RESIDUAL TRANSPORT OF SEDIMENT FROM A PROPOSED DREDGE DISPOSAL SITE ON THE FRASER RIVER DELTA

An instrumented field study was conducted to systematically describe the mechanisms controlling sediment resuspension and the direction of suspended sediment transport in the northern section of Roberts Bank, the sandy intertidal portion of the Fraser River Delta. The study was completed in an area designated for the disposal of dredge material from Main Channel of the Fraser River in order to describe the fate of that material. Suspended sediment concentrations reflect both locally resuspended sediment and sediment advected from non-local sources; the contribution of locally resuspended sediment increases with increasing wave height. The locally resuspended sediment is modeled using the skin friction Shields parameter and is found to dominate the sediment transport at non-tidal frequencies. Wavelet analysis reveals that the residual transport at non-tidal frequencies is episodic and associated with the occurrence of low-pressure systems from the NW that drive wind-generated currents. The direction of residual transport at tidal-scale frequencies is determined by the relative timing of the wave activity relative to the tidal current. In contrast to fine-grained tidal flats, the resuspension of sediment from this sandflat does not increase throughout the tide and necessarily enhance the ebb transport. While the wind-generated currents are not capable of resuspending sediment alone, they have a relatively large impact on the residual transport since they are typically associated with storm waves.