Armoring and Exposure Effects on Wave-Driven Onshore Sediment Transport in Sheet Flow

Wave nonlinearity plays a key role in driving coastal sediment transport. However, it remains unclear how sediment size gradation can affect wave-driven onshore/offshore transport in the coastal zones. Field and laboratory observations suggest wave-driven sediment transport generates inverse grading, where the coarser grains become exposed in the surface layer and finer grains become armored just below the surface. Using a two-phase Eulerian-Lagrangian model, where the fluid phase is based on SedFoam developed in the OpenFOAM framework and the particle phase is simulated with the open-source discrete element method solver LIGGGHTS, we quantified the inverse grading effects of wave-driven onshore transport during sheet flow conditions for medium and coarse grains. We validated the model for a wide range of grain sizes of unimodal and bimodal size distributions from medium (d₅₀ = 0.21 mm) to coarse (d₅₀ = 0.97 mm) using oscillatory tunnel flow experimental data for sheet flows. The armoring and exposure features were also observed in the model results. Due to armoring effect, the entrainment of the fine fraction was hindered, and due to the exposure effect the transport was dominated by the coarse fraction. Investigating the intra-wave sediment transport properties showed that the armoring was more effective during the weak near-bed flow (e.g., wave trough) when the mobility of exposed coarse fraction is low, and the armored fine particles have low chance of getting entrained. On the other hand, during intense near-bed flow (wave crest) both the coarse and fine fractions are mobilized. The observed preferential transport mechanism coupled with wave asymmetry during the onshore and offshore phases led to a large change in the predicted onshore transport rate when size gradation effect was incorporated. Since it is well-known that the ratio of suspended load to near bed load highly depends on the median grain size, we will focus on quantifying the effect of armoring and exposure on wave-driven onshore transport for a range of grain sizes along with analyzing the transport of the fine and coarse modes of sediment with very wide bimodal size distribution (d₉₀/d₁₀ ≈ 6).