Experimental Study Of Point Bar Evolution In Meandering Rivers Under Unsaturated Sediment-laden Flow Conditions

After dam impoundment,downstream reaches experience continuous adjustments induced by unsaturated sediment-laden flow.During the morphological adjustments,pervasive incidents of chute cutoff occur along meandering rivers.Consequent adjustment of bar-pool pattern due to chute cutoff has an important impact on flood control and navigation.Different meandering rivers show various erosion patterns and chute-forming processes on point bars and have different impacts on navigation.Therefore,it is of great theoretical significance and engineering value to systematically study the evolution of convex point bar of meandering rivers under unsaturated sediment-laden flow conditions.In this thesis,laboratory experiments,theoretical analysis and in-situ data analysis were carried out together to investigate the flow characteristics and morphological adjustments in curved flumes with different bend angles.Channel regulation principles and corresponding strategies were also discussed.The principal conclusions of this thesis are as follows:(1)The main stream line moves toward the convex bank with increasing discharge.However,the lateral displacement of main stream line toward convex bank and longitudinal distribution of velocity on the point bar varies with different bend angles.Upstream of the apex,the main stream line moves to the point bar with increasing discharge.Downstream of the apex,the lateral displacement of main stream line toward convex bank decreases with increasing bend angle.Flow velocity on the high-stage point bar reduces more rapidly downstream of the apex with increasing bend angle.The impacts of discharge and bend angle on bed shear stress and sediment transport capacity show a similar rule.(2)Secondary flow increases on the point bar with increasing discharge and secondary flow on the low-stage point bar exceeds that on the high-stage point bar and the concave pool.With increasing discharge the location of maximum secondary flow changes little with varying bend angles and generally lies at the junction between the point bar and the concave pool.With increasing bend angle,the phenomenon of saturation of secondary flow is observed.The secondary flow increases largely with the bend angle increasing from 45? to 90?.When the bend angle further increases to180?,the secondary flow decreases.(3)Unsaturation of sediment-laden flow,discharge and its duration all determine the outcome of scouring on the point bar.Scouring intensity increases on the point bar with increasing unsaturation of sediment-laden flow,increasing discharge and its increasing duration.With increasing discharge,areas of erosion tend to enlarge from the bar magins to higher parts of the point bar,during which a chute cutoff occurs on the point bar.(4)The bend angle affects the erosion pattern on the point bar.Under large unsaturated sediment-laden flow conditions,erosion on the point bar tends to increase upstream of the apex and decrease downstream of the apex with increasing bend angle.For a sharp bend with a large bend angle,chute cutoff occurs on the upper part of the point bar and erosion decreases on the lower part.For a weakly curved bend with a small bend angle,chute cutoff slices through the whole point bar.(5)Current flowing along straight line during high discharge while flowing along curved line during low discharge and sediment grain size being smaller on the point bar are the precondition of chute cutoff in meandering rivers.Unsaturation of sediment-laden flow,discharge and its duration are the main driving factors triggering chute cutoff process in meandering rivers.Those factors all results in the increase of the scouring and sediment-transport capacity of the point bar which exceed that of the concave pool during high discharge.The point bar is prone to erosion and chute cutoff occurs.The adjustment of upstream regime may promote the chute cutoff process to some extent.(6)Chute cutoffs due to unsaturated sediment-laden flow in meandering rivers are likely to cause transformation from the meandering into braided river,during which a chute may develop and even become the main branch,and adversely affect the navigation conditions.Flow is scattered and unable to scour the riffle during the alternation between main branch and anabranch.For a sharp bend,local channel radius of curvature of the junction between convex main branch and concave pool is too small to meet the navigation standards.Channel regulation should be conducted to stabilize the point bar to maintain or improve the navigation conditions of meandering rivers.For sharply-curved bends,stabilization measures should be focused on the upper part of the point bar.For weakly-curved bends,measures should be taken to suppress chutes development on the middle and lower part of the point bar in addition to the stabilization on the upper part of the point bar.