Hydrodynamic and Sediment Transport Analysis for McGlinn Island Causeway Habitat Restoration Feasibility Assessment
SRFB Grant through Skagit River System Cooperative: $150,000 from 2007 to Present
Background and motivation: Dredging of the Swinomish Channel began in the1890s to provide a navigation channel between Skagit Bay and Padilla Bay to facilitate commerce. Dredge spoils were used to build a causeway between La Connor and McGlinn Island to prevent transport of sediments from entering the channel via Dunlop Bay. A jetty was built in 1938 to train the river away from the navigation channel, which further restricted connectivity between the river and the Swinomish Channel. The causeway and jetty have restricted juvenile salmonid access to prominent rearing habitat available in Padilla Bay and have reduced freshwater input to the channel. This has greatly increased channel salinity and created a physiological barrier for juvenile Chinook, which are very sensitive to high salinity. Restoring river-channel connectivity is necessary to allow juvenile salmon access to rearing habitat in Padilla Bay.
Objective: The overall objective of the McGlinn Island causeway feasibility study is to evaluate the potential of restoring historical connectivity between the North Fork of the Skagit River and the Swinomish Channel, which connects Skagit and Padilla Bays. The specific objective of the Pacific Northwest National Lab modeling task was to conduct hydrodynamic simulation of the proposed restoration alternatives to assess whether expected benefits, such as creation of a brackish environment and the desired level of connectivity for downstream fish migration, could be achieved.
Project Team: Pacific Northwest National Laboratory and Skagit River System Cooperative.
Project Highlights and Expected Outcomes: A hydrodynamic model of Skagit Bay, Swinomish Channel, and Padilla Bay was refined to include small-scale geometric features associated with the existing river-training structures at the site, including the jetties and the dikes. The model reproduced existing hydrodynamic conditions and is being used to evaluate two alternative restoration concepts. Concept 1 involves lowering a river training jetty from McGlinn Island across the river channel. Concept 2 consists of creating a causeway and a dredged channel connecting the North Fork of the Skagit River to the Swinomish Channel through Dunlop Bay. Preliminary highlights are as follows:
- Both alternatives provide direct connectivity between the North Fork of Skagit River and the Swinomish Channel, allowing a migration pathway for juvenile Chinook to Padilla Bay.
- The salinity reduction benefit was shown to be modest for both alternatives but reasonable considering that the analysis was conducted during low flow conditions.
- The impact of the proposed alternatives on transported sediments and maintenance dredging requirements is being evaluated.