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Puget Sound Georgia Basin Model

Detailed Hydrodynamic Assessment of the Preferred Restoration Alternatives near Leque Island and zis a ba, Washington (Phase 3)

Map of Leque Island (left) and zis a ba (right).

Ducks Unlimited, October 2016 - December 2017

PNNL has supported Ducks Unlimited Inc. (DU) with hydrodynamic modeling for the Stillaguamish River estuary in connection with feasibility assessment for the Leque Island Restoration project since 2005. Hydrodynamic feasibility addresses the capability of the proposed actions to restore tidal functions such as periodic inundation, suitable currents, desired habitat/salinity levels, and examines sediment impacts, including the potential for excessive erosion or sedimentation requiring maintenance. A hydrodynamic model of the site, including Skagit Bay, Port Susan Bay, and the interconnecting region of Leque Island, was developed. The model was used to simulate tidal inundation, tidal currents, and salinity intrusion in the study area for the existing condition and to evaluate potential hydrodynamic changes following future restoration in Leque Island.

The configuration of the selected restoration designs at Leque and zis a ba sites were incorporated into the modeling framework with a high-resolution grid of selected regions. The model was applied with a focus on site-specific questions related to impacts of the selected alternatives on neighboring infrastructure such as dikes, outfalls, and pipelines with respect to inundation, sedimentation, and erosion. Results were based on a 2-week period from October 5, 2005 through October 26, 2005. The project assessed: (1) hydrodynamic response of the system for the proposed restoration alternative designs relative to existing conditions, (2) hydrodynamic response of the system for high flow condition, and (3) wave and storm induced exposure to extreme high-water surface elevations from Port Susan Bay. The results were examined relative to baseline at specified locations of interest.

Change in mean shear stress from the preferred alternative scenario to the baseline scenario.

Project Highlights

  • The hydrodynamic model was a useful tool in providing a broad understanding of tidal processes (flooding, ebbing, and salinity intrusion) in the Stillaguamish River estuary and near the study sites.
  • The preferred restoration scenario involving full dike removals at the Leque Island and zis a ba sites provided the desired response with sufficient inundation and dewatering without ponding or excessive risk of erosion.
  • From the assessment of wave and storm induced exposure to extreme high-water surface elevations, the full dike removal at Leque indicated a potential exposure to wave and storm induced surge for the City of Stanwood, north of Leque Island.
  • Model results helped alert the team and a detailed wave impact assessment was initiated. The results will dictate the need for a protective dike section along the southern bank of West Pass of Stillaguamish River.

Contacts and Project Team

Study Reports and Publications

Nugraha A, T Khangaonkar. In Preparation. Detailed Hydrodynamic Assessment of the Preferred Restoration Alternatives near Leque Island and zis a ba sites. PNNL-XXXXX, prepared for Ducks Unlimited by Pacific Northwest National Laboratory, Seattle, WA.

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