Hydraulic Structure & Civil: Flood over a Bridge

Flood Over a Bridge — ANSYS Fluent CFD Simulation

A flood occurs when water overflows onto land that is normally dry — when a river exceeds its channel capacity, a levee is overtopped, or rainwater accumulates on saturated ground. Floods are a central concern in hydrology, civil engineering, and public health, and they pose a serious threat to man-made structures in a river's floodplain. This project uses ANSYS Fluent to simulate a flood surging through a dry riverbed and striking a bridge, with the goal of quantifying the forces the flood imposes on the bridge's pillars.

The geometry is built in Design Modeler and meshed in ANSYS Meshing with an unstructured grid of 844,311 elements.

The water–air system is modeled with the VOF (Volume of Fluid) multiphase approach, which tracks the free surface between the advancing flood water and the surrounding air. Water enters the computational domain — a dry river — at a velocity of 10 m/s, representing the incoming flood front. The simulation is run in transient, 3-D form with gravity enabled (−9.81 m/s² in the Y-direction), and turbulence is handled with the standard k-ε model. The transient setup is essential here: a flood is an inherently time-dependent event, and capturing how the water front advances and loads the structure over time is the whole point.

At the end of the solution, the results show clearly how a flood can damage and ultimately destroy a man-made structure like a bridge. One of the most important factors is the shear stress exerted on the bridge's pillars, which the simulation shows to be considerable. This kind of result has direct engineering value: it can inform the design of bridge pillars capable of withstanding extreme events like floods — choosing pillar shapes and structures that reduce the hydrodynamic loading. By the end of this project, you'll be able to set up a transient 3-D free-surface VOF simulation, model a flood front advancing through a domain, and extract structural loads such as shear stress on submerged structures to support resilient civil engineering design.