Laboratory study on sediment distribution of asymmetric Bifurcated river

Abstract

Bifurcation of a river is observed when a river following in a single stream separate into two or more separate streams. Factors involved in bifurcation are the geometry of the bifurcation (cross-sectional area, depth, width, nose angle), sediment characteristics, particle size, fall velocity, bed load, suspended load, etc. Several studies have been carried out on different aspects of channel bifurcation, e.g., dynamics, equilibrium configurations, the evolution of channel bed and bank, and modeling. This study aims to investigate the mutual effect of sediment discharge and flow discharge due to the variation of nose angle in an asymmetric bifurcated channel. Therefore, stepwise methodology has been formulated to achieve the objectives. Hence, an asymmetric bifurcated channel (main channel and two bifurcated channels) with fixed banks and mobile sand bed has been designed and constructed accordingly in the sand bed facility of the Hydraulics and River Engineering Laboratory, WRE, BUET, to conduct experimental runs. A total of thirty-seven runs have been performed for three different nose angles with three different upstream discharges (20l/s, 30l/s, and 40l/s) for varying water levels while feeding sediment into the flowing water. Data have been collected precisely and observation has been noted. The collected data have been analyzed and presented in different forms. The study reveals that accretion is dominant in the main channel and erosion is dominant in branch-1 and branch-2 of the bifurcated channel. It is found that high discharge mostly results in erosion in the channels, except for nose type 2 (at high discharge). It is evident from the results that the reduction of inflow area at the entrance nose to a certain amount is the major variable for the difference in sediment transport through bifurcated channels and also, sediment transport in the bifurcated channels is independent of upstream water discharge. It is found that for a particular discharge ratio, the sediment transport ratio is increased for nose angle variation from negative to positive. Also, the sediment transport ratio decreases for nose angle variation from positive to negative when a discharge is held constant. However, the sediment transport ratio increases for nose angle variation from positive to negative when a discharge is increasing. An empirical equation has been developed that shows the sediment ratio is proportional to the discharge ratio and inversely proportional to the quasi-offtake (bifurcation) angle. The coefficient M and exponent k increase with the increase of discharge. For a particular upstream discharge, M increases as the nose angle changes from negative to positive. When the nose angle is held constant, the value of exponent k in the nodal point relation increases as the discharge increases. It is expected that the outcome of the study will be helpful for the assessment of flow and sediment movement in bifurcated channels.