Data collected in the EU MAST project Circulation and Sediment Transport Around Banks (CSTAB) was analysed with the aim of predicting rates of non-cohesive SPM transport in complex wave-current conditions. The required time-averaged vertical distribution of sediment was derived from detailed measurements of turbulence, surface waves and sediments off the Belgian coast from POL's Sediment Transport and Boundary Layer Equipment (STABLE). Despite the complexity of the instantaneous time-series, the vertical profiles of time-averaged SPM concentration at a single location close to the sea bed. The model performed well over a broad range of conditions and gave in-situ values of grain settling velocity and the wave mixing coefficient in agreement with empirical predictions and theory.
Sediment re-suspension over bedforms was investigated in the Taw Estuary, Devon, using a state-of-the-art multi-frequency acoustic backscatter (ABS)
system (Figure 5), non-instrusively measuring vertical profiles of SPM concentration and particle size and resolving in detail turbulent re-suspension processes.
Electromagnetic and rotor current meters and pumped samples provided an independent check on the ABS results. High resolution profiles of sediment diffusivity, previously available only over flat rough beds in carefully controlled laboratory conditions were derived. Those over a rippled bed were similar to the parabolic form found in early laboratory studies of flow over flat rough beds, suggesting that simple models for SPM transport are applicable (Figure 6a). Over a sandwave, measured profiles were different (Figure 6b) and varied with location, suggesting that complex models accounting for the advection and horizontal variability of turbulence would be required to predict the SPM transport.
The work will continue in new EU MAST projects COAST3D and Inlet Dynamics Initiative - Algarve (INDIA).
Figure 4
a) Typical measured time-series of surface waves and concentration 2 cm above the sea bed of SPM and b) contrasting time averaged vertical profiles of SPM concentration under different wave-current conditions.
Figure 6
Vertical profiles of sediment diffusivity from measurements, a) over a rippled bed (normalised over the suspended sediment boundary layer thickness) and compared to a simple semi-empirical model (solid line), and b) over a sandwave trough.
Figure 5
Experimental apparatus including four current rotors, four electromagnetic current meters, four pump sampling tubes and three acoustic backscatter probes.
Last updated: 12th May 1998. Please send comments to A.Lane@pol.ac.uk