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I I I I I I I I I I I I I I I I I I I back into the main channel at the downstream end of the project. Three trapezoidal channels with bottom widths of 10, 25, and 50 feet were evaluated. An ultimate condition trapezoidal chute channel with a 200-foot bottom width was also evaluated. Riprap inlet and outlet structures, designed for ultimate chute channel conditions, were also part of the pilot channel design. The purpose of the inlet and outlet structures is to regulate the amount of water that flows through the pilot channel, and to control the locations of the endpoints of the pilot channel. The pilot channel alignment, depth, and slope were designed as part of this analysis. Details regarding these designs are included in Appendices Band C. (2) Grade Control Structures. One riprap grade control structure was sized to meet the ultimate chute channel conditions. The structure was designed to prevent excessive bed scour and to help control the future width and alignment of the chute channel. The structure bottom width of 200 feet is consistent with the estimated ultimate chute channel width. Design of the grade control structure followed the methodology described in "Hydraulic Design of Flood Control Channels" (EM 1110-2-1601, US Army Corps of Engineers, 1991). Additional infonnation regarding the riprap design for the grade control structure can be found in Appendix C. (3) BackwaterlWetIand Complex. A backwater/wetland complex was evaluated to allow a hydraulic connection to the Missouri River at the southern end of the bend. It was designed to provide a minimum of 100 acres of wetted area at nonnal open water stages, An average depth of 7 feet would be maintained for 30 percent of the wetland area, while the remaining area would have depths of 0 feet to 4 feet during nonnal open water stages. Four islands totaling 10 acres would be created within the wetlands, with heights of 5 feet or less above nonnal open water stages. Side slopes of the islands would be no flatter than lOH to I V and no steeper than 5H to IV. This alternative would require the dredging and/or excavation of approximately 1.9 million cubic yards of material. The excavated material would be placed over a ISO-acre area within the site, with a maximum fill height of about 10 feet. c. Permits: Section 20 of this report is an evaluation of this project in accordance with 40 CFR 230, "Guidelines for the Specification of Disposal Sites for Dredged or Fill Material (Section 404(b)(1))" , All of the alternatives under consideration except for the No-action Alternative will require the following pennits or authorizations. A Section 404 pennit must be issued by the Corps of Engineers, Omaha District and a Section 401 water quality certification is required from the Nebraska Department of Environmental Quality (DEQ). A Section 402, National Pollutant Discharge Elimination System (NPDES) pennit, also issued by the Nebraska DEQ, will be required if the disturbance is larger than five acres. A floodplain alteration pennit will also be required from Nemaha County. 24