Physical model study on bank protection by revetment in a reach of the old Brahmaputra

Abstract

Bangladesh is one of the largest deltas in the world, which has been fOl1ned mainly with sediment deposited by the three mighty rivers namely the Ganges, the Jamuna and the Meglma. These alluvial rivers are continuously shifting their positions and changing shapes as a consequence of hydraulic and geomorphic forces acting on its bed and banks. The nature of this bank erosion and channel shifting problems demands intensive and careful 10ng-tel1n study both by physical (scale model) and mathematical models. For long term and short term hydraulic research activities including morphological studies on alluvial channels, a physical model facility has been developed the Department Water Resources Engineering, Bangladesh University of Engineering and Technology. This setup covers approximately 61 m length and 11.5 m width. It is equipped with 45.73m long and 10.67m wide sand bed, storage pools for water supply, upstream and downstream reservoirs, sedimentation tank, pumps, recirculating canal, measuring devices like weirs, water level recorders, point gauges, side rails and carriages etc. This physical model has been used for the study on bank protection by revetment. In general, revetment structures are popular as bank protection work in Bangladesh. The different types of revetment, which are mainly used here, are cubical concrete blocks and loose stones. In the present study the perf011l1anCeof hexagonal block for bank protection work was evaluated and was compared with cubical blocks. Previous application of hexagonal block in case of wave attack has shown that it is very effective and less costly as a protection work. In this research work, applicability of the hexagonal block in case of current attack and its performance has been evaluated. With this view, a scale model of an erosion prone reach of the Old Brahmaputra river has been developed. Then the perfol1nance of these two types of blocks i.e. hexagonal blocks and cubical blocks have been evaluated by placing them in the eroded bank keeping same condition for both of the blocks in different scenarios. The variables that are considered for the perfol1nance evaluation are different bank slopes (2H: 1V and 3H: 1V) and different values of velocity ranges. It has been revealed from the study that hexagonal blocks required 50% less amount of material as compared to cubical blocks of equivalent weight for the selected bank protection area. Inspection of the bed profile showed that hexagonal shaped revetment structure suffered less damage than the cubical shapes for both the bank slope conditions. From side slope stability coefficient it can be concluded that flatter side slopes gives more stability for both the cases compared to higher side slope which is close to the angle of repose of the material. On the other hand, comparison between cubical and hexagonal type of revetments on same side slope gives better stability for hexagonal type revetment blocks. Other factors such as, time required for placement of these two kinds of blocks show that cubical blocks require 40% more time than as compared to hexagonal block due to their smaller shape and size.