2009 - Illinois DNR - State of Illinois

water in the sewer to back flow into the new 60-inch diameter culvert and flow to thereservoir on the high school property. This Alternative is intended to greatly reducepeak flows contributed by the Dempster Avenue storm sewer and would control 15% ofthe Prairie Creek watershed.Belleau Lake would be lowered 4.2 feet by gravity through a 24-inch diameter outletpipe at the invert of Farmers Creek at elevation 622.0. This pipe would be gated to allownormal flows to bypass the lake in Farmers Creek. Steep side slopes, exposed along thewestern edge of the lake by lowered water surface elevations, would be graded at 3:1slopes for safety and stability. When stages in Farmers Creek reach elevation 626.0,excess flows would enter the lake over a spillway constructed between the lake andFarmers Creek. The lowered lake would be capable of providing up to 52 acre-feet ofadditional flood storage capacity at the 100-year frequency event.Farmers Creek would be widened to 30 feet between the Farmers and Prairie Creeksconfluence and Belleau Lake. The Dempster Avenue bridge and a private bridgeupstream of Dempster Avenue would be removed and replaced with twenty foot wideCon Span bridges. The Rand Road crossing of Farmers Creek would have an identicalthird barrel added to the existing double 10 foot by 10.5 foot box culvert structure.The existing Rancho Lane Culverts would be replaced by two 7 by 6 foot box culverts.This would relieve the restriction in the flow in the current configuration. The channelabove and below the culvert would be widened to make this reach of Prairie Creek matchthe approximate size of the creek upstream and downstream of this reach.This combination of alternatives would create a project that would provide $92,263 inaverage annual flood damage reduction benefits representing an 64% reduction inaverage annual flood damages in the watershed. Existing flood damages along PrairieCreek would be reduced by 90%. Existing flood damages along Lower Farmers Creekwould be reduced 73%. Existing flood damages along Upper Farmers Creek would bereduced 69%. This alternative would eliminate flood damages for 49 structures currentlylocated in the 100-year frequency floodplain, including most of the floodprone structuresaround Lake Mary Anne on Farmers Creek. Project costs of this alternative areestimated at $4,461,346 including $816,668 in estimated land rights costs, as detailed onTable 32. Although this alternative eliminates many of the flood damages in thewatershed, the benefit to cost ratio (B/C ratio) for this alternative is low (0.31) due tomany overlapping project benefits for the same floodprone structures.120