Entrainment of Sediment Particles in Protection Layers

Abstract

Protection layers are often used to prevent scour and erosion, e.g., prevention of scour around wind turbine foundations. However, several cases exist where installed scour protection has settled, where loss of sediment through the armor layer can explain the failure. This paper presents the use of a detailed large eddy simulation–discrete-element method (LES-DEM) model to study sediment particles in porous media. First, a simple idealized case of the removal of sediment from an idealized cavity beneath a smooth turbulent boundary layer was set up. The model showed the penetration of turbulence, mainly in the form of sweep events, into the cavity. This high momentum would at times reach the bottom and entrain the fine sediments. The sediment would subsequently roll over and form a pile and at times be suspended from the return flow of the penetrating turbulence. Finally, a more realistic armor layer was set up with a series of closely packed spheres. Fine sediments were seeded at the bed. A hydraulically rough boundary layer was developed over the armor layer, where turbulent statistics from the model compared well against experiments. Turbulent structures characteristic of the bursting process were identified in the rough wall case. The penetration of sweep events’ entrainment and suspending the finer sediments is detailed. The flushing of cavities from passing ejection events is also presented.