Modeled Sedimentation in the Lower White River Countyline Levee Setback, Washington State: Comparison of 1D (HEC RAS) and 2D (AdH) Results

The design of the Lower White River Countyline Setback Project in Washington State includes lowering an existing levee and constructing a new setback levee to allow the river to reconnect to an existing wetland. This study used two hydrodynamic and sediment transport models, Hydrologic Engineering Center-River Analysis System HEC-RAS, and Adaptive Hydraulics AdH, to model the river and compare the ability of one-dimensional 1D HEC-RAS and two-dimensional 2D AdH models to simulate hydraulic and sediment behavior in a levee setback project. Overall, both the 1D and the 2D model are well calibrated and indicate that the setback project will increase deposition within the reach. The spatial location of aggradation differs between the two models due to fundamental differences between the 1D and 2D approaches. The 1D model assumes that the river will avulse into the setback area and projects deposition in both the setback and former channel while the 2D model results show most of the aggradation occurring in the setback area. This study shows that, while 1D models can be valuable screening tools for levee setbacks, 2D models of setbacks should be considered when multiple channels may develop, there are lateral processes, or the difference between channel and setback gradations is important.