EFFECT OF STREAM-WISE SPACING OF CYLINDRICAL BRIDGE PIERS ON AFFLUX AND SCOUR

Abstract

The presence of obstructions like bridge piers, abutments etc. changes the flow pattern around these structures. The boundary layer that develops upstream undergoes a three-dimensional separation resulting in the formation of a horseshoe vortex. This leads to the formation of scour hole around the structures. An increase in upstream flow depth, i.e. afflux, is also observed after the placement of obstruction in the flowing stream. The scour depth affects the design depth of bridge piers, whereas the afflux is important in determining the height of upstream marginal embankment. Several investigators have studied the scour and afflux phenomena and developed relationships to estimate the equilibrium scour depth and afflux. But most of the studies are based on a single bridge pier, or piers of a single bridge. Due to increase in population and traffic volume, bridges are being constructed upstream or downstream of existing bridges and not very far from them. The present experimental investigation is conducted to study the effect of stream-wise spacing of bridge piers on afflux and scour. The experiments were confined to clear-water flow and steady flows. The temporal variation of scour depth, the equilibrium scour depth and afflux are studied. For in-line arrangement of piers, the minimum effect on afflux was small and a decrease in scour depth was observed for closely-spaced piers. But with the increase in spacing there is an increase in afflux as well as the equilibrium scour depth. For staggered arrangement of piers, an increase in afflux and equilibrium scour depth was observed, when the piers are closely spaced; with an increase in spacing there is decrease in both afflux and scour. Predictive relationships for the effects of interference of piers on afflux and scour have been developed on the basis of analysis of data.