09A,B,C - Permits, Correspondences, Potential Violations/Enforcement Actions

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Babinski Site Plan February 28, 2005 Page 3 of 4 stormwater basins). Typically,we would expect infiltration basins of this type to contain adequate storage volume for a 100-year, 24-hour storm event. Since the exact shape of the basins is not specified, we assumed that the basin boundaries on the site plans indicate their top contours. We also assumed 4 horizontal to 1 vertical slopes to their specified bottom elevations. With these assumptions, the —' two basins contain approximately 20.1 acre-feet of stormwater storage. _ Expected maximum runoff for the site under a 100-year,24-hour storm event per our calculations is 20.1 acre-feet. The basins, under the assumptions we have listed above, appear to be sized properly for the site. The south basin, according to the contours shown, implies there will be a berm separating it from the rest of the natural depression in its area. It will be necessary to construct this berm to .. retain storm water on the site, as the existing natural depression spans at least one of the adjoining properties. From an analysis of aerial topographic mapping of the area,under more severe storm runoff conditions, it appears that the south basin will overflow to the south,through a number of natural depressions, and eventually into Sibley Lake. The north basin will overflow onto adjoining properties to the north of the site. Overflow to the south,which appears to travel through undeveloped property and along a more or less natural drainage path to Sibley Lake, appears to be much preferred to overflow to the north. A length of what appears to be storm sewer piping is indicated on the site plans. It is labeled _ "Future Storm Sewer If Needed". We recommend that this storm sewer be installed as part of this project,to be used for directing overflow from the north basin to the south basin, and thereby away from developed property and eventually into the natural drainage to Sibley Lake. This would occur only under severe flood-type conditions, greater than the 100-year conditions used to size the basins. .. MPCA Stormwater/SWPPP Requirements The MPCA requires that a storm water pollution prevent plan(SWPPP)be submitted along with the permit applications for a project of this size. If approved,the permit requires certain specific measures to be in place during, and for a period after, construction on the site. These would include,but not be limited to, such items as silt fences,temporary mulch and seeding, or erosion control blankets. When used in accordance with best management practices,these measures are �- intended to prevent erosion and sedimentation and contain these and other contaminants created during construction on the site. An approved SWPPP contains provisions for inspection and monitoring of the best management practices to be used on a given project. While we have not seen copies of the permit applications or the associated SWPPP for this J:\130PQLAK\0000-GENERAL\CORR\DAWN BrTTNER-REVIEW OF BABINSKI SITE PLAN-022805.DOC