Field Measurements and Modeling of Bed Load Transport on the Shoreface and Inner Shelf

Bed load transport rates on the shoreface and shelf are determined by tidal currents, wave stirring and grain size. In currents only, the shear stress is above the critical Shields parameter for incipient motion only for the strongest tidal currents. Turbulent fluctuations or wave stirring strongly increase the net bed load transport rate. There is, however, a strong lack of field data and validated models because bed load transports under waves cannot be measured in the field, and current-only bed load transport is barely measurable in spring tidal conditions. Here, bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 10-20m in fine and medium sands at 1 to 9 km offshore the Dutch coast. Flow velocity fluctuations were recorded at 2 Hz and are log-normally distributed. The measurements are used to derive an empirical bed load model, in which transports are normalized by grain size and density. This model produces bed load transports that are at least a factor 5 lower than existing models predict, but 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 stirring, however, is tenfold. Wave climate data, spectral analysis of measured time series and a shoaling model demonstrate that wave action at the bed diminishes significantly with increasing water depth for intermediate storms. Funded by EC MAST, MAS3--CT97--0086