DESIGN ANALYSIS FOR CHANNEL RECTIFICATION OF GANDAK RIVER IN BIHAR

Abstract

River morphology is one of the most complex natural phenomena,, not only because the flow and flow patterns vary in temporal and spatial dimensions, but also because the coupling to the river valley and channel, in which the flow is contained, varies. This complex morphological nature of alluvial rivers and their effect' on the well being of the human race settled along and around the system has necessitated a wide range of research interest. These interests are driven by a desire to solve and harness the erratic and sometimes destructive nature of these rivers. To understand the physical processes occurring in rivers, the development of physical and mathematical modeling methods have proven to be instrumental. The concept of numerical modeling for flow simulation has found large acceptance over the past decades as a means to simplify and solve these problem. In this present study a hybrid modeling is attempted in the Gandak River around Gopalgunj Area. One dimensional HECRAS 4.1 model is used to study the general sediment flow along the reach. PHYSICAL MODEL study is used to simulate the area adjacent to the bridge site. The study is supplemented by the application of satellite based spatio-temporal variation of channel migration in the vicinity of study area. Major channel migration occurred between the year 1988 to 2010 The channel has shifted its course erratically, resulting in subsequent shifting and bifurcation. The physical model is run for two scenarios, one with clear water condition and the other for sediment concentration of 2000ppm. Model dimension on the ground is 25m by 12m. Geometric scale of Lr =400 and Zr= 50. With clear water simulation the model is verified against observed stage and discharge. The Numerical Model study shows extreme aggradation and degradation all along the study reach. The Physical Model study shows from that deposition has occurred in the middle of the channel between the abutments and erosion while scouring occurred at the toe of the guide bank. And it .is also evident that the left guide bank is under more attack due to the secondary flow generated at the-upstream section of the guide bank. To prevent the damage that may occur on the guide bank and to channelize the flow to the centre of the river, submerged vanes are proposed.