Assessment of equilibrium pressure-flow scour depth using jet flow theory

Most models for predicting pressure-flow scour depth are based on use of the continuity and energy equations. The current study presents a model to predict pressure-flow scour depth using the momentum equation considering the jet flow deflected by the bridge deck. When approaching the bridge deck, the upstream flow acts as a jet flow that deviates toward the bed. Below the bridge deck, a combined jet-flow is created as a result of merging the initial jet-flow and the pressure-flow. The continuity equation is used to determine the thickness of the separation zone and the combined jet-flow angle. Equations are presented to estimate the combined jet-flow velocity and the thickness of the combined jet-flow. The study results reveal that the flow intensity, submergence ratio, and relative separation zone thickness are the principal parameters affecting the pressure-flow scour depth. The equation proposed in this study shows satisfactory accuracy to predict the equilibrium pressure-flow scour depth for a relative opening height of the bridge deck greater than 0.25. For relative opening heights of the bridge deck less than 0.25, insufficient duration of tests is a limitation of the available data to verify the proposed equation.