Abstract
Local scour at bridge piers is a critical phenomenon jeopardizing structural integrity worldwide. This study presents a comprehensive physical modelling investigation into the evolution of local scour in the presence of a protective armor layer, considering both steady and unsteady (hydrograph) flow regimes. A systematic parametric analysis evaluated the effects of pier diameter, flow velocity, and sediment characteristics, the latter comprising three distinct sizes for both the underlying bed material and the armor layer. Experimental observations elucidate the initial scour mechanism: the formation of strong horseshoe vortices that first accumulate armor material upstream of the pier. Subsequent vortex strengthening induces flow instability, leading to the entrainment and downstream transport of bed sediments. Under steady-flow conditions, the maximum equilibrium scour depth shows a strong positive correlation with pier diameter. A 33.3% increase in diameter resulted in a 31.2% increase in scour depth, while a 47.6% increase led to a 46.3% increase. Conversely, the maximum scour depth is inversely proportional to the size of both the armor layer particles and the underlying bed material. For unsteady flows, the peak scour depth was determined to be independent of the sequence of hydrograph events, converging to a value equivalent to the equilibrium scour depth of a steady flow at the hydrograph's peak discharge. However, the temporal evolution of scour depth varied significantly between the flow regimes. A dimensional analysis of the experimental data identified the governing dimensionless parameters. This analysis formed the foundation for deriving new predictive equations to estimate the maximum scour depth at bridge piers protected by an armor layer under both steady and unsteady flow conditions. The proposed relationships provide practical tools for the design and risk assessment of bridge piers.