Turbulenzgesteuertes Erosionsverhalten von Wattsedimenten am Beispiel des Rückseitenwatts der Insel Spiekeroog– Untersuchungen mit hochauflösender Sonar-Technik

This combined flume and field study applies the turbulent kinetic energy (tke) approach to the determination of bed shear stresses ( ) on natural sediments from several sites in the backbarrier tidal flats of Spiekeroog in the East Frisian Wadden Sea, Southern North Sea. Estimations of tke and have been made using Acoustic Doppler Velocimeter (ADV) data in order to determine erosion thresholds under unidirectional and orbital (wave) currents. A prerequisite for the determination of any of the erodability and sedimentation parameters is that the actual bed level changes at the measuring site are known in detail. This information is obtained by an Acoustic Backscatter system (ABS) with 4 transducers (1, 2 and 4 MHz) used as a multi frequency echosounder and a side scan sonar. For the field measurements two different hydraulic situations have been observed: tide dominated and wave dominated conditions. Under calm weather conditions tide induced current velocities up to 20 cm/s did not lead to local sediment erosion. Critical values of tke and bed shear stress from flume experiments were not exceeded. Highest suspension rates occurred at the beginning and the end of the flooding period. At low water depths high turbulent mixing caused resuspension events of mud size particles only. Under rough weather conditions high tke values and bed shear stresses due to the presence of wave induced currents caused high resuspension rates during high water. The acoustic data were able to show the relation between local erosion events and peak values of the turbulence parameters. An additional aim was to describe the influence of biofilms on erosion thresholds under different seasonal and weather conditions. High biological activity during the summer months may increase the critical shear stress so that stabilization effects occur during calm weather periods. But under stormy conditions, especially during winter months, physical parameters, especially under waves, dominate the transport processes and biofilms are easily eroded. So the local fetch and resulting wave heights and orbital velocities are the driving forces for the remobilisation of sediment on the sheltered tidal flats.