Experimental Study On The Evolution Law Of Curved River Channel Under Reduced Sand Conditions

After the impoundment of the Three Gorges reservoir,the discharge water and sediment conditions have changed a lot compared with the natural conditions.In particular,the amount of discharged sediment has been greatly reduced,and the particle size has become smaller,which makes the relative balance of sediment transport and erosion and deposition in the downstream curved channel of the reservoir be broken.The evolution of the downstream curved channel of the Three Gorges reservoir presents new characteristics.It is difficult to recover the side beach erosion,and the main channel and thalweg are shifted to the convex bank side That is to say,the phenomenon of "bend bypassing and shoal cutting" has resulted in a great adjustment of the river regime,which may affect the flood control,navigation and shoreline utilization of the middle and lower reaches of the Yangtze River.Therefore,it is of theoretical and practical significance to carry out the experimental Study on the evolution law of the curved river channel under the condition of reducing sediment inflow.Based on the systematic review of the research results at home and abroad,this paper uses the research methods of prototype data analysis,theoretical interpretation and solid model test of meandering channel(taking Tiaogauan-Laijiapu river section as an example)to comprehensively study the flow movement characteristics of meandering channel,the distribution characteristics of sediment concentration under the condition of sediment inflow and sediment reduction,and the effect of severe subsaturated sediment flow under the reservoir The evolution law of the meandering channel and the coupling mechanism between the flow and sediment conditions and the channel evolution are discussed,and the evolution trend of the meandering channel in the lower reaches of the Three Gorges Reservoir under the new flow and sediment conditions is predicted.The main research results are as follows:(1)Based on the analysis of a large number of prototype observation data and the generalized model test,this paper studies the water surface gradient,the average velocity distribution along the section,the velocity distribution in the cross section,the velocity distribution in the vertical line,the transverse circulation,and the change rule of the water flow force axis of the curved channel with different curvature radius under different flow conditions.It is found that the vertical gradient of the water surface appears "small in flood and large in dry season" at the upper end of the bend,and "large in flood and small in dry season" at the lower end of the bend.The horizontal gradient of the water surface first increases and then decreases from the entrance of the bend to the exit of the bend,which is similar to the normal distribution.The swing amplitude of the hydrodynamic axis isrelated to the curvature radius and symmetry degree of the bend.In the wide and shallow curved river,the horizontal circulation degree is not large,most of which is local secondary flow circulation is dominant.(2)This paper analyzes the water and sediment transport modes of curved channels with different curvature radii.Based on the prototype observation data of Tiaoguan-laijiapu and Xiongjiazhou to Chenglingji curves,the coupling mechanism of water and sediment transport in curved channels with different curvature radii is studied.It is found that in the curve with small curvature radius,the axis of flow power swings greatly with the flow,and the position with the largest swing amplitude mostly appears at the top of the curve and the position with larger sediment concentration is near the axis of flow power.(3)Based on the general model test of Tiaoguan-laijiapu,the change law of erosion and deposition,the change characteristics of river regime,the change of typical cross section and the evolution mechanism of the channel are studied.It is found that under natural conditions,the flow sediment concentration on the convex bank of curved river channel is greater than that on the concave bank,and under the condition of reduced incoming sediment,the flow sediment concentration on the convex bank is less than that on the concave bank.With the decrease of incoming sediment,the convex bank of curved river channel gradually scours,and the main channel of river channel swings towards the convex bank,and the larger the decrease of the amount of incoming sand,the more obvious the trend is.(4)Based on the analysis of the relationship between the water and sediment movement and the evolution law,the coupling mechanism between the two is revealed,that is,the change of the flow and sediment conditions causes the change of the river topography,and the change of the river topography reacts to the water and sediment movement at the same time,and the interaction between the two develops to a new equilibrium model.On this basis,the future evolution trend of the bend section of Tiaoguan-laijiapu is predicted and under the condition of less incoming sand,the curved channel will continue to be scoured,and the convex bank of the curved channel will continue to suffer cumulative scour.