Abstract
The 11 mile (1.6 km) Swinomish Federal Navigation Channel provides a safe and short passage to fishing and recreational craft in and out of Northern Puget Sound by connecting Skagit and Padilla Bays, US State abbrev., USA. A network of dikes and jetties were constructed through the Swinomish corridor between 1893 and 1936 to improve navigation functionality. Over the years, these river training dikes and jetties designed to minimize sedimentation in the channel have deteriorated, resulting in reduced protection of the channel. The need to repair or modify dikes/jetties for channel maintenance, however, may conflict with salmon habitat restoration goals aimed at improving access, connectivity and brackish water habitat. Several restoration projects have been proposed in the Skagit delta involving breaching, lowering, or removal of dikes. To assess relative merits of the available alternatives, a hydrodynamic model of the Skagit River estuary was developed using the Finite Volume Community Ocean Model (FVCOM). In this paper, we present the refinement and calibration of the model using oceanographic data collected from the years 2006 and 2009 with a focus on the sediment and brackish water transport from the river and Skagit Bay tide flats to the Swinomish Channel. The model was applied to assess the feasibility of achieving the desired dual outcome of (a) reducing sedimentation and shoaling in the Swinomish Channel and (b) providing a direct migration pathway and improved conveyance of freshwater into the Swinomish Channel. The potential reduction in shoaling through site-specific structure repairs is evaluated. Similarly, the potential to significantly improve of brackish water habitat through dike breach restoration actions using the McGlinn Causeway project example, along with its impacts on sediment deposition in the Swinomish Navigation Channel, is examined.
Highlights
The Swinomish Navigation Channel is located near the mouth of the Skagit River estuary, which is the largest river in the Salish Sea estuarine system consisting of Puget Sound, Strait of Juan De Fuca, and Georgia Strait (Figure 1a)
We present the refinement and calibration of the model using oceanographic data collected from the years 2006 and 2009 with a focus on the sediment and brackish water transport from the river and Skagit Bay tide flats to the Swinomish Channel
We demonstrate the feasibility and importance of using large-scale 3D hydrodynamic and sediment transport models with local refinements down to fine scale (10 m dike sections) for assessment of and design shoreline modification projects
Summary
The Swinomish Navigation Channel is located near the mouth of the Skagit River estuary, which is the largest river in the Salish Sea estuarine system consisting of Puget Sound, Strait of Juan De Fuca, and Georgia Strait (Figure 1a). The Skagit River estuary is a macro-tidal environment with a tidal range of 4 m and fluctuations around −1 to 3 m during spring tide and −0.5 to 2.5 m during neap tide relative to NAVD88 It has mixed semi-diurnal and diurnal tidal forcing and significant diurnal inequalities. The 2006 Skagit Chinook Recovery Plan identified a total of 26 nearshore restoration opportunities in the Skagit delta [4]. These projects strive to restore hydrologic and hydrodynamic functions in the tidal marshlands largely through shoreline modifications such as dike breaching, dike setback, and dike removal. The numeric criteria for restoration-site hydrodynamic performance are species specific, vary from site to site, and are typically categorized in broad rules such as (a) desired minimum inundation depth and frequency (e.g., >0.3 m and >50% of time); (b) salinity range (e.g., 5–15 ppt); and (c) peak velocity (e.g.,
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