Introduction:
Bridge stability under hydraulic forces is a vital area of study, especially during extreme flow events such as floods. Among the various bridge components, piers play a critical role as they directly interact with river flows. This interaction often results in scouring, a process in which sediment around bridge piers is eroded by flowing water. Scouring poses a significant threat to the structural integrity of bridges, making it essential to understand its mechanisms under different hydraulic conditions. Under high river discharge, flows may become pressurized as they pass beneath the bridge deck, leading to unique scouring patterns that differ significantly from those observed under free-surface flow conditions. Factors such as flow velocity, turbulence, and the arrangement of piers relative to the flow direction strongly influence the extent and characteristics of scour. This research investigates contraction scour around double bridge piers during flood scenarios, focusing on the effects of the presence or absence of a bridge deck. Additionally, the study explores the impact of pier arrangements whether perpendicular or parallel to the flow direction on flow dynamics and scour behavior. Through comprehensive numerical modeling, this study aims to address existing knowledge gaps regarding scouring under complex hydraulic conditions and contribute to safer, more resilient bridge-design strategies.
Materials and Methods:
Advanced numerical modeling was used to simulate and analyze the complex interactions between flow and scour around bridge piers. FLOW-3D, a computational fluid dynamics (CFD) software, was utilized along with the RNG turbulence model, well-suited for capturing turbulent flow characteristics. The experimental flume had a channel length, width and water depth of 9 meters, 0.6 meters, and 0.19 meters, respectively. The piers were modeled as cylindrical structures with a diameter of 0.06 meters and were arranged either perpendicular or parallel to the flow direction. The bridge deck was positioned 0.133 meters above the bed. To simulate sediment behavior, uniform sediment particles with a mean diameter of 0.74 mm and a geometric standard deviation of 1.4 were used. The key parameters included scour depth, turbulent kinetic energy (TKE), and bed shear stress, provided a comprehensive understanding of scour dynamics. Simulations were ran for 600 seconds to capture the development and stabilization of scour patterns under different configurations. The results were validated against experimental data, to confirm the credibility of the numerical approach. This methodology facilitated a detailed assessment of the effects of bridge deck presence, pier arrangement, and flow conditions on scour and associated hydraulic parameters.

Result and Discussion:
The numerical simulations revealed significant differences in flow and scour patterns between scenarios with and without a bridge deck. When a deck was present, flow accelerated beneath the structure, exhibiting jet-like behavior that intensified scour around the piers.For perpendicular pier arrangements, the maximum scour depth in the decked scenario was approximately 2.35 times greater than in the deckless case. For parallel configurations, scour depths around the upstream and downstream piers increased by 2.7 and 3.85 times, respectively, when a deck was present. The study also highlighted the role of turbulence in influencing scour dynamics. The presence of the bridge deck significantly amplified turbulent kinetic energy (TKE), particularly around the piers and beneath the deck. In perpendicular arrangements, TKE values were 2.1 times higher in the decked scenario. In parallel configurations, the highest TKE values occurred between the two piers, with values approximately 2.8 times greater than in the deckless case. Velocity distributions also varied notably between the two conditions. Horizontal flow velocity increased substantially in the presence of a bridge deck, with maximum velocities observed beneath the deck near the piers. For instance, in the perpendicular arrangement, the flow velocity between the two piers was 1.51 times higher in the decked condition than in the deckless one. These findings underscore the bridge deck's impact on increasing flow velocities and turbulent interactions, leading to more severe scour patterns around piers.
This study highlighted the significant influence of bridge decks on hydraulic conditions and scour patterns around double bridge piers. Numerical simulations demonstrated that the presence of a deck not only altered flow structures but also substantially increased scour depth, turbulence, and bed shear stress. These effects were particularly pronounced under pressurized flow conditions, where the interaction between the deck and underlying flow intensified hydraulic forces. For perpendicular pier arrangements, scour depths were approximately 2.35 times greater with a deck. In parallel arrangements, scour depths increased by 2.7 and 3.85 times for upstream and downstream piers, respectively. Turbulence parameters, such as Turbulent Kinetic Energy (TKE), also showed significant increases in the decked scenarios, further emphasizing the deck’s role in modifying scour dynamics. The findings provided valuable insights into the mechanisms of scour under complex hydraulic conditions and stressed the importance of accounting for bridge deck effects in design and maintenance practices. These insights contribute to the development of more resilient and safer bridge structures, particularly in flood-prone regions.