2009 - Illinois DNR - State of Illinois

Dempster Avenue storm sewer, tributary to Prairie Creek, on the Lutheran GeneralHospital property, north of Dempster Avenue. The existing Dempster Avenue stormsewer system would be restricted to a 24-inch diameter orifice to force excess stormwater in the sewer to back flow into the new 60-inch diameter culvert and flow to thereservoir on the high school property. In this alternative, the High School Reservoir isnot only intended to reduce peak flows contributed by the Dempster Avenue stormsewer, but to provide additional storage capacity for Lutheran General Pond when floodstages in that pond exceed elevation 636.0 by accounting for overland flow that wouldback up through the existing outlet pipe at the water tower and back through the junctionbox into the storage reservoir.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.A 60-inch pipe would be routed down Dempster Avenue to divert the existing DempsterAvenue storm sewer, that currently outlets into Prairie Creek downstream of LutheranGeneral Hospital, to a point on Farmers Creek just downstream of Dempster Avenuenear the Tollway.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 $126,493 inaverage annual flood damage reduction benefits representing an 88% reduction inaverage annual flood damages in the watershed. The greatest portion of these benefits(46%) would be realized in the Lower Prairie Creek portion of the watershed. Existingflood damages along Lower Farmers Creek would be reduced 70%. Existing flooddamages along Upper Farmers Creek would be reduced 74%. Existing flood damagesalong Prairie Creek would be reduced by 97%. This alternative would eliminate flood80