Effects of Wetland Restoration on Floodplain Hydrodynamics under Extreme Flooding Conditions

Past dike placement for agriculture has considerably altered the Grays River, Washington, a tributary to the Columbia River, and impaired anadramous salmon habitat. To improve floodplain connectivity, a wetland restoration project was implemented during the summer of 2005 at the Kandoll Farm restoration site (KFS) along the Grays River. This project included breaching the dikes at three locations, installing large culverts at the head of Seal Slough, and altering the access road elevations. The following winter, in January 2006, high flows in the Grays River produced extreme flooding conditions, overtopping dikes along the river and roads upstream of the KFS. We developed a two-dimensional unstructured-grid hydrodynamic model to evaluate the flooding conditions prior to restoration construction (Pre) and following restoration construction activities (Post).

In our analyses at the time of peak water surface elevations (WSE), we found no reduction in peak WSE at the KFS in comparisons between the Pre and Post configurations, and the Post analyses showed a slight increase in WSE within the Grays River channel in comparison to the Pre analysis. Following the passage of the January 2006 flood, the Post WSEs within the KFS and in the main river channel were lower than the Pre WSEs. The velocity at a cross section of the Grays River upstream of KFS did not change between the Post and Pre configurations, but within KFS and at a cross section downstream, there were significant differences in velocities. We attributed the changes to better drainage from the KFS and improved connectivity with the main river channel resulting from the restoration project.