Coastal sand dunes are important natural barriers against storm surges and wave attack, but their protective function can fail rapidly when storm-induced wave runup exceeds the dune crest. This study focuses on wave overwash as a key marine geo-disaster process and investigates how runup, water-level rise, and dune morphology jointly control dune erosion and shoreline retreat. A series of movable-bed laboratory flume experiments were conducted under irregular-wave and solitary-wave conditions, with high-speed video imaging used to continuously monitor wave runup, overwash flow, shoreline migration, and dune-profile evolution.

The results show that overwash leads to rapid adjustment of the dune-beach system, including dune-crest lowering, landward sediment transport, and measurable shoreline retreat. Under irregular waves, water-level rise accelerates the overwash process and increases sediment loss from sand dunes by more than 40%, indicating that rising water levels can substantially amplify coastal erosion hazards. The experiments also reveal a close relationship between hydrodynamic forcing and morphological response: overwash water volume is linearly related to deposited sediment volume, while dune erosion is approximately proportional to wave energy flux. In addition, secondary dunes are shown to reduce shoreline retreat by more than 40%, highlighting their potential role in nature-based coastal protection.

To improve hazard identification, wave runup observations were compared with empirical runup formulas and XBeach numerical simulations. A dimensionless overwash threshold parameter based on wave runup and dune crest elevation was introduced to distinguish collision and overwash regimes. This runup-based criterion provides a practical indicator for identifying the transition from relatively stable dune response to overwash-dominated erosion. The findings offer a physically grounded framework for assessing coastal dune stability, predicting shoreline-retreat risk, and supporting marine geo-disaster prevention under future storm intensification and water-level rise.