Hydrodynamic Modeling Analysis of Skagit Bay – Fir Island Farm Restoration Project

Washington State Department of Fish and Wildlife: $37,000, from 2010 to Present

Background and motivation: The Fir Island Farm project site is located on the Skagit Bay front along the Fir Island dike in between Brown Slough and Dry Slough. Considering losses in saltwater marshes along the dike front, this project represents an important opportunity to restore and protect a portion of the habitat. The restoration would involve removing or breaching the dike along the bay front and installing tide gates to prevent tidal inundation of the farmland surrounding the project site. To evaluate potential effects on vegetation, groundwater conditions, flooding, habitat and effects on adjacent farms, the empirical site-data and study information would need to be augmented with more appropriate models showing hydrodynamic flow conditions of the tides, river, and slough. The proposed approach to utilize the 3‑D hydrodynamic model already developed by Pacific Northwest National Labs for the Skagit Bay area to provide data and information on tidal stages, flow rates and velocities, and salinity for existing and proposed conditions.

Objective: The overall objective for this project is to develop a high-resolution hydrodynamic model to cover Fir Island Farm in the Skagit River delta and use it to examine the restoration potential of the Fir Island Farm.

This study includes two major model-application tasks: (1) hydrodynamic modeling analysis of the baseline condition; (2) hydrodynamic modeling analysis of the preferred alternative (restoration) condition after selected dikes are opened or built. Simulated oceanographic properties in the restored condition will be compared with baseline conditions.

Project Team: Shannon and Wilson, Washington State Department of Fish and Wildlife (WDFW), and Pacific Northwest National Laboratory (PNNL)

Project Highlights and Expected Outcomes: The modeling study is expected to provide key hydrodynamic information on flow, water depth, velocity, and salinity around Fir Island Farm site under both existing (baseline) and the preferred alternative (restoration) conditions. The simulations will be conducted during a one-month representative flow condition for smolt outmigration and for peak habitat use in the estuary. The results are expected to provide important guidance to evaluate the restoration potential of Fir Island Farm.