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river channel every after five minutes to keep the portions of the main parts of the sediment
sediment concentration constant. The sediment depositions with an exception only in case of
samples were collected longitudinally and laterally embankment overtopping. In this case no coarse
from the main pad of the sediment deposition with sand (2.0-0.6 mm) and even no higher medium
a circular area of 10 cm diameter surrounding each sand (0.6-0.4 mm) were observed to have been
of the coordinates (X, Y) of the floodplain of the deposited over the floodplain as these coarser
model. In total, 1138 sediment samples were sands were at the lower depth of water flow in the
collected and analyzed in the laboratory to have the river channel due to their heavy weight, hence they
grain size distribution curves for each of the could not overtop the embankment and settle over
coordinates (X, Y) from where average d 5() values the floodplain during experimental runs. In case of
were determined. Finally, six soil (grain size overland flow higher medium sand (0.6-0.4 mm)
distribution) contour maps were drawn based on and lower medium sand (0.4-0.2 mm) were
the average d 50 values. Results of the model runs observed to have been deposited respectively at the
were compared with those obtained from the d 50 upstream and downstream of the main part of the
values of the grain size distribution curves of the sediment deposition. In case of embankment
sediment materials deposited over the floodplain of breach, piping, overland flow and variable (higher
the Dakkhin Patiata Beel due to the actual failure & lower) discharges the higher medium sand (0.6-
of the Chalan Beel Embankment during the 0.4 mm) and the lower medium sand (0.4-0.2 mm)
devastating flood of 1998. occupied the maximum area of the main part of the
sediment deposition over the floodplain with an
The results of the analyses showed that the exception in case of embankment overtopping. In
sediment deposition over the floodplain could be this case fine sand (0.2-0.06 mm) occupied the
divided into two parts: main part and tail part. The maximum area of the main part of the sediment
sediment deposition was the maximum in the main deposition.
part and minimum in the tail part. The most of the
silts and clay particles of the sediment materials 328 ISLAM, M.Z. & DATTA, A.R.
were observed to have been deposited at the tail (Institute of Water and Flood Management, BUET,
parts of the sediment depositions. The soil (grain Dhaka). AN EXPERIMENTAL STUDY ON
size distribution) contour maps of the sediment FORMATION OF SCOURS IN THE
materials deposited over the floodplain due to COUNTRYSIDE FLOODPLAIN DUE TO
embankment breach, piping and variable EMBANKMENT FAILURE. IWFM Res. Bull.,
discharges could be divided into four grain size 2003, 4, 1-12.
distribution zones corresponding to the grain size
(d 50) values of 2.0-0.6 mm, 0.6-0.4 mm, 0.4-0.2 Experimental runs 1-16 were conducted in a
mm and 0.2-0.06 mm considered for coarse sand, distorted model of the Institute of Water and Flood
higher medium sand, lower medium sand and fine Management (IWFM), Bangladesh University of
sand respectively. But in case of embankment Engineering and Technology (BUET) during 2001-
overtopping the same could be divided into two 2002 with the objective to observe the formation of
grain size distribution zones corresponding to the scours in the countryside (c/s) floodplain due to
grain size (d 50) values of 0.4-0.2 mm and 0.2-0.06 different lengths (L b) and depths (d b) of
mm representing lower medium sand and fine sand embankment breaches. Each group of experimental
respectively. In case of overland flow the same runs 1-4. 5-8, 9-12 and 13-16 was conducted with
could be divided into three grain size distribution different L b of 5 cm, 10 cm, 20 cm and 30 cm for
zones corresponding to the grain size (d 50) values particular d b of respectively 2 cm, 4 cm, 5 cm and
of 0.6-0.4 mm, 0.4-0.2 mm and 0.2-0.06 mm 6 cm of embankment breaches. Each group of
representing higher medium sand, lower medium experimental runs 1,5, 9 & 13; 2, 6, 10 & 14, 3, 7,
sand and fine sand respectively. In case of 11& 15 and 4, 8, 12 & 16 was also conducted with
embankment breach, piping and variable different d b of 2 cm, 4 cm, 5 cm and 6 cm for
discharges respectively coarse sand (2.0-0.6 mm) particular L b of respectively 5 cm, 10 cm, 20 cm
and higher medium sand (0.6-0.4 mm) were and 30 cm of embankment breaches. Each model
observed to have been deposited at the downstream
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