Abstract:
River bank erosion has always been a challenging problem in Bangladesh. Conventional
method of designing erosion protection structures are governed by the hydraulic loads
resulting from currents and waves. The effectiveness of design of protection works
mainly depends on its constructional aspects. The appropriate method of construction
depends on mechanism of settling behavior of protection elements. In practice, toe
protection elements are dumped into flowing water and settle somewhere on the river bed
to form an apron. But the placement of elements at designated positions is difficult to
ensure.
The present study has been undertaken to investigate experimentally two important aspect
of underwater construction such as the settling behavior and threshold condition of toe
protection elements. The experiments are conducted to determine the fall velocity in a
square shaped settling column of 30 cm X 30 cm X 130 cm and in the large tilting flume
of the Hydraulics and River Engineering Laboratory of Water Resources Engineering
Department, BUET. Sixteen different sizes of elements ranging from 1.6 cm X 1.6 cm X
1.3 cm to 4.1 cm X 4.1 cm X 2.7 cm have been used to conduct 64 experimental runs
with the discharges range from 0.033 m3/s to 0.203 m3/s. During experimentation various
observations are made and the measured data are used to obtain various relationships for
the settling behavior of the toe protection elements.
Experimental results are analyzed to develop relationships between the relative size and
flow parameters. Developed empirical relationships can be used to predict the settling
velocity, horizontal settling distance and 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 relationships is found to be satisfactory i.e. for fall velocity prediction
equation, the error is 3.91% for CC block and 2.38% for geobags. To estimate settling
distance results show that the developed equation also performed better compared to Zhu
et al. (2004). Also verification of the proposed equation has been done using the
independent set of laboratory data and result shows satisfactory agreement with an error
of 3.80%.
It is hoped that the outcome of the present study can be used as a tentative guideline for
under water construction of toe protection elements in river bank protection works.
