Experimental Study On The Characteristics Of Incipient Motion And Erodibility Of Cohesive Sediment Based On Sediment Rheology

Almost all sediment related problems in water are associated with the critical conditions of sediment motion.Numerical or physical hydro-morphological modeling often requires a rigorous validation of the critical condition of sediment motion to ensure model reliability.However,due to the complex properties of cohesive sediments,what factors influence the cirtical condition of cohesive sediment motion is still a complex and hot problem.Because of the shear thinning and thixotropy,the cohesive forces between fine particles are always in a dynamically changing state.When dealing with such cases,the cohesive sediments may be regarded as a non-Newtonian Fluid,so that the rheological theory should be introduced when studying the flow behavior of the sediments.Hence,based on the theory of mechanics of sediment movement and sediment rheology,a series of experiments are carried out and the angle of repose(AoR),threshold of sediment motion,and erodibility of cohesive sediments are systematically investigated in this dissertation,the details are as follows:Firstly,the angles of repose(AoR)of submerged cohesive sediments with different fluidization degrees are investigated in a glass water tank using a tilting plane.The results indicate that the water content or the wet bulk density is not enough to directly reflect the effect of the interparticle force between sediment particles on the AoR of submerged cohesive sediments.The yield stress and particle size are the factors that has major influences on the AoR.Furthermore,the sin(?)(? is the AoR of submerged cohesive sediments)linearly increases with the increasing yield stress.Meanwhile,the effect of the yield stress on the AoR is significant for the cohesive sediments and becomes progressively weaker with the increasing median grain size.By introducing the yield stress and median grain size as the key parameters,an empirical formula is proposed to calculate the AoR of submerged cohesive sediments.This formula contains a rheological term and a gravity term,which reflects the effects of the rheological properties and effective gravity on the AoR of cohesive sediments,respectively.Secondly,the critical condition of incipient motion of cohesive sediments is investigated in a water-circulating,closed-pressure transparent(Perspex)rectangle duct.The results indicate that the water content and wet bulk density are not enough to determine the critical condition of incipient motion of cohesive sediments.The yield stress and grain size are the key factors that affect the critical Shields parameter.The critical Shields parameter is found to be exponentially proportional to the yield stress and inversely proportional to the median grain size.The effect of yield stress on the critical Shields parameter is significant for the cohesive sediments and becomes progressively weaker with increasing median grain size.According to rational analysis,an empirical formula for calculating the critical Shields parameter of cohesive sediments that includes a rheological term and a gravity term is proposed by introducing the yield stress.According to this formula,a modified Shields diagram is drawn which can visually show the yield stress effect on the criticle Shields parameters.Thirdly,the erodibility of fluidized cohesive sediments is experimentally investigated in unidirectional open flows.It is found that the water content and wet bulk density are not enough to determine the erodibility of cohesive sediments.The yield stress is the key factor which affects the erodibility.Again,the erodibility of cohesive sediments exponentially decreases with the increasing yield stress,and a lower yield stress of sediment indicates higher potential of being eroded.Based on the Partheniades' s(1962)equation,the erosion parameter(M)is modified by introducing the yield stress,and two models are proposed to calculate the scour rate of cohesive sediments.Furthermore,the reliability of the critical Shields parameter formula proposed in this study is verified by the results of the scouring tests.Finally,based on the relationship between rheological properties and the characteristics of sediment motion of cohesive sediments obtained in this study,a fluidization technique for the cohesive bed and a related vibration fluidizing device are proposed.This device is used to fluidize the sediments during the flushing,and the enhancement on the erosion of test sediments is investigated.The experimental results indicate that the fluidization of the cohesive bed by the mechanical vibrations can significantly enhance the erodibility of cohesive beds.Moveover,the longer the mechanical vibration applies on the sediments,the higher the fluidization degree is,which leads to a greater increase of the scour rate.