Computational fluid dynamics modeling of abutment scour under steady current using the level set method

Scour around bridge foundations is one of the most consequential phenomena in hydraulic engineering. When river flow encounters an obstruction — a bridge pier, an abutment, or an embankment — the resulting flow acceleration and vortex systems erode the surrounding bed material. If the scour hole deepens beyond the foundation embedment, the structure is at risk of collapse. Scour is, in fact, the single largest cause of bridge failure in many countries, making accurate prediction essential for safe infrastructure design.

This research presents a three-dimensional numerical investigation of local scour around vertical-wall abutments under steady current conditions. The computational approach uses REEF3D, an open-source CFD solver developed for hydraulic and coastal engineering applications. REEF3D solves the Reynolds-Averaged Navier–Stokes (RANS) equations on a structured grid using a finite difference scheme. Turbulence is modelled with the k–ω model.

A key innovation in this work is the use of the level-set method for interface tracking. In scour simulations, two interfaces must be tracked simultaneously: the free water surface and the evolving sediment bed boundary. The level-set method represents each interface as a signed distance function, enabling the solver to capture complex interface deformations — such as the development of a scour hole — without remeshing. This is a significant advantage over body-fitted mesh approaches, which require expensive re-gridding as the bed geometry changes.

The sediment transport is modelled using established bed-load and suspended-load formulations coupled to the hydrodynamic solver. The bed shear stress computed from the flow solution drives the sediment pickup, and the bed elevation is updated at each time step based on the Exner equation for sediment mass conservation.

The numerical results were validated against experimental flume data for scour around a vertical-wall abutment under steady, unidirectional flow. The model successfully captured the primary flow features responsible for scour: the horseshoe vortex system at the base of the abutment, the flow acceleration around the abutment face, and the wake vortices downstream. The predicted equilibrium scour depth and scour hole geometry showed good agreement with the experimental measurements.

For practising engineers, this research provides a validated computational framework for assessing scour risk at bridge abutments, river training works, and embankment structures. Rather than relying solely on empirical scour formulae — which are often calibrated to narrow ranges of flow conditions and geometries — 3D CFD modelling enables site-specific, physics-based scour predictions that account for complex flow–structure interactions.