DESCRIPTION
Walla Walla County Conservation District used this grant to fund a geomorphic assessment of a reach of the Touchet River from ½ mile above the town of Waitsburg, WA to the Hwy 124 Bolles Bridge (river miles 40-45). The assessment examined channel, vegetation, and floodplain conditions. The upper part of this study reach is leveed for to protect the town of Waitsburg and other features along the river. The Highway 12 and Main Street Bridges cross the river along this reach. Generally, the levees transport sediment through the channelized reach, allowing for little deposition, and sediment is deposited in downstream areas, past the confluence with Coppei Creek. The excess accumulation of sediment in areas downstream of the levee system generally has caused higher channel width to depth ratios, leading to extensive bank instability, and a higher probability of channel migration.
Objectives of this study were to provide recommendations for accommodating human uses along the Touchet River, while also suggesting ways to improve channel and riparian health and river stability through active restoration and river management. The assessment proposes several areas for restoration, including these activities: bank stabilization, bar stabilization, creation of a bank-full bench, relief channels for routing high flows around or away from structures, or other treatments. Restoration designs are consistent with geomorphic information that has been gathered at each site. Information on pool and riffle dimensions, total scour depth, and meander shape determine the appropriate restoration design. Total station surveys, information from LIDAR imagery, and hydraulic and sediment modeling information have also been used to determine restoration designs.