COMPUTER SIMULATION STUDY ON DREDGING AND DYKING IN A BRAHMAPUTRA RIVER REACH

Abstract

Brahmaputra river system is very large and complex because of its braided nature, high discharge, and heavy silt load. It is one of the most braided rivers in the world in which the width varies from 1.5 km to more than 20 km. Sedimentation is one of the major problems in Brahmaputra river. Due to high sedimentation the river is becoming shallow and flood carrying capacity is rapidly reducing. A part of the solution is to dredge the river along with other complementary measures. Dredging removes the sediment from river bed mechanically or hydrulically. Moreover, the Government of Assam, India, is planning to construct two express highways along the entire length of the Brahmaputra in Assam state from Sadiya (at east) to Dhubri (at west), which in turn would help to control the river bank erosion. as well Dumping of dredged materials is the major issue while dredging the river. The dredged materials can be placed at both banks of river for two express highways. It solves the problem of space requirement for disposal of dredged materials. The embankment made by disposing the dredged materials would perform two functions i.e. flood dyke as well as highway on the both side of river banks. Flood dykes are very good alternative for disposal of materials which are unsuitable for open water placement. Combination of dredging and dyking is undertaken where river is braided as well as aggradation occurs. The past trend of the river form year 1957 to 1998 in terms of change in overall waterway and change in bed level has been found out using the available cross section data from Brahmaputra Board, Assam. The ongoing study focuses on analyzing the effects on fluvial parameters of the river due to dredging and dyking in Brahmaputra river reach. Study reach consists of 64 cross sections. USACE Hydrologic Engineering Center’s River Analysis System (HEC-RAS 5.0.3) model for the selected study area was prepared. Steady flow simulation has been conducted for the calibration of Manning’s n value. Observed and simulated water surface level has been compared for the validation. One-dimensional quasi-unsteady sediment transport simulation using realistic boundary condition and approximate model parameter is conducted with three different transport functions; viz Ackers-White, Yang and Engelund Hansen which are employed to evaluate the iv water surface elevation and bed level changes. The calibration and validation of the model is done by comparing the observed water surface elevation with the simulated water surface elevation. Goodness of fit has been found out to select the transport function which had given the better results. After selection of the transport function, the dredging and dyking analysis has been started. By using the same quasi-unsteady sediment transport model, the dredging and dyking scenario has been developed. As proposed by government, the river width has been kept in between 5.0 km to 7.0 km by using the longitudinal dyking. The depth of the dredging is found by iterative procedure until the flood conveyance capacity of the dredged channel section is to match with the original river channel. Since dredging cannot be done in whole river reach at a time, it is a prerequisite to identify the fluvial consequence of the partial dredging. For this study two dredging cases have been adopted; first one is the dredging commencing from upstream end and the second one is dredging operations from downstream end. Dredging from upstream end is done up to 186.1 km reach from upstream section 65(Sadiya). And dredging from downstream reach end is simulated up to 83.6 km from downstream section 2(Dhubri). After developing the dredging scenario for both cases, the simulation has been performed separately considering only one scenario at a time. From this study the result shows that dredging removes the sediment from bottom and water surface elevation has been accordingly depressed. Due to constriction of river width by dyking the channel velocity, shear stress, stream power and sediment transport capacity have been increased and hence the rate of deposition is curtailed proportionately. Moreover, the upstream and downstream impact due to partial dredging is also investigated. The upstream impact is studied for the scenario with dredging from downstream end (CS 2- 11). The results show the increase in velocity, shear stress and sediment transport capacity at upstream section for the case with downstream end of the dredging which is due to resultant degradation at immediately upstream end. Similarly, the downstream impact is studied while dredging commencing from upstream end (CS 64-46). The results show the decrease in velocity, shear stress, stream power and sediment transport capacity at downstream beyond the section from dredged reach, and this can be attributed to bed aggradation that has been observed at downstream end, from the simulations.