Numerical feasibility study for dredging and maintenance alternatives in Everett Harbor and Snohomish River

Snohomish River, which supports the third largest complex of tidally flooded estuarine and freshwater wetlands in Puget Sound, Washington, USA, delivers a large amount of sediment load to the lower estuary including the federal navigation channel and the Everett Harbor. In recent years, progressive sedimentation and diminished dredged material placement site capacity have led to economic impacts, particularly near the local service facilities maintained by the Port of Everett. An assessment of the performance of existing navigation features including jetties, dikes, and sedimentation basins was conducted using the latest generation of hydrodynamic and sediment transport numerical models to inform long-term sediment and navigation channel management decisions. This study assessed the relative merits of available alternatives for improving dredging and maintenance needs using hydrodynamic and sediment transport models: Finite Volume Community Ocean Model (FVCOM) and Adaptive Hydraulic model (AdH). In this paper, we present the refinement, setup, and calibration of morphological models using available data collected from the years 2017 and 2018 with a focus on in-channel sedimentation basins and the Port of Everett marinas. The potential for reduction in sedimentation through site-specific structure construction and repairs was evaluated. The results show that while it is feasible to modify in-channel sedimentation through river training structures, deposition at off-channel marinas along the banks and shelves may require sediment exclusion alternatives such as curtains and walls. Construction of training structures at the boat launch and reconstruction/repair of an existing training structure near the Port of Everett marinas provided promising response and merits further exploration. This study demonstrated that sediment transport simulation provided valuable information on the efficacy of proposed solutions and could help avoid unnecessary costs with alternatives likely to fail or be ineffective.