Suspended Sediment Transport Driven By Tides,Winds And Waves In The Coastal Waters Adjoining The Abandoned Yellow River Mouth

The coastal sea from the abandoned Yellow River mouth to the central Jiangsu coastal plain is an ideal place for researching source-sink and process-product issues.The abandoned Yellow River subaquatic delta was constantly eroded by hydrodynamic forces,after the Yellow River flew into the sea through Bohai Bay in 1855.Since that,a huge amount of fine-grained sediment eroded from the abandoned Yellow River subaquatic delta was transported southward along the coastline,and became a significant sediment source for sedimentation and morphological evolution in the southern Yellow Sea.'However,the temporal and spatial characteristics,the exact quantity,and the hydrodynamic mechanisms of this longshore sediment transport trend are poorly understood by researchers because of the absence of long-term hydrological observations in winter,and researches on high suspended sediment concentration events.In this paper,hydrological observations including a tripod observation system,a wave buoy,and vessel hydrological profiles were conducted for a month in winter in the coastal waters adjoining the abandoned Yellow River mouth and the Xinyanggang Coast,respectively.Observed data analysis and 2DH Delft3D simulations were applied to quantitatively investigate the influence of tide,winds and waves on the suspended sediment transport processes in this region,and the hydrodynamic mechanisms behind this transport trend.The results showed that:(1)Tidal asymmetry is obvious in the offshore area of Xingyanggang.The tidal duration is 1 h shorter during the flood tide than during the ebb tide,and thus the velocity of flood currents is greater than ebb currents.Tidal currents move along the coastline,with flood currents flowing from northwest to southeast and ebb currents flowing from southeast to northwest.The calculation results of the vertically 5-layer residual currents at the Xinyanggang station showed that all layers have a component transporting southward along the coastline,which has a larger value near the bottom layer than the upper layers.(2)The averaged suspended sediment concentration(SSC)of the near bottom layer(0.2 m above seabed)is about 1.5 kg/m3 at Xinyanggang station.When high SSC events occur,the SSC of this layer will exceed 4.0 kg/m3 and there will be a SSC stratification phenomenon in the water column.The start time of the high SSC event has a one-day time delay compared with the start time of the wind event.(3)The modelled water levels and flow fields showed reasonably good agreement with the observed values in the study area.Simulation results showed that tidal currents move along the coastline and tidal asymmetry is obvious in the entire study area.The coastal waters have a net transport trend to the south.(4)The surface sediment at the Xinyanggang station is very fine sand,which cannot significantly increase the SSC of the water column with strong sediment resuspension under wind events.Therefore,the high SSC events are not caused by local sediment resuspension but strong advection.Both observed data and numerical simulations showed that wind-driven currents and waves moving southward caused by strong north wind can strengthen the flood currents and weaken the ebb currents.In this situation,the suspended sediment stirred up by storms events in the coastal waters adjoining the abandoned Yellow River mouth could reach the Xinyanggang Coast within 1 d.(5)The idealized simulation showed that if the only sediment input in this coastal system comes from the fine-grained surface sediment in the abandoned Yellow River delta which has a thickness of 2 m,the sediment there will be transported southward constantly by tidal currents,and a muddy deposition belt will occur in the longshore area.The sediment eroded from the abandoned Yellow River delta will also deposit on the shoreline front to the south of the Xinyanggang Coast and become a significant sediment source for tidal flats accretion.(6)The calculation results of the suspended sediment flux output from the model showed that there is a strong suspended sediment flow transporting southward from the abandoned Yellow River subaquatic delta to the Xinyanggang Coast.The suspended sediment flux reaches the highest value along the 10 m isometric line and decreases on each side.The value ranges from 0.177 to 0.486 kg/s/m.