Wave-Driven Vertical Sorting of Density-Varying Particles

Abstract

It has been recognized that the vertical sorting of polydispersed sand grains via the “Brazil Nut effect” can lead to inverse grading (upward coarsening) in the surface layer of a sand bed. However, the addition of nonnative particles not inherently observed on the beach, characterized by different densities and sizes, may complicate the vertical sorting processes and the fate of the nonnative particles. For example, olivine particles, which may be released on natural sandy beaches to facilitate carbon dioxide capture via weathering from wave action, have a density about 25% larger than the typical native sands. An Eulerian-Lagrangian model, which couples Computational Fluid Dynamics for the fluid phase and Discrete Element Method for the particle phase was utilized to investigate how the so-called “Reverse Brazil Nut effect” due to the larger density of nonnative (olivine) particles may be counteracted by the so-called “Brazil Nut effect” for a range of nonnative particle sizes. Numerical simulations showed that the higher-density nonnative particles tended to sink into the sand bed consistent with the “Reverse Brazil Nut effect”; however, the vertical sorting structure of native sand, driven by the “Brazil Nut effect,” also was a controlling factor determining the fate of nonnative particles. To maintain the presence of the nonnative (olivine) particles in the active layer of sediment transport, the higher-density nonnative particle size must be larger than the mean native sand size found at the bottom of the active layer.