Sediment Dynamics Within an Urbanized Pacific Northwest Estuarine Embayment

Abstract:

Coastal sediment dynamics are crucial to understanding the transport and dispersal of contaminants in urbanized environments, predicting coastal resilience in the face of rising sea levels, and synthesizing integrated ecosystem responses. Here we present analyses of six months of observations recorded by three bottom-mounted moorings in Bellingham Bay, Washington, USA, sea-bed sediment samples obtained across the bay, and results from a high-resolution 3D coupled hydrodynamic and sediment-transport model of the bay and its fluvial sediment source the Nooksack River. Despite a large (> 4 y return interval) observed river-flood event in February 2020, water-column suspended-sediment concentration (SSC) near the bed was not correlated with Nooksack River runoff at sites 2–3 km from the mouth on the outer delta topset but was closely correlated with local wind-wave events. Water samples show strong suspended-sediment stratification within the water column during flood events. Model results indicate the Nooksack River produced an initial flood-event sediment delivery to the seabed that was concentrated around the river mouth; the plume was strongly influenced by wind forcing. Grain-size and geochemical (⁷Be) results from sea-bed sediment obtained across the bay shortly after the large flood indicate gradients of fluvial influence and dispersal. Studies of this type can help unravel outstanding ecological questions in urbanized coastal settings regarding the role of contaminants preferentially adsorbed to fine-grained sediment by comparing the remobilization of legacy contaminated sediment relative to modern fluvial sediment delivery. These results are being used to assess longer-term delta morphological evolution and to inform managers planning future coastal restoration efforts.