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.
