Flow and scouring around a pile with a punched-hole collar in steady flow

• Innovative collar design disrupts vortex formation, reducing bed shear stress. • Novel punched collar reduces scour depth by 14.29 % over traditional designs. • Punched collar reduces scour pit width by 21.43 % at front and 40 % at sides of pier. • Optimal collar performance achieved when the hole diameter is 27–30 % of the collar's downstream width. • An empirical formula was developed to optimize punched collar design efficiency. Local scour poses a significant threat to bridge safety globally, leading to substantial economic losses. This study employs a three-dimensional hydrodynamic and sediment transport model in FLOW-3D to investigate collar functions in reducing scour around piles in uniform flows, combined with flume experiments. Collars exhibit dual hydrodynamic effects on scouring around piles: (1) they positively reduce scour by blocking downward flows, and (2) they create negative pressure zones at the collar's leeside, which can accelerate near-bottom flows and potentially increase local scour. A punched hole on the collar’s leeside mitigates negative pressure, enhancing scour reduction. Based on the findings of this study, an appropriately designed punched collar outperforms a traditional plain collar, achieving a 14% decrease in scour dimensions. The width of the scour pit in front of and at the side of the pile was reduced by 21% and 40%, respectively. Pressure distribution analysis confirms that the punched collar reduces the pressure gradient beneath the collar around the pile, thereby lowering the maximum pressure difference and reducing erosive forces on bed materials, which improves scour protection. The innovative design addresses both horseshoe and wake vortices, potentially enhancing the safety and longevity of hydraulic structures in river and marine environments. These findings contribute valuable knowledge for engineers designing pier foundations with improved scour protection.