Morphodynamic Processes In The Yellow River Estuary And Their Responses To Variation Of Riverine Supply

River estuaries are the interaction zones between land and coastal ocean,playing a critical role in materials and energy transfer in geosphere,atmosphere,hydrosphere and biosphere.River estuaries are of profound socio-economic and ecological significance due to their particular features,such as low and gentle topography,high productivity and biodiversity,as well as navigational value.The dynamic evolution of river estuaries are also direct reflections of the relative strength between riverand ocean dynamics.In recent decades,under integrated impact from climate change and intensified human activities in river basin,the river delivery has entered into a new regime,which significantly influences the morphodynamic evolution processes of river estuaries.Among the mega estuaries worldwide,the Yellow River estuary(YRE)is regarded as a typically constructive estuary dominated by fluvial dynamics in the past thousand years,as formed in a micro-tidal estuary with weak wave activity by a heavily sediment-laden large river.In this study,multiple datasets are utilized,including the annual water and sediment discharges of the Yellow River,remote-sensing images,in situ measurements and surface sediment sampling.The main objectives of our study are to:1)explore the variations of water and sediment discharges into the sea of the Yellow River over different timescales,2)investigate the distributions of sediment grain size,sedimentary environment and the bottom sediment transport trend around the YRE,3)examine the hydrodynamics and sediment transport pathways of the YRE under both low and high river discharge,and 4)reveal morphological evolution of the YRE at multi-timescales.The main results and conclusions are as follows:(1)There exist significant intra-annual and inter-annual variations,long-term reduction trends and responses over and across different timescales of water and sediment discharges into the seaThe water and sediment discharges into the sea exihibit great variability within a year.The sediment discharge in flood season is much higher than that in non-flood season.Monthly water discharge has greater fluctuations than monthly sediment discharge.Meanwhile,the sediment discharge increases with power index as the water discharge increases.At inter-annual scale,the water and sediment discharges both show a drastic decreasing trend,with a higher fluctuation and decreasing amount in sediment discharge.The amount of annual sediment delivery is not only closely related to the annual water discharge,but also the distribution of water discharge within a year.When the distribution of water discharge is at a relative stable level within a year,the water discharge in flood season accounts for a higher percentage,inducing a potential increase in the sediment load.Over the past few decades,the inter-annual fluctuation of sediment discharge is highly controlled by the inter-annual change in water discharge in flood season.(2)The characteristics of sedimentary dynamincs in the current and old estuaries are of great difference,and the current estuary significantly responds to the change of runoff,while the old estuary insignificantly responds.The characteristics of sedimentary dynamics in the YRE show obvious spatial and temporal differences in sediment grain size,sedimentary dynamic environment and sediment transport trend.The changing trends of component contents,type and grain size parameters of bottom sediment from shore to sea are distinct in the active and old estuaries.There both exist strong sedimentary dynamic environment in the active and old estuaries,but the sedimentary dynamic environment in the active estuary is weaker than that in the old estuary.Sediment tends to transport towards southeast and northeast in the active and old estuaries,respectively.The deposition centre is within the area between the active and old estuaries.The change of river deliverysignificantly influneces the characteristics of sedimentary dynamics in the activeestuary,but insignificantly influences that in the old estuary.With the increase of water discharge,the sand and silt contents show an increasing trend,whilst the clay content shows a decreasing trend.Meanwhile,the results reveal that the median grain size increases with riverine input increasing,and the Sorting,Skewness and Kurtosis tend to decrease.The increase of riverie input enhances the hydrodynamic environment in the activeestuary,and leads to the clockwise rotation of the sediment transport trend and the southward movement of the depo-center.(3)The spatial distributions of water and sdiment transport processes in the bifurcated estuary are of significant difference,and part of the processes are greatly influenced by river dynamicsUnder both low and high water discharges into the sea,the hydrodynamics in the bifurcated estuary both exihibit reciprocating flow pattern.The tides near the YRE mainly belong to the irregular semidiurnal type.Time-average current velocity and residual current decrease from the surface layer to the bottom,and time-average suspended sediment concentration(SSC)and suspended sediment transport rate increases with the water depth.Time-average current velocity,residual current,SSC and suspended sediment transport rate in the near-field is much higher than those in the far-field.With the increase of river flow discharge,the current direction rotates anti-clockwise both in the flood phase and ebb phase.Meanwhile,flow velocity and residual current are enhanced by the increase of water discharge.Time-average SSC from the surface layer to the bottom all increase,but the distribution of SSC in the far-field area of the YRE is weakly influenced by the increase of riverine input.The results also show that the net sediment flux per width increases with water discharge increasing.(4)The inter-annual and decadal morphological evolutions are extremely dynamic with clear regularity of spatial distributions,which are closely related to the riverine water and sediment supply at or across different timescalesThe dynamic feature of inter-annual variability and long-term morphological change of the YRE can induce spatial variability in erosion and accretion trends in subaerial and subaqueous estuary.At the inter-annual and decadal timescale,the shoreline dynamics in propagation and retreat has positive feedback in underwater bathymetric changes.At the inter-annual timescale,the morphologic evolution is closely related to river discharge.Every 1×10~8m~3 increase of annual water discharge will cause an accretion of 1.79×10~6 m~3,whilst every1×10~8 t increase of sediment load will induce an accretion of 6.12×10~4 m~3.The critical annual water and sediment inputs for maintaining the deposition/erosion equilibrium state of the YRE are 102.79×10~8 m~3 and 1.98×10~8 t,respectively.At the decadal timescale,the morphological change of the YRE is also related to the amount of river input.Every 1×10~8 m~3 increase of annual water discharge will induce an accretion of 1.69×10~6 m~3,whilst every 1×10~8 t increase of sediment load will induce an accretion of 5.53×10~4 m~3.The critical average annual water and sediment discharges for maintaining the deposition/erosion equilibrium state of the YRE are89.35×10~8 m~3/yr and 1.10×10~8 t/yr,respectively.The inter-annual morphological evolution process of the YRE is not only related to annual runoff,but also related to the distribution of water discharge within a year.In the period of relatively stable annual runoff,the higher the water discharge proportion in flood season,the larger the annual volumetric change.The increase of annual volumetric change caused by the per unit volume increase of water discharge in flood season is obviously higher than that in non-flood season.The long-term change process of inter-annual morphodynamic evolution is mainly controlled by the iner-annual fluctuation of water discharge in flood season,but weakly influenced by that in non-flood season.The thesis systematically studies the morphodynamic processes of the YRE and their responses to the change in river water and sediment supply.The relevant achievements can provide valuable information for sustainable development and coastal protection of the YRE.Meanwhile,it is also believed to be a potential reference for estuarine and coastal theory study.