Experimental study on the equilibrium scour pattern around isolated piers of varying diameter and inflow velocity

Scouring around bridge piers is the prominent reason for the failure of the bridge. The present study examines the entire scour hole geometry in detail, contrary to a majority of past studies that focused on only the equilibrium scour depth. Laboratory experiments were conducted in an open channel and consisted of measuring the equilibrium scour geometry around isolated piers of four different diameters, viz., 10 mm, 12 mm, 13.5 mm, and 15 mm, at three different inflow velocities with U/Uc= 0.4, 0.67, and 0.9. The scour was found to be initiated at U/Uc=0.4 irrespective of the pier size. All dimensions of scour hole were found to increase with the increase in size of the pier and the incoming flow velocity. The scour hole was found to be unsymmetrical about the pier, with steeper slopes at the upstream compared to downstream and greater longitudinal spread than transverse spread. Decreasing the flow velocity resulted in steeper deposition profiles with lesser spread and lesser difference between the longitudinal and lateral extents of the scour hole. Finally, a set of five empirical equations was proposed to predict the equilibrium scour depth, lateral and longitudinal extents of the scour hole, scour surface area, and scour hole volume as a function of pier size and the ratio of inflow and critical velocity.