Abstract:
Bank protection works are essentially important parts of river training works. In
recent times, out of all types of river training structures, bank revetments works are
common in Bangladesh. The purpose of bank revetments is to prevent bank erosion as
a structural measure. They have to be designed to resist the current and wave and to
protect the river bed and river bank against erosion. Toe scour is probably the most
frequent cause of failure of riprap revetments. This is true not only for riprap, but also
for a wide variety of protection techniques. A flexible toe protection in the form of a
falling apron finds many applications in Bangladesh. For designing of bank protection
works in perennial rivers, where low water level prevails, the construction works have
always been a difficult task. Placing of underwater apron materials requires a
considerably higher skill, equipment demand and standard and practical approaches.
Such scenarios become more challenging as the channel bed composed of sand. In the
sand bed channel flowing under equilibrium condition may be considered as live bed
channel.
The present study has been undertaken to investigate experimentally two important
aspect of underwater construction such as the placing behavior and incipient condition
of toe protection elements under live bed condition. The experiments are conducted in
the Hydraulics and River Engineering Laboratory of Water Resources Engineering
Department, BUET. Three types of CC block and five types of geobag have been used
to conduct fifteen experimental runs with two different hydraulic conditions to
investigate placing behavior. For the incipient condition experimentation, four types
of CC block and five types of geobag have been used to conduct nineteen
experimental runs with various conditions. The measured data have been used to
obtain various relationships at the incipient condition of the toe protection elements.
Experimental results are analyzed to develop relationships between shear velocities,
depth averaged velocity, relative depth, turbulent intensity, depth factor, boundary
Reynolds number and Shield’s number. Developed empirical relationships can be
used to predict the shear velocity, depth averaged velocity, turbulent intensity and
depth factor at incipient condition for selected types and sizes of toe protection
elements. The proposed relationships are also compared with the equations available
in previous studies. Comparisons show that the predictive capacity of the proposed
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relationships is found to be satisfactory. Both CC block and geobag shows greater
shear velocity and shear stress in case of live bed than that of fixed bed. Shear
velocity on a live bed is about 2% greater than that of fixed bed for CC Block. For
geobag shear velocity is about 8% greater on a live bed than that of fixed bed. The
shear stress on live bed for CC block and geobag is about 4% and 16% greater than
fixed bed respectively. However, a comparison has been made between underwater
and dry bed toe protection to observe the incipient parameters.
It is suggested that the designer should consider the hydraulic parameters which
should be commensurate with underwater constructional aspects. Therefore, this study
will be helpful in designing of toe protection in case of underwater construction in
real life practice.
