Experimental Investigations of Scour Characteristics and Hydrodynamics around Bridge Piers under Pressure Flow Conditions

Bridges are often designed under the assumption of free-flow conditions under the decks. However, during the high flood events, the flow condition can change to a pressure flow as the deck becomes submerged, leading to vertical contraction of the flow. Therefore, it is important to understand the flow dynamics and scour characteristics under pressure flow conditions. In this regard, the present study quantified the effect of contraction height (Hb) relative to contraction length (L) on the flow pattern and the length and frequency of occurrence of vortices. Additionally, turbulent characteristics such as turbulent kinetic energy and Reynolds shear stress were also evaluated. The effect of the L/Hb ratio on scour development and equilibrium scour depth was analyzed across various pressure flow conditions. It was observed that the frequency and strength of vortices increased as L/Hb decreased, leading to higher turbulence intensity and scour depth. Irrespective of the L/Hb value, high vortex and turbulent strength were observed near the downstream edge of the deck. Notably, the scour depth in the combined pier–deck condition was observed to be substantially greater than that predicted by the HEC-18 superposition principle, indicating that the existing design guidelines through the existing superposition technique underestimate scour under pressure flow conditions. The findings highlight the need for improved guidelines that account for nonlinear pier–deck interactions in the submerged bridge flow conditions.