ACOUSTIC OBSERVATIONS OF FINE SEDIMENT DISPERSAL AND DEPOSITION DURING DREDGED SEDIMENT RELEASE IN THE YANGTZE ESTUARY

摘要

ASSM-II Acoustic Concentration Profiler and Acoustic Doppler Profiler were deployed to concurrently observe the concentrated suspension dispersal provided by dredged material in the Changjiang Estuary. Field measurements were conducted at the flood, moderate and neap tides in June 2002, respectively. Results show: (a) Vertical profiles of suspended sediment concentration are serrate curves in three types, i.e. L-shaped, exponential and floating-like. Both deposition from suspension and horizontal advection of dense, high concentration layers contribute to the emplacement of cohesive sediment, (b) Two modes of sediment suspension dispersal and deposition coexist, i.e. the upper low-concentration plumes and the lower high-concentration density currents, and the latter is the major mechanism for suspension dispersal, (c) The low-velocity patches at the local transverse fields become weakly dispersed downstream with little vertical displacement at the moderate tide, whereas at the neap tide they rapidly move down to the bottom. The behavior of upper lutocline layers responds to that of the horizontal dispersal of the low-velocity patches, (d) Two kinds of internal waves are generated by the lutocline interfacial instability and the interaction of tidal flow with subaqueous topography, respectively. The former evolves along the flood currents from instability to stability according to calculated Richardson numbers, and the concentration profiles collapse, forming L-shaped structure and resulted in benthic density currents. Tidal internal waves and surface waves travel at different wavelengths and velocities across the water column, producing different degrees of shear damping for the high-concentration underflow spread. The density current moves far away in the approximate phase between the internal wave and surface wave, whereas it diminishes immediately out of phase.