Temporal evolution of local scour around bridge pier-abutment configurations: effects of pier-abutment spacing and blockage ratio

Local scour around bridge foundations remains a major threat to structural stability, particularly when bridge piers and abutments are placed within the same contracted flow region. This study experimentally investigates the temporal evolution and final morphology of clear-water scour around coupled pier-abutment configurations, with emphasis on the combined effects of pier-abutment spacing and blockage ratio. The experiments were conducted in a recirculating laboratory flume using a rectangular vertical-wall abutment and single and double-pier arrangements. The results show that pier-abutment spacing has a non-monotonic effect on abutment scour. At the closest spacing ( x/Dₚ=1.5 x / D p = 1.5 ), the pier partially shields the abutment by reducing the direct erosive action of the approach flow. In contrast, the intermediate spacing ( x/Dₚ=3 x / D p = 3 ) produces the most severe abutment scour because of constructive interaction between pier-induced and abutment-induced flow structures. Under this condition, the maximum abutment scour depth increases by 10.9% compared with the close-spacing configuration and by up to 55% compared with the far-spacing ( x/Dₚ=6 x / D p = 6 ). Increasing the blockage ratio further intensifies scour, raising the maximum scour depth by approximately 7.4% at the abutment and 10% at the first pier. The temporal scour response follows a clear biphasic pattern, with 65–75% of the maximum scour depth developing during the initial 10–15% of the test duration, followed by a slower asymptotic approach toward equilibrium. The final scour morphology reveals pronounced upstream–downstream asymmetry, with deeper upstream scour holes controlled by downflow and horseshoe-vortex action and shallower downstream depressions associated with wake vortices and sediment redistribution. These findings demonstrate that pier-abutment spacing and blockage ratio jointly control both the rate and magnitude of scour development, highlighting the need to consider their combined effects in bridge-foundation design and scour-risk mitigation.