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Puget Sound Georgia Basin Model

Leque Island Restoration Feasibility Study, Washington

Leque Island Project Site - Stillaguamish River/Port Susan Bay

Ducks Unlimited, May 2007 - 2008

Ducks Unlimited Inc. (DU) has proposed restoration of nearly 50% of the Leque Island historic tidal marsh area, located near the mouth of Stillaguamish River in Washington. This area is currently shielded from tidal action by a dike, and proposed restoration would be accomplished by removing sections of dikes or dike setbacks along the South Pass and Davis Slough branches of the river near its mouth. The project would restore 115 acres of intertidal wetlands on the Leque Island wildlife area. The goal of the project is to restore fish access to the estuary habitat and to create a self-sustaining estuarine marsh area. Battelle was contracted by DU to conduct a hydrodynamic modeling analysis of Leque Island as part of the restoration feasibility study. The overall objective of the study was to evaluate whether the proposed restoration of Leque Island is feasible with respect to hydrodynamic and sediment transport impacts on the restoration site and the surrounding parcels of land.

For this study, a three-dimensional hydrodynamic model for the Leque Island restoration site was developed using the Finite Volume Coastal Ocean Model. The model was used to simulate tidal inundation, tidal currents, and salinity intrusion in the study area for the existing condition as well as for the restored the Leque Island condition, and to evaluate changes in hydrodynamics and circulation at the site. The potential impact on sedimentation in Leque Island under the restoration scenario was also evaluated based on predicted bottom shear stress distributions near the Leque Island restoration area.

Example of predicted surface salinity contours and velocity distribution

Project Highlights

  • Model simulations show that the tidal functions can be successfully restored at the Leque Island project site. The duration of tidal inundation in the restoration site would be about 4 to 7 hours per day because of the relatively high bottom elevation of the restoration site.
  • Salinity distributions within the restoration site would be in the brackish range because of the influence of freshwater discharge from the South Pass. However, a sharp salinity intrusion was shown to occur during high tide as the restoration area is inundated by the tide from Port Susan Bay.
  • Model results indicate that the bottom shear stress distributions in the restoration site are smaller than 0.1 Pa during most of the tidal cycle. Therefore, it is unlikely that erosion would occur in the Leque Island restoration site.
  • Because of the low estimates of bottom shear stress distribution at the restoration site, sedimentation could occur if the sediment source is available.

Contacts and Project Team

Study Reports and Publications

Yang Z, T Khangaonkar, and C Lee. 2008. Hydrodynamic Modeling Analysis for Leque Island Restoration Feasibility Study. PNWD-3813, prepared for Ducks Unlimited by Battelle, Pacific Northwest Division, Richland, WA.

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