Exploring Flow Dynamics in Overtopping Induced Dike Failure Processes through 3D Numerical Modelling

River dike failure events can have catastrophic consequences for the areas near the dike breach. Overtopping is one of the main causes of dike failures in rivers and understanding the complex flow dynamics in such cases is of vital importance for the modelling of dike failure processes and the prediction of the breach discharge that is required for flood warning systems and evacuation plans. This study investigates the intricate flow dynamics in different instances of an expanding dike breach due to overtopping with the aid of a 3D nonhydrostatic numerical model that solves the Reynolds-averaged Navier-Stokes equations. The dike breach evolving geometry was taken from a previous experimental study, which the present study complements by investigating the associated flow patterns. The numerical model was validated by reproducing fairly well the flow depth at the upstream part of the main channel (difference of ∼ 6% on average) and discharge partition at the breach (difference ∼6% - 15%). The flow surface patterns at upstream main channel differ from those below the surface for all tested cases. Specifically, the velocity magnitude close to free surface is approximately 50% to 70% higher than that near the bottom. Breach-induced secondary flow is observed at both horizontal and vertical cross-sections downstream of the breach while the distribution of velocity in the transverse direction is not uniform. Meanwhile, the level of water surface increases along the channel direction at the levee breach at the earlier stage of breaching. The expansion of the breach increases the turbulent kinetic energy peak locally near and downstream of the breach depending on the breach size. In addition, the simulated wall shear stress at the breach generally surpasses the critical erosion threshold of the dike material, and the zone of high shear stress extends downstream as the breach widens. This uneven distribution of shear stresses explains the asymmetrical lateral expansion of the breach. This study shows the complexity of the flow patterns, turbulent characteristics and the exerted wall shear stress during a dike failure process. From a practical perspective, 3D numerical simulations provide information that is challenging to measure in physical experiments and could also be used to calibrate lumped or 2D dike failure models, which are typically used in real-world applications.