in. Ho\vever, there is a substantial body of information gathered nationwide over the last decade or so indicating that water quality impacts in streams begin to appear \vhen the impervious surface area exceeds about 10 percent of the watershed area for a particular stream. As the impervious surface coverage reaches 25 percent or so, water quality degradation becomes evident, and as impervious surface coverage exceeds about 50 percent, stream ecosystems essentially disappear and the remnants become an urban drainage system. The deleterious effects include sedimentation, bank instability and changes in aquatic in\rertebrate and fish community structure, as well as nutrient and pollutant delivery. Imperviousness similarly affects lakes and wetlands. The contours on the U.S. Geological Survey quadrangle map (Exhibit B) show Twin Lakes do\Vl1 gradient of the project site. What controls \ViII be used to protect these resources from the increased impervious surface stormwater runoff? Section 17- This section states, "Peak discharge rates are likely to increase. Stormwater detention ponds will be constructed to mitigate quantity, quality and discharge rates." As a point of information, the following is taken from "Protecting Water Quality in Urban Areas," MPCA's Stormwater Best Management Practices Manual: "By reading the output from models such as TR20, \ve see that ponds can reduce peak discharges to less than the predevelopment rate. But ponds do not usually decrease the total ,rolume of runoff. Analysis at most sites clearly sho\vs an increase in runoff volume from predevelopment conditions, even \vhen the peak discharge from developed conditions is held to predevelopment peak discharge rates or less. This is because the duration of discharge is ex'1ended proportionately. Since 1.3- to t1vo-year events are approximately bankfull, and bankfull events are usually the most erosive per unit offlo\v, this increase in duration can significantly increase erosion problems for the do,vnstream ,vatershed, even for small storm events." Please clarify how these ponds are "expected to fully mitigate the impacts to the receiving streams" and please identify what the receiving stream is. This item only identifies '\vetland basins and ditch systems" as receiving water bodies. 520 Lafayette Rd. N.; Saint Paul, MN 55155-4194; (651) 296-6300 (Voice); (651) 282-5332 (TTY); www.pca.state.mn.us 81. Paul · Brainerd · Detroit Lakes · Duluth · Mankato · Marshall · Rochester · Willmar Equal Opportunity Employer. Printed on recycled paper containing at least 20 percent fibers from paper recycled by consumers.