| River discharges fresh water with suspended particles and contaminants into the sea,forming river plume and inducing the erosion and accretion of the river delta.River plumes supply nutrients to the sea.Also,the turbidity of the river plume affects the transparency of the sea,which has a great impact on the ecological environment of the sea.The Yellow River is famous for its high suspended sediment concentration(SSC).The Yellow River course has shifted several times due to the accretion of the riverbed during the past 60 years,resulting in three typical river mouths:the Diaokou river mouth in southern Bohai Bay,the Qingshuigou river mouth in northern Laizhou Bay,and the current river mouth,which discharges water and sediment into the central area of the Bohai Sea.The migrations of river mouth influenced the transport and diffusion of water and sediment off the river,affecting the hydrology and the erosion-accretion pattern in the coastal areas.A high-resolution water and sediment transport 3-D numerical model considered river discharge,tide,wind,topography,wave,and heat flux et al.were established in this study to investigate the water and sediment transport and diffusion in the Yellow River mouth and the adjacent sea,which is of great scientific importance and application value to the protection of ecological environment and fishery resources in the ocean.Simulations of the water and sediment transport and diffusion in the adjacent sea near the Yellow River estuary during the Diaokou course age(1964–1976),Qingshuigou course age(1976–1996)and Q8 course age(1996–present)were conducted and the dynamic mechanisms were clarified.In summary,the main works and conclusions are:(1)The extension and dynamic mechanism of the Yellow River diluted water(river plume)during the current course age.If only the river discharge was considered,the river plume had an unrealistic extension,that was it firstly flowed upstream(the opposite direction of the Kelvin wave propagation)after leaving the river mouth,and then rotated clockwise under the action of Coriolis force to flow downstream(the direction of the Kelvin wave propagation),forming a low salinity water mass on the northside of the estuary.When the tide was added,there was a strong baroclinic gradient on the northside of the river mouth due to tidal mixing,which blocked the northward extension of the river plume.Sea surface heat enhanced the stratification,resulting in the river plume spreading further offshore.The wind stress drove the Yellow River plume extension in the direction of Ekman transport.(2)Effects of the different river courses on the river plume extension.During the Diaokou course,the Yellow River plume was mainly concentrated near the the river mouth in southern Bohai Bay.Part of the diluted water expanded downstream along the coast under the action of the north wind in winter,and expanded far eastwards into the central area of the Bohai Sea in summer.The water depth near the Diaokou river mouth became shallower from 1964 to 1976,which leading to the enhancement of vertical mixing and the narrowing and lengthening of diluted water.During the Qingshuigou course,the river plume was confined within the head of Laizhou Bay under the Coriolis and buoyancy forces induced by the salinity front in winter,and occupied the whole Laizhou Bay in summer,then extended eastward out of Laizhou Bay.During the Q8course,the river plume firstly extended upstream and then rotated clockwise under the Coriolis force and flowed into Laizhou Bay in winter,and part of the river plume extended downstream into Laizhou Bay and part of it extended into the central area of the Bohai Sea driven by the eastward Ekman transport induced by the southerly wind in summer.(3)The dynamic process and mechanism of transport and diffusion of the sediment discharging from the current Yellow River mouth.Taking 2012 as the calculation year,the suspended sediment concentration(SSC)was higher near the Yellow River mouth,at the southside of the river mouth and northern Laizhou Bay in summer.The sediment transported northeastward on the northside of the current river mouth,downstream on the southside of the river mouth.The net sediment flux along the coast at the southside of the river mouth(across Sec 3,located between the current river mouth and Qingshuigou river mouth)was 1.7×10~8 kg during spring tide and 3.2×10~8 kg during neap tide(positive value indicates downstream direction).The bottom SSC along the coast of Yellow River delta was higher due to the enhanced bottom shear stress induced by strong waves in winter.As a result,the sediment downstream transport was enhanced.The net sediment flux along the coast at the southside of the river mouth was 1.3×10~9kg during spring tide and 2.2×10~9 kg during neap tide(Sec 3).The tidal effect on sediment transport was mainly reflected by tide-induced vertical mixing.Without tide,the bottom sediment resuspension was restricted as a result of the decrease of bottom shear stress.Also,the bottom sediment upward diffusion was hindered due to the weak tidal mixing.Additionally,the downstream sediment transport along the coast was weakened.The net sediment flux along the coast at the southside of the river mouth became 2.9×10~7 kg during spring tide and-2.6×10~7 kg(negative value indicates upstream direction)during neap tide in summer,and-1.9×10~8 kg during spring tide and 7.7×10~7 kg during neap tide in winter(Sec 3).Wave increased the bottom shear stress along the Yellow River delta,resulting in the increase of SSC and enhancement of sediment transport along the coast.The effect of wave was more significant in winter.Without wave,the net sediment flux along the coast at the southside of the river mouth reduced to 3.7×10~8 kg during spring tide and 2.0×10~8 kg during neap tide in winter(Sec 3).(4)The transport and diffusion of SSC in the different river courses of Diaokou river(1965)and Qingshuigou river(1994).During the Diaokou river course,the SSC was higher in southern Bohai Bay,and along the coast downstream to the current river mouth in summer.The sediment transported northeastward in the surface layer driven by the southerly wind,and transported to the Diaokou river mouth in the bottom layer under the baroclinic gradient force.The net sediment flux along the coast at the Diaokou river mouth was 9.5×10~7 kg during spring tide and 9.0×10~7 kg during neap tide in summer(across Sec 6,located at the Diaokou river mouth).In winter,the stronger wind induced higher bottom shear stress,resulting in higher SSC along the coast of Yellow River delta and southeastern Laizhou Bay.The sediment transported eastward in the surface layer and westward in the bottom layer.The net sediment flux along the coast at the Diaokou river mouth was-2.5×10~8 kg during spring tide and-1.5×10~9 kg during neap tide(Sec 6).During the Qingshuigou river course,the SSC near the river mouth and along the coast of Yellow River delta was lower in summer.The sediment transported northward in the surface layer under the southerly wind and transported to the Qingshuigou river mouth in the bottom layer.The net sediment flux along the coast at the Qingshuigou river mouth was 2.3×10~8 kg during spring tide and 7.6×10~7 kg during neap tide(across Sec 8,located at the Qingshuigou river mouth).In winter,the SSC along the coast of Yellow River delta and inside Laizhou Bay was higher as a result of the increased wave-induced bottom shear stress.The sediment transported offshore in the surface layer and towards Qingshuigou river mouth in the bottom layer.The net sediment flux along the coast at the Qingshuigou river mouth was-1.7×10~7 kg during spring tide and 1.0×10~9 kg during neap tide(Sec 8). |
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