In situ estimation of erosion and deposition thresholds by Acoustic Doppler Velocimeter (ADV)

Field-based estimations of bed shear stress have been made using SonTek/YSI 10 MHz ADVs (Acoustic Doppler Velocimeter) at the Kongsmark mudflat, Danish Wadden Sea, in order to test if it was possible to estimate erosion and deposition thresholds in situ by use of unidirectional tidal and orbital (wave) currents.

The results were promising and erosion thresholds were in the same range as those observed in EROMES erosion experiments carried out at the same site. Similarly, the short-term erosion rates which could be calculated matched closely those obtained with EROMES, and were in the same range as those published for a large annular flume (Sea Carousel) from another fine-grained site.

This indicates that the erosion rates obtained with those two erosion-instruments are reasonable estimates of the actual erosion taking place under natural tidal current and waves. One advantage of the use of ADVs to characterize sediment erodibility is that measurements can be carried out for long periods, which makes it possible to examine the temporal variation of erosion threshold, erosion rate and sedimentation.

The maximum bed shear stresses, which was observed during a strong storm in January 2005, was only 0.7 N m(-2) which is three to four times lower than the highest thresholds measured by EROMES experiments On sediment with biofilms during previous studies. This shows that such strong biofilms at this site will not get eroded unless degraded or physically disrupted by some other mechanism.

A prerequisite for the determination of any of the erodibility and sedimentation parameters is that the actual bed level changes at the measuring site are known in detail. This information is obtained automatically by the ADVs used in the present study. Accurate estimates of the bed shear stress under waves in very shallow water are difficult to obtain and, in the present study, a simple method based on instantaneous velocities and assumption of a logarithmic velocity profile was used.

Further field-studies are required of the boundary layer under these extreme conditions in order to refine this simple method of calculating bed shear stresses.