Run-up Controls on Low-Tide Terrace Beaches Revealed by Non-Hydrostatic Wave Modeling

Wave run-up on low-tide terrace (LTT) beaches is strongly modulated by tidal water depth and local morphology, yet quantitative field observations in these environments remain scarce. Here we combine multi-sensor coastal observations with phase-resolving numerical modeling to investigate swash and run-up dynamics on an LTT beach in Nha Trang, Vietnam. A dense instrumentation array—including a 2D LiDAR system, an offshore ADCP, and camera imagery—was deployed to characterize free-surface fluctuations, shoreline excursions, and short-term morphological evolution. Using these measurements, we validate the non-hydrostatic version of CROCO through wave-resolving simulations forced by observed offshore conditions and time-varying beach profiles over 7 days. The model accurately reproduces tidal modulation of significant wave height, inner-shore hydrodynamics, and run-up statistics ($R^2$ > 0.7), including the nonlinear transfer of incident energy to the infragravity band. Sensitivity experiments reveal that run-up variability is governed primarily by wave period and surf-zone slope, rather than offshore significant wave height, within the observed forcing range. These depth-controlled mechanisms are characteristic of LTT beaches, where long-period swell and tidal modulation jointly influence breaking patterns and swash responses. We further develop an adapted Stockdon-type parameterization based on surf-zone slope, which performs comparably to CROCO and at times better once the terrace forms. This highlights the potential of simple, morphology-aware predictors for operational applications. Our results demonstrate the value of integrating remote sensing and wave-resolving models to characterize swash processes in tide-modulated environments, and provide new constraints for developing transferable run-up parameterizations on low-tide terrace beaches.