Coastal Erosion And Sedimentary Environment Of The Modern Yellow River Delta Under Land And Sea Interaction

With the sharp decrease of.sediment supply from the Yellow River,marine dynamics have become more dominant in reshaping coastal topography along the subaerial Delta of the Yellow River.During recent years,some parts of coasts have undergone notable erosion.It leads to a great challenge for coastal protection and has become a hot subject for the scientific study.This paper presents a data-based study of waves,currents,sediments and subaqueous bedforms by applying Regression,Empirical Orthogonal Function (EOF),Levenberg-Marquardt methods and GIS techniques to the measured coastal profiles in order to understand the evolution of topography and sedimentary. environments of the subaerial Delta of the Yellow River.The results are shown as follows:(1) Since 1997,the coasts along the Yellow River Delta have shown extensive erosion except that there is slight accretion at the current river mouth.(2) The coast of the Yellow River Delta can be divided into three areas: abandoned estuary area,current estuary area and Laizhou Bay.For abandoned estuary area,the coasts of Wanwangou were relative stable because it has been abandoned for a long time and the geomorphology of the coast had already adjusted itself to resist further erosion.But Diaokou estuary and Shenxiangou estuary,which are abandoned in 1976 and 1964 respectively,have shown continuous erosion since 1976.The erosion processes can be divided into three stages:rapid erosion stage (1976-1980),accretion-erosion adjustment stage(1981-1998) and slow erosion stage (1999-2004).During the accretion-erosion adjustment stage,there was an erosion-accretion balance zone(EABZ) on every profile,whose depth mostly depended on the local tidal current velocity.The higher the tidal current velocity,the deeper the depth of EABZ.The coastal profiles near the Qingshuigou estuary showed continuous progradation.The evolution process exhibited three distinct stages:rapid progradaion phase(1976-1985),stable phase(1986-1995) and slow accretion phase(i996-2004).All the profiles have been prograded seawards generally.The distance of the seaward movement was controlled by the relative strength of riverine and marine dynamics.The coast of Laizhou Bay was generally stable.Due to the influence of the tidal current field,little suspended sediment from the Yellow River was transported to Laizhou.Bay.Erosion and accretion tended to occur in the.shallow area while slow deposition prevailed in deep.water area alternatively.(3) From the forecast of coast topography at abandoned subaerial delta after they have been abandoned 0,10,20,40,60,80,100 and 150 years,respectively,we find that the longer time the subaerial delta have been abandoned,the more slowly the erosion velocity of coastal profile.The coastal types also switch from destructive type to stable type slowly.Furthermore,the maximum erosion,depth is about -15m.(4) Based on measured data of bottom sediment samples,combined with sediment texture,topography and hydrodynamics,a new triangular diagram advanced by Fleming is applied to reveal the sedimentary environments and their hydrodynamic processes.The results indicate that nearshore zone of the Yellow River delta can be divided into three sedimentary environments:abandoned estuary area,current estuary area,and Laizhou Bay.At the abandoned estuary area,there are two parts,Wanwan'gou and Feiyantan,which were abandoned respectively in 1953 and 1976.Due to long term scouring of wind wave,the grain composition of the majority bottom sediment near Wanwan'gou is relatively coarse with strong erosion-resisting characteristics.For Feiyantan,because of the combined effect of wave and tidal current,the bed sediment contained a high proportion of sands at the water area shallower than 5-6m.The distribution of various sediment fractions did not seem to change noticeably with the water depth,which probably indicates the strong mixing effects of wave breaking on the bed sediments in this area.In contrast, the,bottom sediment was finer with low percent of sand and high percent silt and clay at the deeper water area seaward of 5m to 6m contours.For current estuary area, the grain size had a changing tendency of coarse-fine-coarse which resulted from the effect of strong tidal current zone in front of sand spit.For the area near Laizhou Bay, because of the influence of tidal current field,grain size of bottom sediment.particles was coarser in the area south of Guangli River estuary than the area north of that. (5) According to the variations of grain size and magnetic parameters of Core Q1 at Qingshuigou estuary and Core D1 at Diaokou estuary,their sedimentary facies have.been established.The difference between their facies was the result of the sediment and water discharge from the Yellow River and the marine dynamics.