Modeling Transport of Disposed Dredged Material from Placement Sites in Grays Harbor, WA

The US Army Corps of Engineers Seattle District (NWS) is addressing short- to mid-term dredge material management strategies for the Federal Navigation Project at Grays Harbor, Washington. This includes evaluating navigation channel realignment in the Point Chehalis/Entrance reach. It is hypothesized that relocating the channel to align with natural channel migration will reduce future annual dredging quantities. However, the most heavily used dredged material placement sites lie in proximity to the federal navigation channel. Thus, the goal of this numerical modeling study was to assess the impact of the three dispersive dredged material placement sites on future channel maintenance dredging. The modeling study simulated tidal and wave-induced sediment transport in the Grays Harbor estuary using a surface water modeling system, EFDC, under four different hydrodynamic and wave forcings that corresponded to 0.5-, 2-, and 5-year events and the most extreme event from a 38-year wave hindcast record. A coastal wave model, CMS-Wave, was used to simulate wave conditions in the model domain, and a regional circulation model, ADCIRC, was used to provide the clamped tidal boundary conditions along the ocean boundaries of the EFDC model domain. Model results using both the existing and realigned channel configurations were analyzed to estimate residence times of dredged placed sediment at the three placement sites and channel infilling rates from sediment eroded from the placement sites for the existing and realigned channel configurations. The findings from the analysis indicated that there were significant differences in the erosion rates of the dredged material between the existing and realigned channel simulations at one of the three placement sites, and that there were no significant differences in the fate of the eroded sediment between the existing conditions and realigned channel configuration.