May 15, 2020

Hydrologic and Hydrodynamic Modeling of the Skagit River Estuary - Rawlins Road Restoration Feasibility Study


The Skagit Watershed Council (SWC) initiated the Rawlins Road Restoration Feasibility Study to evaluate the potential for improving habitat for migrating salmonids along the bayfront region of the Skagit River delta. The bayfront region of interest includes the mudflats and marsh habitat located adjacent to the dike on the seaward side. Fir Island is the region of the Skagit River delta enclosed inside a long perimeter dike that was constructed nearly 150 years ago for agriculture and flood protection. Historically, the Skagit River delta has provided rich estuarine and freshwater habit for salmon and other fish and wildlife. However, construction of the perimeter dike around the delta isolated Fir Island between the North and South Forks of the Skagit River and eliminated the pathways of freshwater and natural sediment to the mudflats and tidal marsh areas.

In a collaborative effort between the affected parties represented by the Western Washington Agricultural Association and the project sponsors including the Washington Department of Fish and Wildlife, Seattle City Light, and Skagit River System Cooperative in a technical advisory role, SWC selected the northwest corner of Fir Island as the target of this feasibility study. The region north of Browns Slough and around Rawlins Road tide gate became the focus of the Rawlins Road Restoration Feasibility Study. The expectation was that restoration alternatives in this region would have minimum impact on the agricultural lands, allow repair of the leaking tide gates, and provide additional benefit of improved drainage along with restoration of habitat.

Battelle’s Marine Sciences Laboratory (Battelle) conducted a hydrologic and hydrodynamic assessment in support of SWC’s efforts to evaluate the feasibility of achieving restoration goals through modifications of the Fir Island dike near the Rawlins Road project site at the northwest corner of the delta. The study had four major goals that included restoring marsh habitat, improving salinity conditions in the nearshore habitat, increasing conveyance and passage, and minimizing impact on current land use. In addition, the project team saw this as an opportunity to improve drainage conditions in the farmland interior of the Fir Island dikes.

Battelle developed predictive numerical models for the Skagit River estuary, Skagit Bay and the Fir Island watershed associated with the Rawlins Road study area. The hydrodynamic model was constructed using the Finite Volume Coastal Ocean Model (FVCOM) developed by the University of Massachusetts. The hydrodynamic model was driven by a combination of tides, freshwater discharge, and surface-wind stresses. The model was set up using observed tide, current, and salinity data collected for this study for a period of 14 days. The hydrologic models included the U.S Army Corps of Engineers HEC-HMS model to provide runoff from rainfall and the UNET model, which routed flows in the agricultural drainage canals interior to the Fir Island dikes. Both models were calibrated to data collected in the summer of 2005. The models were then applied for five different alternatives that looked at crossisland diversions, breaches, and dike removals near the Rawlins Road study area.

Published: May 15, 2020

Yang Z and T Khangaonkar. 2006. Hydrologic and Hydrodynamic Modeling of the Skagit River Estuary - Rawlins Road Restoration Feasibility Study. Battelle PNWD-3692,prepared for Skagit Watershed Council, by Battelle Pacific Northwest Division, Richland, WA.