**Zhaoqing Yang, Taiping Wang, Dave Cline and Brian Williams**

**Abstract:** To re-establish the intertidal wetlands with full tidal exchange and improve salmonid rearing habitat in the Skagit River estuary, State of Washington, USA, a diked agriculture farm land along the Skagit Bay front is proposed to be restored to a fully functional tidal wetland. The complex and dynamic Skagit River estuarine system calls for the need of a multi-facet and multi-dimensional analysis using observed data, numerical and analytical methods. To assist the feasibility study of the restoration project, a hydrodynamic modeling analysis was conducted using a high-resolution unstructured-grid coastal ocean model to evaluate the hydrodynamic response to restoration alternatives and to provide guidance to the engineering design of a new levee in the restoration site. A set of parameters were defined to quantify the hydrodynamic response of the nearshore restoration project, such as inundation area, duration of inundation, water depth and salinity of the inundated area. To assist the design of the new levee in the restoration site, the maximum water level near the project site was estimated with consideration of extreme high tide, wind-induced storm surge, significant wave height and future sea-level rise based on numerical model results and coastal engineering calculation.

Reprinted from *J. Mar. Sci. Eng.* Cite as: Yang, Z.; Wang, T.; Cline, D.; Williams, B. Hydrodynamic Modeling Analysis to Support Nearshore Restoration Projects in a Changing Climate. *J. Mar. Sci. Eng.* **2014**, *2*, 18-32.

#### **1. Introduction**

Estuarine wetland provides important fish habitats for salmon during their juvenile rearing period before they migrate from freshwater to the saltwater environment [1]. However, population shift and land use change over the past century have resulted in a significant impact on the coastal ecosystem and the associated marine wildlife. Construction of dikes in estuarine and coastal systems for protection of agriculture land use has eliminated the natural tidal exchange to the agriculture farm land, altered tidal prism, and changed the sedimentation pattern in estuarine systems. The Skagit River, located in the Whidbey Basin, is the largest river in Puget Sound and is responsible for about 34%–50% of the total freshwater flow into Puget Sound (Figure 1). Over the past century, urban development and construction of dikes for agriculture land use have caused significant losses of nearshore habitat and impact on salmon migration in the Skagit River, Port Susan Bay and Lower Snohomish River in Whidbey Basin of Puget Sound. To protect and improve estuarine habitats that are vital to marine wildlife, many nearshore restoration projects are currently underway to restore estuarine habitats and improve fish migration pathway through dike breaches, setbacks, and removals in the Puget Sound region.

Coastal hydrodynamic models have been used extensively to support nearshore restoration actions and provide vital hydrodynamic information to guide the restoration engineering design in Whidbey Basin. For example, Yang *et al*. [2] applied a coastal hydrodynamic model to help select and design restoration alternatives in Skagit River Delta for improvement of estuarine habitats and salmon migration. Lee *et al*. [3] conducted a hydrodynamic and sediment transport modeling analysis to evaluate the feasibility of restoring natural fish habitat in Cottonwood Island, approximately 16 km upstream from the mouth of Skagit River estuary. Yang *et al*. [4] investigated the cumulative effects of multiple nearshore restoration projects on estuarine hydrodynamics in the Lower Snohomish River estuary, the second largest river in Puget Sound. Yang *et al*. [5] conducted a hydrodynamic and ecological assessment of a nearshore restoration project in Port Susan Bay in Whidbey Basin. Yang and Wang [6] simulated the drainage process of a restored intertidal wetland in the Snohomish River estuary using a high-resolution hydrodynamic model with spatial varying bottom roughness to better represent the high bottom roughness due to the presence of dense vegetation. An integrated modeling approach was taken to link the inundation process in the upstream river floodplain and the downstream intertidal zone for fish habitat restoration and flood management in the middle Skagit River, estuary and bay system [7].

**Figure 1.** Puget Sound estuarine system (**left**) and Skagit River Estuary (**right**). Red circle indicates the location of the restoration project site.

In this paper, a three-dimensional (3D) unstructured-grid finite volume coastal ocean model (FVCOM) was applied to simulate the tidal circulation in the Skagit River estuary and assist an estuarine restoration project in Fir Island, a large river delta bounded by the North Fork and South Fork of Skagit River. The numerical model was used to evaluate the hydrodynamic response of different restoration configurations and assist the selection of a preferred restoration alternative for

engineering design. A set of hydrodynamic parameters were calculated based on model results to quantify the outcome of nearshore restoration. To assist the design of a new levee, a practical approach based on numerical model results and coastal engineering calculation was proposed to estimate the maximum water level that accounts for the combined effects of extreme high tide, significant wave height, wind-induced storm surge and future sea-level rise as a result of climate change.
