Experimental Study On Incipient Motion Of Cohesive Fine Sediment Under Different Saline Conditions

Cohesive sediment are widely exist in estuaries,lakes and silty coasts.Affected by runoff,tidal current and seasonal climate,the cohesive sediment in these areas is often in the water environment of salinity change.The salt content in the water have great impact on the flocculation and agglomeration characteristics of cohesive sediment.This has an important impact on the sediment incipient motion,flow and sediment transport,fluvial process and pollutant diffusion in the area.In order to enrich the theory of sediment incipient motion,improve the design code of estuarine,port and coastal engineering,and promote river ecological management,we have to study the incipient motion of cohesive fine sediment in saline environment.A research of inception of cohesive fine sediment under different saline conditions was carried out based on theoretical analysis and flume test.Firstly,the influence of salinity on turbulence characteristics was analyzed;then,the basic properties of cohesive sediment were introduced;then,Taking the sand samples from the Yellow River Huayuankou and Pearl River Estuary as the research objects,three series of muddy clay incipient flume tests,including the incipient motion tests of sand samples with salt under freshwater condition(only sand samples contained salt),the incipient motion tests of sand samples with salt under saline conditions(both water and sand samples contained salt)and the incipient motion tests of sand samples with salinity of 0‰ under saline conditions(only water contained salt)were carried out.On this basis,the incipient discrimination method of cohesive fine sediment is proposed,the incipient law of cohesive fine sediment is clarified,and the flow turbulence characteristics of cohesive fine sediment in the incipient stage under different salinity conditions are analyzed.Finally,the incipient velocity formula of cohesive fine sediment with salinity variable is established.The main results are as follows:(1)The salt content in the water has no obvious effect on the turbulence characteristics of water body,and the influence of salinity on the incipient motion of cohesive sediment is mainly manifested in the influence on the physical and chemical properties of cohesive sediment.(2)Based on the PIV laser camera system and the digital image gray processing technology,the gray value curve of the bottom water near the sediment bed surface with the velocity is drawn,than through the inflection point of the relation curve between the gray level of water body and the velocity,the quantitative discrimination of the incipient velocity of sediment is realized.(3)The higher the salinity of water body or cohesive sediment is,the more difficult it is to start.There is a logarithmic relationship between the incipient velocity of cohesive sediment and the salinity of water or sediment.The incipient velocity increases sharply in the range of 0-10 ‰ and slowly in the range of 10-40 ‰.For the same salinity,the incipient velocity of sand samples with salt under freshwater condition is the largest,followed by that of sand samples with salt under saline conditions,and that of sand samples with salinity of 0‰under saline conditions is the smallest.(4)Based on the flume test data,the incipient velocity formula of cohesive fine sediment with salinity variable is established by analyzing the stress of cohesive fine sediment.Through theoretical derivation,the critical conditions of sediment incipient motion based on incipient velocity,incipient friction velocity and critical incipient shear stress are unified,and their mutual conversion relationship is obtained.