Abstract:
Local scour occurs around embankments, spur dikes and abutments because of the
obstruction to flow caused by such structures. The scour hole develops as overbank
flow reenters the main channel and scts up large vortices to wash sediment away. But
in laboratory tests, scour hole seldom stays constant under a constant discharge,
because the depth of scour fluctuates with time when there are dunes moving on the
alluvial bed. Embankments those are directed downstream have smaller scour holes
than those of angled upstream becausc the scour depth is directly related to the extent
of the obstruction to flow. Scour dcpth around embankment-like structures increases
as the value of blh (b= length of the structure in the lateral direction and h=approach
flow depth) increases up to certain extent. A number of scour depth prediction
formulae available so far have been analyzed in order to find out appropriate scour
predictor around such structurcs in Bangladesh. The study was made under clearwater
condition. The hydraulic parameters were set so that the flow is close to the
critical condition of sediment transport that is clear-water condition.
For scour study, three sloping-wall embankment type structures and three verticalwall
embankment type structurcs wcre used. These structures were made of half-inch
thick particleboards and wood. Around these structures the scour depth, scour contour,
scour pattern, scour area, velocity vector and veliical velocity distribution were
observed. Bed level variation of each structure was compared with others. Each test
was run for 7 hours and 30 minutes. This test running time was selected based on the
limitation of laboratory pum]) and in an attempt to reach equilibrium stage. Results
were compared with some existing prediction methods (LaceyI930, Melville I 992,
Rahaman and Muramoto 1999, Liu 1961).
It was found that for vertical-wall structures the results were close to Lacey's (1930)
method. Particularly, the value of vertical structure with larger width, b=60cm was
very close to Lacey's (1930) method. For sloping-wall structures experimented value
were near to Lacey's method. For Melville's (1992), Rahaman and Muramoto's and
Liu's (1961) cases scour values were higher than Lacey's.
Finally, suggestions have been put forward for further extension of the study.
