Pier scour and thin layered bed scour within a long contraction

Abstract

The paper presents an experimental investigation of (i) scour at a pier within a long contraction and (ii) scour of a thin layered bed within a long contraction. The scour depth at piers within long contractions increases with an increase in sediment size and a decrease in channel opening ratio. A theoretical calculation proposed to estimate the maximum equilibrium scour depth suggests that it is the summation of the individual equilibrium scour depth within a long contraction and the equilibrium scour depth at a pier under critical flow conditions in the upstream bed. The scour depth (relative to the approaching flow depth) within long contractions with thin gravel layers increases with an increase in the ratio of the diameter of the surface gravel to that of the bed sand and a decrease in the channel opening ratio. The scour depth within a channel contraction with a gravel layer, however, is greater than that with a unlayered bed of uniform sediment. Further, the maximum equilibrium scour depths within long contractions with gravel layers calculated theoretically using the energy and continuity equations are in agreement with the experimental data.