2009 - Illinois DNR - State of Illinois

normal 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.Combined additional flood storage at various sites in the watershed created by thisalternative, would provide $121,444 in average annual flood damage reduction benefitsin the watershed representing an 84% reduction in average annual flood damages. Thisalternative would benefit all 96 floodprone structures in the watershed. Existing flooddamages along Lower Farmers Creek would be reduced by 72%. Existing flooddamages along Upper Farmers Creek would be reduced by 77%. Existing flooddamages along Prairie Creek would be reduced by 87%. This alternative wouldeliminate flood damages to 46 structures currently impacted by the 100-year frequencyfloodplain, including 7 of the 8 floodprone structures around Lake Mary Anne.Project costs of this alternative are estimated at $3,832,341, including $1,233,084 inestimated land rights costs, as detailed on Table 17. Although this alternative eliminatesmany of the flood damages in the watershed, the benefit to cost ratio (B/C ratio) for thisalternative is 0.50 due to many overlapping project benefits for the same floodpronestructures.55