Non-Hydrostatic and Shock-Capturing Modeling of Free Surface Flow Driven Sediment Transport around Bridge Foundations

Sediment transport problems in rivers often arise under conditions involving a dynamic and complex free surface. Local scour around hydraulic structures can pose significant threats to the stability and safety of riverine infrastructure during extreme discharge events. To date, computational fluid dynamics (CFD) software using the two-phase approach are used for such scenarios, which comes at the cost of significant computational resources. This contribution presents a non-hydrostatic Navier-Stokes equations solver on a σ-coordinate grid that allows the grid to follow the variations of the free surface as well as the bottom. The approach is significantly more efficient then said CFD models. The model is developed within the open-source hydrodynamics framework REEF3D, which allows for use of the parallelization and high-order finite difference frameworks. For discretization, it uses a Godunov-type scheme for shock-capturing properties, allowing for stable and accurate representation of complex free surface conditions, such as hydraulic jumps. Bed load and suspended load transport formulations are implemented based on standard formulations. The possible sediment transport and scouring effects around the large bridge piers of a relatively old bridge over the river Nidelva in Trondheim, Norway are investigated. Due to the contraction effects of the piers, subcritical flow is forced for certain conditions. The numerical model captures the hydrodynamics and the free surface realistically, showing the possibility for a more efficient alternative to two-phase flow CFD simulations in such scenarios.