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In the offshore environment, monopile foundations are facing significant local scour problems, which would have a serious negative impact on their stability and safety. The numerical simulation provides us with an efficient tool to investigate the patterns and underlying mechanisms of local scour. In this study, an Euler-Lagrange two-phase flow (CFD-DEM) model was employed to simulate local scour around monopiles. The CFD-DEM model was utilized to simulate several different scenarios, including sediment transport under undisturbed flow, flow around monopiles, and local scour around monopiles. In the first scenario, the LES model was incorporated to simulate turbulent flow, and it was observed that the CFD-LES-DEM model accurately predicted flow characteristics and sediment behavior within the water–sediment two-phase system. In the second scenario, the flow around the monopile was simulated under rigid bad conditions, and the characteristics of the flow field, including the horseshoe vortex and the wake vortex, were studied and validated. In the last scenario, the local scour around monopiles was simulated under different mean flow velocities, with simulation results being validated through model experiments. Based on experimental and numerical results, the phenomena and mechanisms of local scour were studied from a particle-scale perspective. [ABSTRACT FROM AUTHOR]

Backward erosion piping (BEP) is a failure mechanism of hydraulic structures like dams and levees on cohesionless foundations subjected to seepage flows. This article models the time-dependent development of BEP using numerical simulation of the erosion process. A 3-dimensional finite element equilibrium BEP model is extended with a formulation for the sediment transport rate. The model is compared to and calibrated with small- and large-scale experiments. Finally, a large set of simulations is analyzed to study the effects of factors such as grain size, scale (seepage length) and overloading on the rate of pipe progression. The results show that the development of BEP in the small-scale experiments is predicted well. Challenges remain for the prediction at larger scales, as calibration and validation is hard due to limited large-scale experiments with sufficiently accurate measurements. The results show that the progression rate increases with grain size and degree of overloading and decreases with seepage length, which is consistent with experimental observations. The model results provide a better physical basis for incorporating time-dependent development in the risk assessment and design of levees. [ABSTRACT FROM AUTHOR]