BED TOPOGRAPHY, BED SHEAR STRESS DISTRIBUTION AND VELOCITY FIELD IN A CONFLUENCE

Abstract

Laboratory experiment in 60-degree confluence was conducted using well-graded sand under active bedload condition for a discharge ratio of 0.6. The equilibrium channel bed was fixed and detailed measurements of mean flow were then carried out. Thereby, bed topography, two component velocity measurements, bed shear stress distribution and two counter rotating sec-ondary flow cells are reported. Also, flow mechanism and bed morphology development in a confluence are explained. Bed shear stress maxima appear over the upward slopes along which bedload is transported away from the scour hole. LIntroduction River confluences where two upstream channel flows of different hydraulic condi-tions experience changes in channel alignments, mixing of two flows to form a single river channel of different hydraulic conditions are present in every river system. This adoption occurs in a comparatively short river length and is characterized by bed scour, bank erosion, bar formation and water surface superelevation. Therefore confluences are critical elements in a river system and thorough understanding of them is essential in flood control, irrigation systems, navigation etc. The studies of river confluences have been reported by many researchers. Tay-lor(11) theoritically investigated the channel junction flow by one-dimensional ap-proach. Two-dimensional theoritical approaches have been employed to predict the size of the recirculation zone as documented by Webber and Greated(12), Modi et.al(7) and Fujita and Komura(4). Moreover, three-dimensional numerical models of river conflu-ence flows, based on k-e turbulence model, are documented by Tamai and Ueda(10), Weerakoon and Tamai(14). These models have provided the detailed flow field with two counter rotating secondary flow cells. As for the experimental investigations , Best and Reid(3) reported the size of recirculation zone with discharge ratio, Fujita and Komura(5) documented detailed measurements of velocity in a 90-degree confluence. Tada(9) performed velocity measurements in a confluence with a training levee. Ve-locity field in a 60-degree confluence with a recirculation zone was documented by Weerakoon et .al (13) . The available studies under movable bed conditions are however very limited. Mosley(8) carried out detailed laboratory experiments on confluence scour under active bedload conditions. He observed the two back-to-back vortices that cause the lens-shaped scour hole, and also the water surface superelevation over the scour hole. He also reported the increase of confluence scour depth with the increase of angle of incidence and discharge ratio. Also the decrease in scour depth with the increase of sediment load carried by the branch channels. Ashmore and Parker(1) analysed large amount of field and laboratory model data and reported that local geometry,i.e. the angle of incidence of the upstream branches, is the major factor for confluence scour and not valley parameters at all. They also found the influence of gradation and cohesivness of bed material in the reduction of scour depth. The possibility of modelling of gravel braided streams with low model Reynolds number , coarse noncohesive bed material has also been concluded. Best and Reid(3) investigated the sediment transport distribution in confluences and found that the strong helicoidal motions transport sediments from the upstream branches passing around the scour hole than through the center of it . • \307 • \