Hydrodynamic and Sediment Transport Analysis for Cottonwood Island Restoration Assessment

Salmon Recovery Funding Board Grant through Skagit Watershed Council: $50,000 from 2007 to 2008

Background and motivation: Cottonwood Island is located near the bifurcation of the Skagit River into the North and the South Forks, approximately 10 river miles upstream from the mouth of the river. This area is a riverine tidal and floodplain forest zone that is influenced by tide and is periodically flooded. Historically, the Skagit River meandered around the west side of Cottonwood Island until 1889, when the existing levee was relocated to the present location. This relocation constrained flow and developed a complex cut-off channel to form the present river. Consequently, the old west channel was blocked because of sedimentation at the head of the channel, and in its current state, it does not allow flow to pass when the river is low.

Objective: The overall objective was to evaluate the feasibility of restoring the historic connection around Cottonwood Island such that the fish have access to refuge in the west channel habitat. The specific task was to evaluate whether the proposed restoration of Cottonwood Island is feasible with respect to hydrodynamic and sediment-transport impacts at the restoration site. For this study, a 3‑D hydrodynamic model was developed for the Cottonwood Island restoration site using FVCOM. The model was calibrated with field-observed tide, current, and salinity data. The wetting and drying process in the Cottonwood Island and the overbank regions was successfully simulated in the model. The calibrated model was then used to simulate flood inundation, velocity distribution, and sedimentation in the study area for the existing conditions as well as for the restoration alternatives.

Project Team: Pacific Northwest National Laboratory (Battelle), Skagit Watershed Council, Seattle City Light, and Washington State Department of Fish and Wildlife

Project Highlights: The study found that Cottonwood Island is partially or completely inundated only during flood events such as 10-yr and 25-yr flood flows. Under average- and low-flow conditions, the west channel remains dry.

• Tidal inundation in the large tidal flats region in the bay and estuary mouth was successfully simulated using the wetting/drying feature of the model.
• An alternative design that uses dredging provides the least costly means of restoration. It is especially attractive because it does not require a dike setback. It also reduces the sedimentation at the entrance of the west channel more than other alternatives
• The results indicate that habitat functionality may be successfully restored in the west channel, with respect to flow connectivity at low-flow conditions, by dredging the channel up to an optimal elevation for juvenile fish habitat, just below the low-water level.