Tide- and wave-generated sediment transport in a shallow tidal channel and adjacent tidal flat

Alyson Day

Tidalflats occur in low energy coastal environments where mobile sediments are plentiful, water depths are shallow, and large breaking waves are absent. Sediment accretion and erosion in tidal flats and other shallow estuarine environments creates essential habitat for shore birds and juvenile fish, transports nutrients from upland sources, and can undermine engineered coastal structures like jetties and sea walls. To examine the processes responsible for transporting sediments and shaping these remarkable environments, we present field observations from a small (~30 m wide, 1.5 m deep) tidal channel and adjacent shallow, sandy tidal flats. Data collected near the north fork of the Skagit River estuary in June of 2009 included GPS bathymetric surveys, water velocity profiles at eight locations, water salinity, water temperature, and wind velocity. Calculations made from bathymetric surveys indicate that, between June 25 and 30, erosion along the channel edge removed approximately 27 kg of sediment across a 10m transect spanning the flats and channel, per meter of channel length. Sediment transport was estimated by combining standard models (Meyer Peter and Muller, 1948, and Bagnold, 1966) with flow velocities measured along an across channel transect. Near the channel iv edge, simulated across and along channel fluxes had comparable magnitude, and channel edge erosion was consistent with simulated net across channel flux away from the channel edge. In contrast, in the deepest part of the channel, simulated along channel fluxes dominated over across channel fluxes, and measured channel edge erosion was not consistent with the simulated net across channel flux away from the channel edge. Modeled fluxes revealed the importance of waves to sediment mobility in this environment, despite sheltering by nearby saltmarshes. The importance of waves persisted even during a case of moderate offshore directed winds when fetch was less than 40m. When averaged over many tidal cycles, simulated net transport of coarse silt was onshore over the flats, whereas simulated net transport of coarse sand was offshore. Therefore, the fine sand present on the flats had a grain size near a balance point between simulated offshore and onshore transport.

Tide- and wave-generated sediment transport in a shallow tidal channel and adjacent tidal flat

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