Geomorphic Evolution of a Levee Setback in a Gravel–Sand Channel in Washington State

Levee setbacks can create space for side channels, reduce flood impacts, and provide environmental benefits. Guidance is needed on how side channel design affects geomorphic changes over time. This paper explores application of existing research on natural and forced side channels to a levee setback with side channel on the White River (Washington). The side channel maintained a high bifurcation angle into the setback for 2 years. High flows then reset the bifurcation to a lower angle. Flow and sediment volumes and deposition patterns from each 2-year period matched results from existing studies for either a high or low angle. Application of existing research insight into expected flows and sediment patterns was dependent on a static bifurcation angle. In this case study, because the entrance area did not restrict the bifurcation angle and the flow regime included high flows, the bifurcation angle could adjust to maintain the side channel. Designing a large setback entrance enabled the channel to self-adjust to maintain flow and sediment transport into the side channel without the need for engineering intervention.