Hydrodynamic Instability And Short-term Evolution Properties Of Bed Surface In Curved Channel

Curved river is the basic river form and the multi-bending river with continuous curvature is the most common and necessary stage of river evolution.Under the influence of different hydrodynamic environment and human activity factors,most rivers in China have serious siltation and undercutting problems in some river reaches.The "serious deposition and serious undercutting problem" is the product of typical bed surface morphology evolution.Therefore,it is very important to study the dynamic characteristics of the near-bed water and the evolution of the surface morphology of the bed surface.In this paper,based on the research background of continuous curvature curved channel,in theoretical modeling,different influencing factors are generalized as the coherent disturbance to the flow structure.According to the formation mechanism of the bed shape which will lead to the change of the bed surface due to the unstable disturbed flow,the weak nonlinear control equation of water and sediment is established,and the bed response elevation under the disturbance velocity is analyzed.And flow characteristics near the bed surface,the bed shape and its short-term evolution characteristics were measured and analyzed with high precision and multi angle under the conditions of different flow rate,width to depth ratio,bed-sand property and bed surface flexible grass coverage density.The main research contents and conclusions are shown as follows:(1)The method,including theoretical modeling,calculation and analysis,were used to determine the relation of the coherent disturbance structure and bed response in Microbend channel.By searching the modal and analyzing the characteristic spectrum of wave number-growth rate in the curvature and disturbance wave number in a certain range,we obtained the response range of the unstable Reynolds number influenced by a growth rate greater than zero.We then obtained a quantitative relationships which can be used as a criterion to judge whether the bed elevation response continues to develop under certain conditions among the Reynolds number,the disturbance wave number and development of bed form.(2)The hydrodynamic instability of curved channel is related to the bending degree,incoming flow rate and aspect ratio.Under the condition of different aspect ratio,the flow field and the stress distribution on the bed surface of the bend are different.The hydrodynamic response under different aspect ratio is analyzed by bend flume model experiment.According to the different flow conditions,the flow velocity is higher,it is closer to the side wall and more violent;the flow velocity of the convex bank is large,the flow velocity of the concave bank is small,and the small flow velocity area exists near the concave bank,the flow velocity is smaller,and the area of the small flow velocity district is larger;the characteristics of bed shear stress distribution reveal the characteristics of bed sand starting;the distribution of water flow dynamic axis,bed shear stress and vorticity are all changed by the existence of small velocity region.(3)Using the Trimble 3-D scanner,a high-precision scanning of the surface morphology in the short-term evolution with small width to depth ratio in bends channel was carried out.The results are as follows: under the same flow,water depth and bed sand conditions,with the increase of scour time,the location of shoal and deep groove is staggered,and the position of deep groove is more inclined to the upstream.This distribution form of shoal and deep groove adapts to the position of hydrodynamic axis;In the bed shape curve,the large-scale fluctuation with the wavelength of about 2 m reflects the alternation change between the side beach and the deep groove,and the change has a direct relationship with the curvature of the curved channel,while the wave shape of the high-frequency oscillation wave with the wavelength of less than 2 m in the curve is the sand wave form,and the irregular sand wave form is gradually attenuated while the bed surface is migrated to the downstream,and the mutual adaptation of the water flow and the bed surface has the filtering effect on the high-frequency oscillation wave of the bed surface;the discharge of river section has the greatest influence on thalweg,followed by median grain size,width depth ratio and scour time of bed sediment;there is a flat area of bed shape near the side wall,which corresponds to the velocity vector of corresponding working conditions in Chapter 3 and the small velocity area in the flow dynamic axis.(4)Natural planting method with flexible vegetation was used with various planting density conditions.And the 3-D laser scanning measurement technology is introduced to study the short-term evolution of the bed in the bend,the results are shown that the Shoals and the deep groove are staggered distribution in the deposition and scour on both sides of the bend.And the maximum deposition depth and scour depth are greater than that of the whole bed covered with Zoysia japonica.Under the same discharge condition,the smaller the bed sand size is,the easier the sediment starts,and the greater the suspended sediment concentration.The greater the coverage density of Zoysia japonica on the bed surface in the bend,the greater the resistance in the flow,and the smaller the suspended sediment concentration.The root and the plant of Zoysia japonica both affect the bed sediment and water flow,and increase the resistance of riverbed.According to the laboratory tests data,the fitting formula of comprehensive roughness n value in small width-depth ratio channel can be directly obtained by considering the conditions of planting flexible Zoysia grass on the river bed and bank,and the silt grain size covers fine sand,medium sand and coarse sand.