A multi-scale numerical study on coastal hydrodynamics, sediment transport, and morphodynamics

Abstract

This dissertation reports studies that advance the scientific understanding of coastal processes by elucidating the coupled dynamics among waves, flows, sediment transport, and morphodynamics across multiple spatiotemporal scales in the coastal environments. Utilizing high-fidelity computational fluid dynamics (CFD) with process-based morphodynamic models, this multi-scale numerical investigation spans three representative scales of coastal dynamics: (1) fine-scale sand particle sorting driven by grain-turbulence interactions, (2) small-scale wave-driven sand ripple evolution and the mobility of underwater munitions, and (3) intermediate-scale storm-induced cross-shore beach profile changes. By integrating insights across these scales, the dissertation seeks to reveal the fundamental coastal processes and underlying physical mechanisms observed in laboratory and field settings. ☐ At the fine scale, simulations using an Eulerian-Lagrangian two-phase model, CFD-DEM, investigate the vertical sorting of polydispersed native sand and denser nonnative particles (e.g., olivine for coastal carbon removal) under oscillatory sheet-flow conditions. Results show that competing upward and downward migration mechanisms control nonnative particle fate, offering insights for deploying the optimum size of nonnative particles that can stay in the active layer to maximize their weathering and carbon capture. ☐ At the small scale, large-eddy simulations (LES) using SedFoam, an Eulerian two-phase model, resolve turbulent coherent structures (TCS) that drive sub-orbital ripple formation from an initially flat sand bed under oscillatory flow. The results demonstrate that TCS are the dominant mechanism initiating the formation of three-dimensional (3D) bed features. At a later stage, when ripples grow sufficiently larger than the integral length scale of turbulence, the wave orbital motion takes over and becomes the dominating driver for the subsequent ripple evolution to equilibrium. These findings elucidate the fundamental coupling between TCS evolution and sediment transport during ripple development. Furthermore, by extending SedFoam to incorporate six-degree-of-freedom for object motion with complete flow-sediment-object interaction coupling, the new model was validated to simulate the onset motion behavior of underwater munitions. The simulation reveals that hydrodynamic forcing and object properties, such as object density, size, and initial burial depth, jointly influence the motion behavior of small objects driven by oscillatory flows. ☐ At the intermediate scale, cross-shore hydrodynamics and morphodynamics in the surf zone are first investigated using the process-based model XBeach-Surfbeat (XB-SB) and large-wave flume data for an erosive event. Simulations of storm-induced berm erosion, sediment transport, and sandbar formation reveal that default model settings overpredict undertow, leading to excessive berm erosion. Systematic calibration produces optimized coefficients that improve morphodynamic predictions based on a well-calibrated undertow. Extending this work to field conditions, XB-SB is applied to two 2023 experiments at the Field Research Facility in Duck, North Carolina, representing an accretive (March 2023) and an erosive event (November 2023). Results indicate that adjusting existing model parameters alone cannot achieve consistent agreement across the shoreline and sandbar regions, highlighting the need to incorporate geotechnical properties into morphodynamic models to represent increased sediment strength in the intertidal zone and to stabilize the foreshore under energetic wave conditions. ☐ Collectively, the findings of this dissertation establish a coherent linkage of physical processes across scales, demonstrating that integrating multi-scale insights yields a more unified understanding of coupled coastal dynamics and enhances the predictive capability of reduced-complexity models.