SIMULATION OF MORPHODYNAMIC ACTIVITIES DUE TO DENSITY EFFECTS IN CASE OF DREDGED MATTER DISPOSAL

摘要

The time dependant spreading of dumped matter was numerically simulated for a dumpsite in the partially mixed estuary of outer river Weser in the German Bight. A coupling of a hydrostatic threedimensional hydrodynamic model and a morphodynamic model was extended with respect to density-dependant momentum exchange. The simulations are based on substantial field measurements carried out by ship-mounted acoustic (ADCP) and optical attenuation and backscatter instrumentation. The dumpsite is located at the outer edge of the turbidity maximum. In addition to the tidal currents strong barocline stratification during the first half of the flood tide dominates the local hydrodynamic situation. The dumped matter was of mainly cohesive kind. The dumping of cohesive matter leads to density currents and stratification due to concentration gradients as well. Therefore the applied numerical morphodynamic model was extended to consider stratification effects due to local density gradients. The estuarine flow dynamics with respect to tidal velocities, barocline stratification and turbidity are well reproduced. A multi-fraction approach also allows to discriminate suspended load due to natural background turbidity from dumped matter. In combination with the field measurements the spatial and temporal distribution of the dumped matter is calculated. The observed and calculated spatial and temporal distributions of suspended sediment in the nearfield of the dumpsite are compared. The simulation results can be used to extend the spatial concentration distribution gathered by measurements.