Abstract:
Scour around structure is a well known problem for engineers aroun'd the world, It has been
studied extensively for a long time as a serious threat to human life safety and sever structural
damage due to scour results huge economical loss, Most of rivers in Bangladesh are compound
in nature and alluviaL Pier like structures are often constructed at floodplain of compound
channel which are also subjected to local scour and may cause failure of structure, This current
experimental research has been carried out to investigate general behavior of local scour around
pier like structure at floodplain of compound channeL Also effect of river bed material on local
scour and comparison with main channel local scour characteristics was studied in present work,
A compound channel of 22 m long and 20 cm deep having one adjacent floodplain of 80 cm
wide with 165 cm wide main channel has been constructed to conduct present experiment. Flow
depth was kept 7 cm on floodplain and 27 cm in main channel considering relative depth ratio
(Y,) 0.26. Uniform flow and live bed scour condition has been maintained throughout
experiment. General behavior of local scour at floodplain has been investigated for three locally
available bed materials having sediment size (dso) of 0.75 mm, 0.18 mm and 0.12 mm
respectively. Commonly used circular and round nose shaped structures were used in this study.
In total 36 experimental runs has been conducted for three discharges (200 lis, 175 I/s, 150 lis)
and for four length-width ratio (l1b=1 for circular pier and 11b=2,3,4for round nose pier). To
evaluate comparison between floodplain and main channel scour, one structure was placed on
floodplain when another one in main channel at a time. Scour map, bed profile, velocity profile,
VNc contour map and 3D perspective plot were analyzed for both floodplain and main channeL
Generally maximum scour depth was found for finer bed material at higher discharge. Scour
depth significantly decreases with increasing length-width ratio and increases from coarse
sediment to finer. Finer bed material resulted more steeper, uniform and deeper scour comparing
with coarse sediment. Circular pier scour depth was highest than any other structure shapes.
Scour depth varied with velocity variation of flow and an increasing tendency of scour depth has
been observed with increasing flow intensity. Flow velocity and scour was always higher in
main channel than floodplain. Floodplain velocity distribution is associated with vortex
generation as mid of channeL Maximum scour depth has been varied almost linearly with
Froude number irrespective of pier shape. An empirical relationship was developed using
experimental data to predict floodplain scour depth. Proposed equation has been compared with
available scour equations.
