Coastal Evolution And Its Influencing Factors In Gudong Nearshore Area Of The Yellow River Delta In Recent 40 Years

Since the channel avulsion of Yellow River shifted from Diaokou course to Qingshuigou course in 1976,the morphological processes of Gudong have undergone tremendous changes owing to variations in fluvial water and sediment discharge,human activities and marine dynamics.Based on measured bathymetric data and hydrological data at Lijin station and hourly wind speed and tide level data from Gudong tide station,systematically analyzes the morphological evolution processes and hydrodynamic change of Gudong near-shore area on the inter-decadal scale(1976-2014)and the inter-annual scale(2002-2014),mainly focus on the analysis of morphological evolution of near-shore area,isobath change and ocean dynamic characteristics,then explore the evolution mechanism and prediction of the morphological evolution of Gudong near-shore area.The main conclusions are summaried as follows:(1)According to the main natural changes and human activities events,the four periods of morphological evolution of Gudong near-shore area are divided into1976~1986,1986~1996,1996~2002,2002~2014.Gudong has undergone four stages:rapid accretion stage,accretion-erosion adjustment stage,erosion stage and rapid erosion stage.This area has transformed from deposition to erosion since 1996,the erosion depth was about 6 to 8 m.The erosion degree near the north side of Gudong dike is the largest,and the maximum erosion depth is more than 8 m.(2)The isobath change has spatial and temporal differences,the erosion began on the north,and then gradually developed to the south.The change of erosion and deposition within 5 m depth area is more sensitive than 10 m.The enveloping area of the 5 m and 10 m isobath increased before 1996,the near-shore area siltation,but the degree of siltation slowed down gradually.After 1996,the enveloping area of the 5 m isobath began to erode and the erosion increased.From 2002 to 2014,5 m isobath continued to advance to the shore,and the area of the envelope of the isobath was the most obvious,and the erosion intensity in the north was significantly stronger than the south.In contrast,the envelope area of the 10 m isobath is relatively flat.(3)Four stable intervals were identified through the stability analysis of the Gudong area from 2002 to 2014:type I(strong erosion type),type II(general erosion type),type III(normally stable type),type IV(high stability type).The erosion of near-shore area first appeared in the north of the dike of the No.2 station in 2005,then the erosion center gradually expanded to the north and south sides.From 2005 to 2010,the strong erosion area expanded rapidly.After 2010,the Gudong area evolved toward a balance of erosion and siltation.By 2014,the water depth in front of the dike of No.2Station and its north side has reached 6 m.(4)The large reduction in the amount of sediment load is an important cause of erosion in the Gudong area.The annual average amount of sediment load is closely related to the annual average amount of deposition and erosion in the study area.It is estimated that the sediment discharge threshold value in the study area was 3.78×10~8t/a.Maintaining water and sediment discharge conditions and the marine dynamic environment,it is predicted that the scouring and siltation of the near-shore waters in the future will remain basically stable,but there is still the possibility of further deepening of the depth near the northern levee.Coastal protection and human intervention activities should be strengthened.