Field observations of current and suspended sediment concentration during sediment disposal in the Changjiang (Yangtze) Estuary

Acoustic Doppler current profile and acoustics concentration profiler are applied respectively via fixed-point serial and sailing observations to measure temporal and spatial variations of current and suspended sediment concentration at the North Passage in the Changjiang (Yangtze) Estuary.Field observations show that(1) near-bed high concentration events occur in various tides,with different frequencies and formation mechanisms.They take place frequently in a moderate tide,especially at the slack or maximum tides.(2) At least three types of vertical profiles of suspended sediment concentration are found out,that is,L-type with the upper concentration smaller and the lower larger,exponential type,and floating type with the upper concentration larger and the lower smaller.(3) low-velocity zones occur owing to the effect of dredged sediment disposal on the flow,which are located at the surface column or through the whole column,for the latter case,forming flow fronts.The intensity of the zones tends to decrease with the diffusion and settling of the disposal sediment.(4) The behavior of ebb currents differs at various tidal phases.The ebb currents are directed mainly to the north in the spring and neap tides,while they are directed mainly to the south in the moderate tide.(5) Both sediment transport diffusion and settling coexist during dredged sediment disposal,under the combined action of ebb current and gravity.Disposal sediment deposit is immediate in the neap tides,and it disperses far away in the moderate and spring tides.