Experimental study of local scour at the toe of protected embankment

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.