Abstract
Scouring around bridge piers is one of the major reasons for bridge failure. A total of 20 experiments of scour around tandem piers were conducted in an open channel, at a subcritical inflow velocity (U/Uc=0.67) by placing an equal or smaller diameter circular bridge pier downstream of an existing pier with relative diameters in the range 1.00 to 0.67, at varied normalized spacings in the range 4 to 9.33, and detailed measurements of the equilibrium scour beds were taken. Separate scour holes with distinct deposition between the piers was observed in the majority of cases. Highest reinforcement effects were observed for pier combinations with Dd/Du=0.90 and 0.80, resulting in larger scour hole dimensions at the upstream pier, compared to isolated condition. The spacing corresponding to the minimum scour hole dimensions at the downstream pier was observed in the range L/Du=4 to 9.33 for each pier combination and it decreased with the decrease in diameter of the downstream pier. Variations for the normalized equilibrium scour depth, lateral and longitudinal extents of the scour hole, scour surface area, and scour hole volume as a function of normalized spacing for different ratios of downstream and upstream pier diameters, are presented in the form of graphs and equations.