Experimental Study Of Effects Of Emergent Rigid Vegetation On Flow And Sediment Movement In The Curved Flume

Mechanism of flow and sediment movement in a river bend is one of the fundamental parts among the contents of Hydraulics and River Dynamics,on the other hand,riparian vegetation has played a significant role in the fluvial process and river evolution.In addition,following the development of economic and society,more and more attention has been paid on the sustainable development of river ecosystem and harmonious relationship between human and water,which puts forward a higher requirement for the studies about vegetation effects on the river dynamics.However,the existed research in these fields were only focused either on the channel bend without vegetation,or on the effects of vegetation in a straight channel flow,the comprehensive effects of these two factors are not extensively studied.In this thesis,under the supported of a National Natural Science Foundation of China(NSFC)project,"Mechanism of influence of riparian vegetation on the flow structures and sediment transport in a river bend",a series experiments were conducted in a 90m length,1m width and lm height flume with a 180°bend.Uniform silica sand with diameter of 0.44mm were laid in the bottom of the flume,and a cluster of plexiglas rods were set in the convex side of mobile bed to resemble simulate the emergent rigid vegetation in a river bend.Effects of the vegetation on surface velocity fields,main and secondary flow structure,turbulence distribution,and sediment transport characters were studied by the measured data.The main results of the thesis are as follows,(1)A novel PTV(Particle Tracing Velocimetry)matching algorithm named Pyramid Lucas-Kanade "Corner Centroid Method"(PyrLK-CCM)is proposed based on the Optical Flow Method.In this method,the velocity of the particles was obtained only depend on the image information itself,other than the hypothetical particle movement rules which necessary for most of the existing methods.Calculation results in a measured simple straight flow field and a complex bend flume flow field based on the new method and two well-known methods,namely the Matching Probability Method(MPM)and Nearest Neighbor Method(NNM)were compared extensively.Results shown that proposed PyrLK-CCM was suited to both simple and complex conditions with very few mismatching.MPM was only capable for simple flow condition,otherwise there would be too many mismatching in the complex condition in which different particles with less resemblance movement character.Moreover,the MPM results were very sensitive to the specific parameters selection.Among the three methods,NNM was less capable and with the most mismatching,and NNM is only suitable for the particles with limited displacement between two successive images.(2)A surface flow field analysis software was developed by using C++ language in Visual Studio 2010 development environment on the basis of OpenCV(Open Source Computer Vision Library).Three PTV matching algorithm methods mentioned above,PyrLK-CCM,MPM,and NNM were encapsulated in the software.The flow filed results can be previewed directly in the software when choosing different parameters,and the other functions such as to determine the particle area manually were also included in the software.Calculation practices shown that the software was fairly stable,and with relatively high running speed,which is very capable for PTV flow filed analysis.(3)Flume experimental shown that the longitudinal velocity of surface flow around the 90°cross section were significantly influenced by the vegetation.Velocity near concave area were larger than in the center and convex area of flume.Compare with the condition without vegetation,longitudinal velocity was larger in concave area and smaller in the convex area.Meanwhile,the transverse velocities from convex to concave at the convex area of 45° and 90°cross sections were enhanced by the vegetation,and the transverse velocities from convex to concave at 180° cross sections were also enhanced by the vegetation.(4)The structure of water flow along the upstream and downstream were influenced by the vegetation.The flume experimental shown that:circulating flow were strengthened by vegetation,especially near and prior to the top of the bend;longitudinal velocity profile in the downstream areas of the bend were influenced significantly by vegetation,and the incidences were different at different flow condition;transverse velocity reduces gradually and approaches to zero along the downstream without vegetation,while when the flow discharge equal to 25 1/s and 301/s with vegetation,the transverse velocity were increased gradually at the downstream of flume bend.(5)Longitudinal,transverse and vertical velocities in 90° cross section at B/2 and B/4 location were significantly reduced by the vegetation.Among them,the longitudinal velocity decreases more than 60%compared with at the same position without vegetation,while the vertical velocity is along depth is nearly reduced to zero.The profile of longitudinal velocity and the transverse velocity were similar as shape of letter "J",and the direction of transverse velocity were always from convex bank to concave bank.In addition,the profile of velocities were similar at different discharge conditions.The longitudinal flow velocity distribution on the 3B/4 location of 90° cross section is significantly affected by the vegetation,which the graph of distribution combined with a plurality of upper convex and concave curves along the depth.The transverse velocity towards the concave bank also increases.But the change degrees of longitudinal velocity distribution graph and the increase degrees of transverse velocity towards the concave bank are related to the flow discharge.The smaller the flow discharge,the smaller the inertia,the more obvious the longitudinal velocity distribution deformation,the more the transverse velocity towards the concave bank increases.(6)Turbulence intensity of all the three direction velocities at the location B/4 and B/2 in 90° cross section were significantly reduced by the vegetation,as well as the turbulence intensity near the bed(y/h?0.2).The maximum turbulence intensity near the bed with non-vegetation is reduced by more than 60%in the three directions.The coverage of the vegetation also reduce the turbulence intensity in the three directions on the B/4 location of 45° cross section upstream and the 135°cross section downstream.(7)Bedload transport rate at the B/4 location of the 45°,90°,135°and 180°cross section,the B/2 location of the 45°,90°and 135° cross section were significantly reduced by the vegetation.(8)As one expected,the deformation processes of flume bed were completely changed by the vegetation.Firstly,an apparent scour pit emerged near the top of concave bank when the other side was covered by vegetation.The scour pit was located at the top of the bend under the condition of flow discharge of 20l/s,whereas it was near downstream the top of the bend when flow discharge were 25 1/s and 30 1/s.Secondly,the deformation of bed in the vegetation area were negligible under every flow condition.Thirdly,bedform shown tongue-shaped at the downstream of bend in the absence of vegetation,while when the convex bank was covered by vegetation,bedform at same section shown three different forms,which were flat bed,sand wave with straight or curved ridges,and tongue-shaped sand waves,respectively.(9)The bed elevation were measured under flow discharge of 30 1/s and the results show that:the scouring degree of concave bed with vegetation were much larger than that in the absence of between the 90° cross section and 135° cross section,and the scouring degree of the concave bed with vegetation were smaller than that in the absence of vegetation between the 135° cross section and 180° cross section.The convex area of bed without vegetation were deposited along the flume,but the vegetation covered make the convex area of bed deposited firstly and then scoured and micro-deposited finally,the scouring area of convex side were mainly between the 90° cross section and 150° cross section.In addition,the bed elevation in the vegetation area were relatively consistent.