Scour at River Bend: A Parametric Study

Abstract

Scour at river bend is a complex process involving various parameters. Experiments were conducted in two laboratory flumes having 90° and 180° bends to study the influence of different parameters on scour at bends. Uniform non-cohesive sediments having median diameters varying from 0.28 to 1.18 mm were used as bed materials under different flow conditions. Three different cross-sectional shapes of the bed, namely flat, trapezoidal and parabolic, were used to study the effect of shape on scour. From the experiments, it is found that, at 90° bend, maximum scour and deposition occurred on concave and convex bank, respectively, between 50° and 60° azimuthal sections, whereas for 180° bend, the location was between 130° and 140° sections. Magnitude of scour was maximum for parabolic bed on the concave bank under flooded condition and deposition was maximum for flat bed on convex side. The deposition pattern on trapezoidal and parabolic bed was also different than that of flat bed. Again, for the same shear velocity ratio, the bigger size bed sediment produced larger scour depth but, for the same bed particle size, scour was more for greater approach shear velocity ratio. Further, measured three-dimensional time averaged velocity components before and after the scour on the 180° bend having initial parabolic bed shows the dominance of a spiral flow at the first half of the bend, and cross-flow from concave to convex at lower layer of the second half of the bend. The combined effect of vortex, cross-flow and accelerated flow is responsible for the erosion on the concave bank and deposition on the convex bank.