River Hydraulics and Sediment Transport

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Table 5-26.-Channel Restoration Plans* Alternative Mechanical Action Total Acres Affected Locations by River Mile Governance Committee, Island clearing 29 acres 235, 212, 199, 192, 169, 162 Scenario 1 Governance Committee, Scenario 2; Water Leasing; Island clearing and leveling 417 acres 235, 226, 220, 212, 199, 169 and Wet Meadow Water Emphasis Island clearing and leveling 482 acres 235,192 *As noted in chapter 3, "Description of the Altematives," the locations of land management activities are displayed for illustrative purposes. Actual sites are not known since this is dependent on willing sellers. In the scenarios analyzed in this DEIS, the mechanical clearing, or clearing and leveling of islands, begins in Year 4 at the furthest upstream Program cross section. The work progresses downstream during 100day summer field seasons, reaching the last location at RM 162, 169 or 192 depending on the alternative, in Year 13. In addition to mechanical changes to the channel for the land plans that level islands, additional sediment input occurs at those locations during the 100-day summer field period in each year that leveling changes occur. Figure 5-17 shows an example of how a river cross section might be modified by the clearing and leveling of a river island. In this example, the vegetation would be cleared from a 750-foot- wide river island between two channels. The sand from this island would then be mechanically pushed, in stages, into the two nearby river channels. For this example, the water surface elevation of a 5,000-cfs flow occurring after the clearing and leveling activity would only increase by 035 feet, assuming that none of the cleared and leveled river island sand is eroded. The creation of a wider river channel would reduce average river velocities and the hydraulic capacity of the river to transport sand. This will help to offset channel degradation processes. If an area of river channel is widened too much, then sediment would begin to deposit in the over- widened reach, and vegetation would likely grow back. The increased river width that can be sustained over the long term, without vegetation clearing, depends on the annual peak flows, as indicated by the 1.5-year flow, and the prevention of channel degradation. After the clearing and leveling of selected river islands, the river island sand would be available for transport to downstream reaches. The sand transport rate through these widened reaches would be limited to the hydraulic capacity of the riverflows through these reaches. Some of the sand, from the cleared and leveled river islands, may be quickly eroded and transported downstream, while some of the sand may erode very slowly or not erode at all. The riverflow velocities and the sand transport capacities through reaches that are widened would be less than the velocities and transport capacities through the narrower reaches where no land management actions are taken. Therefore, sand that is transported from widened reaches would tend to remain