Application of a Hydrodynamic and Sediment Transport Model for Restoration Feasibility Assessment at Cottonwood Island, Washington

A 3D hydrodynamic and sediment transport modeling analysis was conducted to evaluate the feasibility of restoring natural fish habitat in the Skagit River estuary, located in the northern Puget Sound, Washington. The Cottonwood Island restoration site is located where the north and the south forks of the Skagit River bifurcate approximately 10 river miles upstream from the mouth of the river. The study site is a riverine tidal and floodplain forest zone which is influenced by tide but not mixed with marine water, and is periodically inundated by flooding events. Fish habitat and migration pathway in the area was lost due to sedimentation in the channel around Cottonwood Island. Restoration actions were proposed to reconnect the channel sloughs to the main stem of the river to restore fish habitat. The analysis focused on estimating the potential for success associated with the proposed three restoration alternatives including dike setback and channel modification. In this study, the unstructured Finite Volume Coastal Ocean Model (FVCOM) was used to evaluate the effects of proposed alternatives on hydrodynamic response and potential channel re-sedimentation. The numerical model was calibrated with ADCP and CTD measurements. The calibrated hydrodynamic model was used to drive the sediment transport model to calculate sediment erosion and deposition patterns under different flow conditions. The model was applied to baseline and alternative conditions for 5 different flood hydrographs (1-yr, 2-yr, 5-yr, 10-yr, 25-yr). Model results for the alternatives were compared with baseline condition to assess the relative effects and potential re-sedimentation.