Erodibility Of Bed Sediments In The Yangtze Estuary

Erosion in estuarine and coastal areas is becoming a global concern in coastal management.Continuous decline of river-borne sediment load will lead to the reduction in accretion and even a change to erosion,such as in the Yangtze Delta.Delta erosion will not only cause loss of valuable tidal flats and wetlands,but also increase the coastal flooding risk.Study of sediment erodibility and erosion resisitance is a basic theme in sediment transport dynamics and it is of vital importances for understanding and prediction of geomorphologic evolution.This thesis focuses on the spatial variability and controls of bed sediment composition and erodibility in the Yangtze Estuary.Based on large-scale field observations and sediment samplings from 2015 to 2017,the grain size composition,water content,and bulk density of the bottom surface sediments were mapped.Combined rheological study with a U-GEMS(UMCES-Gust Erosion Microcosm System)system,the incipient motion and rheological properties of the bed sediments were examined;the erosion processes as well as the influencing factors were discussed.We then suggest a formula to estimate the critical erosion shear stress based on information of grain size and water content.The critical shear stress of the surface bed sediments in the Yangtze Estuary and its adjacent sea areas are mapped.The results and findings provide essential scientific bases for study of erosion and deposition processes and geomorphologic evolution in the Yangtze Estuary.The main conclusions obtained are as follows:1.The critical erosion shear stress distributions in the Yangtze Estuary were mapped.It is smaller,i.e.,0.35~0.6 Pa,in the turbidity maximum(TM)zone and the depocenter,and larger inside the estuary and in the north region off the estuary.Bed sediments in the North Branch had a smaller erosion resistance,with a critical erosion shear stress of 0.45~0.6 Pa.In the South Branch,the South Channel and the upper North Channel,the critical shear stress is>0.75 Pa.In the TM zone,the critical erosion shear stress varies in space with a mean value<0.6 Pa.It is~0.8 Pa in the seaward segment of the North Channel and the front areas of the Eastern Chongming Island and the Hengsha shoal owing to low mud contant and low water content.In the South Passage,it varies between 0.5 and 0.7 Pa.In the adjacent sea areas,the critical erosion shear stress in the north is overall>0.8Pa,which indicates that the bed sediments were hard to be eroded.It is 0.35~0.6 Pa in the south regions around the depocenter.This smaller critical shear stress implies that erosion and sediment resuspension has a larger poential in the depocenter.The critical erosion shear stress along the north shore of the Hangzhou Bay increased from 0.6 Pa to 0.8 Pa from east to west,suggestting landward increased erosion resistance in this area.2.We mapped the water content in the surface bed sediments throughout the Yangtze Estuary.It shows that water content in the north is higher than in the south inside the estuary,while it is higher in the south outside the estuary.The water content is positively correlated with clay content.Landward the TM,the water contents in the north area are~70%higher than those in the south area.The water content has a larger spatial variations in the TM,i.e.,in the range of 30-150%.Water contents are higher in the south regions of the adjacent sea outside the estuary,with the largest water content of~130%detected near the depocenter off the South Passage.The bulk density ranged from 1000 to 1800 kg/m~3and showed a good correlation with water content.The water content increased accordingly with larger clay and silt contents.The water contents were controlled by the sediment composition,consolidation and the change of pore water saturation.3.We identify that sediment composition and water content are the two key controls on surface erosion of bed sediments,while the mass erosion was mainly controlled by the cohesion between the fine particles.The critical shear stress for surface erosion increases with decreasing water content.When water content is<90%,the bulk density is>1250 kg/m~3 and the consolidation is enhanced,the influence of sediment particle size on the critical shear stress began to increase.The clay content has a significant effect on water content and sediment cohesion,particularlly when the clay content is>20%,sand content is<20%or median grain size is<20?m.The critical shear stress has a large variability,generally increases with the decrease in grain size.On the contrary,when the proportion of coarse particles was high,the cohesion weakened,while the gravity control became prominent,as a result,the critical shear stress increased with increasing grain size.The threshold for mass erosion is measured by the undrained shear strength.When the proportion of fine particles was high and the cohesiveness was significant,the undrained shear strength was large,and vice versa.Mass erosion was easier to occur for bed sediments in the Yangtze Estuary when the clay content was less than 20%,sand content was larger than 20%and the median grain size was larger than 20?m.The water content could change the cohesion of the fine sediments,and influence the undrained shear strength.When water content was>50%and bulk density was<1500kg/m~3,the undrained shear strength declined with the increase of water content and a larger clay content would intensify this process.