Alongshore bed load transport on the shoreface and inner shelf

Abstract

Bed load transport rates on the shoreface and shelf are determined by tidal currents, wave-current interaction and grain size. There is, however, a strong lack of field data and validated models because bed load transport under waves cannot be measured in the field, while bed load transport by currents without waves commonly is barely measurable in spring tidal conditions. Herein, bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13-18 m in fine and medium sands at 2 to 8.5 km offshore the Dutch coast. Near-bed flow velocity was recorded at 2 Hz. The measurements are used to derive an empirical bed load model, in which transports are normalized by grain size and density. The model produces bed load transports that are at least a factor 5 smaller than predicted by existing models. However, they agree with a large laboratory data set of sand and gravel transport in currents near incipient motion. Cohesion of sediment due to mud in-mixing or biological activity was excluded. Including turbulence probabilistically in bed load models strongly improves predictions near incipient motion, and predict 20% more alongshore transport annually for currents only. The effect of wave-current interaction is predicted to be twice as large, and the combined effect results in 100% larger transports. The effect of wave stirring is gives much larger flood and ebb transports but the net transport is the same as for the combined wave-current interaction and turbulence case. An overestimation of the current velocity leads to much larger transports than any of the model combinations. Concluding, the effects of turbulence, wave-current interaction and wave stirring are of secondary importance compared to the choice of empirical or existing bedload predictor and the representation of the current climate