EFFECT OF DOWNFLOW ON SCOURING AROUND BRIDGE PIER

Abstract

Scouring around bridge pier is the process of removal of material around the pier due to two mechanisms of flow, viz. horseshoe vortex and downflow. There is controversy in literature as to the relative importance of the two mechanisms. While the recent study of Virender Kumar (1996) suggested that downflow may not contribute much to scour, the success of the slot of a scour-reducing device suggests otherwise. The main objective of this study was to conduct specially designed experiments on scour around pier to clearly understand the role of downflow on scour. Grids with spacing varying over the height were used to modify the approach velocity distribution significantly and thereby reduce the downflow. The effect of placement of such grids on scour has then been studied. The experiments were carried out in a tilting flume of width 0.6m, length 8m and depth 0.4m. A working section of lm length and 0.6m wide was made at 4.25m from the upstream end of flume and filled with sediment of size 1.4mm. The sediment of same size was glued on the bed everywhere else. Experiments were conducted for determination of velocity distribution upstream of working section with and without grid for different discharges in the flume. The scour depth around a 60 mm pier was studied for different discharges with and without grids and the data analysed to study the effect of placement of grid on scour. It was seen that the grids are quite effective in reducing downflow by modifying the velocity profile. The scour data indicated either no change in scour depth or even an increase in scour depth due to placement of the grid. It is concluded that downflow is not an important agent so far as pier scour is concerned.