Spatiotemporal Evolution Of Yellow River Estuarine Channel And Its Influencing Factors Since 1976

Estuarine delta is a transition zone where the river and the ocean interact,and with significant social and economic value as well as ecological service function.The estuarine channel is an important passage for rivers to supply terrestrial materials for the delta.It not only constitutes the basic framework of the delta landform,but also directly controls the evolution of the delta coast,the transportation process of sediment,and the maintenance and restoration of coastal wetlands.The Yellow River is well-known for its low discharge,high sediment concentration and frequent migrations of the channel.In recent years,affected by climate change and human activities,the riverine delivery regime in the Yellow River has markedly changed,which significantly influences the morphodynamic evolution processes of the riverine estuary.Under this scenario,the systematic study of the temporal and spatial evolution characteristics of the Yellow River estuarine channel,is conducive to the understanding of the evolution of the Yellow River Estuary under the environmental changes.This dissertation takes the Qingshuigou channel of the Yellow River Estuary as the research area.based on the measured elevation data of the channel transects as well as Landsat remote sensing images from 1976 to 2017,a long time-series channel DEM based on a curved orthogonal grid is established.Combining with riverbed shape variations and changes in water and sediment discharges of the Yellow River,the relationships between the scouring and silting evolution process and their influencing factors are also been discussed.The main results and conclusions are as follows:(1)Since the Yellow River migrated to Qingshuigou in 1976,the channel had undergone a complex process of erosion and deposition.According to the characteristics and change rates of erosion and deposition,the evolution pattern of the estuarine channel can be divided into four stages:Accretion(1976-1978)–Severe erosion(1979-1985)–Alternate period(1986-2001,the overall performance is siltation)–Erosion(2002-2017,the erosion rate tends to decrease).The channel mainly experienced erosion in flood seasons and accretion during the non-flood seasons,and this seasonal pattern became more obvious after 2002.(2)According to the evolution of the shape of the Qingshuigou flow path along with the adjustment of the cross-section shape,the evolution of the Qingshuigou riverbed shape from 1976 to 2017 can be divided into four stages.In stage 1,drastic aggradation happened during1976-1979 and the main channel was forming,with the decrease of cross-sectional area,width,and the width-to-depth ratio;In stage 2(1980-1985),the main channel was deepened and widened,the cross-sectional area increased,while the width-to-depth ratio decreased,increased of the channel transportation capacity;During stage 3(1986-2001)the riverbed accreted and the main channel shrunk due to the drastically decreased discharges entering the channel,accompanied by the cross-sectional area and width reduced while the width-to-depth ratio fluctuated;Stage 4(2002-2017)is characterized by continuous erosion and deepening in the main channel,the river width remained unchanged overall,which is different from stage 2.The cross-sectional area increased and the width-to-depth ratio decreased,resulting in the increase of flow capacity of.(3)At a long-term scale,the scouring and accreting characteristics of the river channel mainly depends on the incoming water and sediment conditions and the water-sediment collocation.The larger the incoming discharges and the lower the incoming sediment coefficient,the easier the river erosion will occur,and vice versa.At the interannual scale,there is no obvious quantitative relationship between the river erosion and sedimentation and the various water and sediment parameters,which indicates the complicated influencing factors existing in the estuary channel.(4)The implementation of water and sediment regulation since 2002 significantly changed the annual distribution of water and sediment regime in the estuary channel of the Yellow River,resulting in the increase of the transport compacity of the channel.However,the erosion rate was constantly changing.The channel evolution process since 2002 can be divided into three stages according to the rates of erosion or deposition,namely:rapid erosion(2002-2005),slow erosion(2006-2014),and slow deposition(2015-2017).Approximately 80%of the total eroded sediment was depleted in the first stage.The erosion flux has shown a significant decreasing trend after 2006,and the overall characteristics of the channel transferred from erosion into deposition afterwards,the riverbed no longer deepened down.The erosion efficiency reached the maximum(6.67 kg/m~3)in 2002 and dropped sharply afterwards.It shows that the impact of the Water and Sediment Regulation Scheme on the erosion and deposition evolution of the estuarine channel is unsustainable,and the river bed has adapted to the present water and sediment conditions through riverbed adjustment.(5)The river base level change caused by the extension of estuarine accretion and the diversion of the branch has been an important impacting factor on the evolution of the river.The seaward stretch of the mouth caused the relative rise of the base level,which reduced the erosion efficieny and aggravated the siltation of the channel nearing the mouth,resulting in a decrease in the longitudinal gradient of the river channel and increasing the instability of the near mouth section of the estuarine channel.The artificial refurbishment will lower the base level and cause upstream erosion of the river,but this kind of effect is unsustainable.