Experimental Study On Incipient Scouring Of Cohesive Sediment In The Lower Reaches Of The Yellow River

There are obvious layered structures(sand,loam,silt and clay layers)in the riverbed of the lower Yellow River.It is because of this layered structure that different parts of the riverbed have different scour resistance,which makes the bottom of the river exposed and abnormal river situation appear in the lower Yellow River.The predecessors have done a lot of research work on the bottom of the river and the abnormal river regime,but at present,there is no systematic study on the anti scourability of the cohesive sediment after cementation,and this work is of great significance to the evolution of the river regime on the plane and the flood control safety of the Yellow River,a sediment laden river.In this study,the undisturbed clayey soil in the abnormal River Bay near the Huayuankou of the Yellow River is selected and remolded by the rapid sedimentation method and natural settlement method.The physical and mechanical properties of remolded clayey soil are tested and the starting erosion test of clayey sediment is carried out.The interdisciplinary intersection of river dynamics and soil mechanics is emphasized,and the starting conditions and anti erosion characteristics are analyzed.The main research results are as follows:(1)Through the analysis of shear strength of cohesive sediment,it can be seen that the shear strength,cohesion and internal friction angle of cohesive sediment decrease with the increase of water content,which satisfies the exponential relationship of decreasing.(2)According to the results of the rapid deposition and natural deposition tests,the water content of the clayey soil samples can be reduced rapidly by about 16% from the saturated state,and the water content of the rapid deposition test is 13-29 times of that of the natural deposition test.(3)The starting test of cohesive sediment shows that the starting mode of cohesive sediment is obviously different from that of bulk sediment,and it starts in the form of pieces or blocks at the initial starting;with the increase of velocity,cohesive sediment is gradually lifted by large blocks,or even destroyed as a whole.In this study,the starting velocity range of a small amount of movement of clay particles is 0.741m/s ? 1.296m/s;for different graded cohesive sediment,the starting velocity increases with the increase of deposition time,dry density,shear strength and clay content,and decreases with the increase of water content and median particle size.Finally,the starting velocity formula of viscous sediment is established.(4)The results show that the erosion rate of viscous sediment increases with the increase of shear stress,water content and median particle size,and decreases with the increase of dry density.By analyzing the relationship between the erosion rate and relative residual shear stress,the linear relationship between the erosion coefficient of viscous sediment and the increase of dry density,and the linear relationship between the erosion coefficient of viscous sediment and the increase of clay content are obtained.Based on the analysis of the influencing factors of the scour rate,the formula of the scour rate of viscous sediment is established.