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I I I I I I I I I I I I I I I I I I I 5.2.2 Assumptions A number of assumptions were made during the conceptual alternative stage. These included: . Use of the 100-year discharge for design. . Improvements from the U.S. 40 bridge upstream will be coordinated to avoid generating some type of system-wide instability. . The U.S. 40 bridge will be protected, and will act as a grade control for the reaches immediately upstream. . Some excavation of the two large point bars that exist immediately upstream of U.S. 40 and opposite the sharp bend would take place. 5.2.3 Bed Stability Measures Several alternative measures are available for stabilizing the channel bed. These measures include drop structures, grade control, or channel armoring, and would be used to reduce or limit the amount of erosion or degradation of the channel bed. Drop structures are used to effectively reduce slopes by providing energy dissipation along steep reaches that have high erosion potential. Grade control structures include construction of a hard point or sill that will effectively resist erosive forces and control the grade or control of the channel bed. These structures may be used to limit headcut migration upstream, or maintain the grade of a channel experiencing degradation. Two types of grade control structures were considered for this study. The first is a pre-formed boulder grade control that would extend across the channel and into the adjacent bank a distance of 20 ft. This concept is illustrated in Figure 5.1. It anticipates that the channel bed will degrade as much as 2 ft. Boulders would provide the drop with a plug of smaller (Type M) riprap across the front to protect against scour, and provide the opportunity for some habitat development while providing armor protection. The structure would be pre-formed, and could either be backfilled, or left to fill in naturally. This would allow the existing channel bed to flatten to around 0.004ftJft through this reach. The second grade control concept is a riprap grade control, and is illustrated in Figure 5.2. This concept provides a trench across the channel that is filled with a riprap volume adequate to protect against anticipated degradation and local scour. This measure relies on the natural degradation processes to launch the stone as needed for protection of the bed. It is anticipated that some type of grade control will be necessary upstream of the sharp meander bend between Sections 5 and 6. This would provide protection for the upstream area by maintaining the bed elevation and help guard against mass bank failure that could result from significant channel degradation. Copies of backup calculations for the grade control measures are included in Appendix J. 30