Pressure-Flow Scour Due to Vertical Contraction: A Review

Abstract

Climate and land-use/land-cover changes are amplifying hydrological uncertainties, which are increasing the probability of flood magnitudes exceeding their design values. Consequently, bridges are more likely to get submerged. The vertical contraction of flow during a submergence event causes the flow beneath the bridge to accelerate and transit from atmospheric flow to pressure flow. The pressure flow increases the velocity gradient and bed-shear stresses, thereby increasing the scouring potential of the flow and the risk of bridge failure. Laboratory and numerical experiments have been performed to relate the magnitude of pressure-flow scour to the flow geometry and flow, bed, and fluid properties. These developed prediction equations have found a place in design manuals. Nevertheless, many aspects of the pressure-flow scouring are confounding and are still unknown after 3 decades of research. In this article, an overview of the pressure-flow scour phenomenon is presented. The focus is on the important but special case of pressure flow with no pier that has received maximum attention in the literature. The experimental studies on pressure-flow scour are summarized, and inconsistencies and gaps in the reported results are highlighted. A compendium of data compiled from all existing data sets is used for evaluating the design equations and investigating their relative strengths and shortcomings. Based on the anomalies identified in the literature, the article questions the applicability of empirical and semitheoretical models for the pressure-flow scour phenomenon and suggests directions for future research.