Sedimentation in the Yellow River delta, part II: suspended sediment dispersal and deposition on the subaqueous delta

Abstract

Rapid deposition and suspended sediment dispersal off the modern Yellow River mouth has been examined from the data of topographic maps (1 : 25,000), suspension and bed sediments, current and salinity, which were measured in the field. The results show that the sedimentation rate of the mouth bar is the highest of all the environments of deposition in the modern Yellow River delta. About 30–40% of the sediment that the river transports into the sea has been deposited within the mouth bar area. Rapid deposition of the mouth bar results mainly from kinetic energy dissipation of the river under the influence of three ambient mechanisms. These three mechanisms occur simultaneously as shear front movement, which obstructs the river's jet-like flow, small-scale circulations where large velocity gradients occur and the `bulldozer' effect of the tide sensitive zone in the estuary at ebb. Based on the analysis of morphologic changes, the mouth bar can be divided into two morphologic types: one is the double-lobe and single-channel type and the other double-channel and single-lobe type. Weak hyperpycnal plumes with small density differences between the river flow and ambient water dissipate rapidly and deposit about 20% of the river sediments on the delta front slope, while hyperpycnal underflows from the river mouth can go through the steep delta front slope to form a rise apron along the upper part of the prodelta. Grain size analysis of suspended sediment shows that the flow can be divided into three layers: a tide layer in the upper part, and a hyperpycnal layer in the lower part, (these two layers are normally graded), and a transition layer with inverse grading in the mid-part. The fine suspended sediments have been transported in the hypopycnal plume layer into the prodelta and deep sea, such as the North Huanghai Sea (North Yellow Sea). This study indicates that the tidal current field has dominated the distribution of deposition of sediment on the subaqueous delta and the movements of the river effluent and hyperpycnal underflows.