Three-Dimensional Numerical Simulation of Pressure-Flow Scour

An estimate of scour depth is required in the design of bridges. Much of the research on scour estimation has focussed on the local scouring, i.e. scouring near abutments or around piers. In comparison, contraction scouring has received less attention. Contraction scouring occurs when the waterway is contracted either laterally or vertically. During extreme flood events, the water level may rise above the bottom chord of the bridge, creating a pressure-flow situation in the passage below the bridge. The pressure scour thus induced is not clearly understood in terms of its location, geometry and time evolution. Some recent experimental studies have tried to establish empirical relations between scour parameters and various properties of flow, fluid and bed material. However, numerical approach has not been employed to study the pressure scour. The present study is an attempt to numerically simulate the pressure scour caused due to vertical contraction. International River Interface Cooperative (iRIC) solver NAYSCube is used to simulate the flow over mobile bed. NAYSCube, an open source 3D flow solver, employs finite volume method to solve Navier–Stokes equations and couples turbulence and bed-load models. Due to the limitations of the computational capacity, the simulations are performed to generate the flow and bed data for a period of one hour. The approaching flow velocity, channel slope and the sediment size of the bed are kept constant. The results show that bed shear has the maximum value inside the contraction section. The scour hole is observed to develop within the contraction towards the upstream face of the contraction. The magnitude of maximum scour is found to increase with increase in degree of contraction or length of contraction or both. The study establishes the need to conduct more numerical simulations to investigate pressure-flow scouring.