Quantifying overwash in a laboratory experimental analysis using bichromatic waves

Overwash influences dune resilience and sediment transport, yet the physical parameters governing this process remain poorly understood due to complex hydrodynamic and morphodynamic interactions. This study investigates how broad and narrow-band wave groups and water level variations govern the feedback between overwash dynamics and dune morphological response. Three overwash scenarios were identified: (1) early-stage overwash, where the system begins in overwash and transitions to collision; (2) intermittent overwash; and (3) persistent overwash, characterised by stronger onshore sediment transport.

Results indicate that small initial water level variations ($<$4% of the water depth) can significantly influence overwash intensity and frequency, with the highest tested water level (0.775 m) producing the most persistent overwash conditions. Longer wave groups enhance overwash initiation by maintaining elevated infragravity swash across successive waves, increasing the likelihood of crest exceedance. In contrast, shorter groups generate more frequent individual runup events that interact with the dune, increasing overwash occurrences. However, under high water levels, wave group effects become secondary.

The Overwash Potential (OP) metric is assessed as an indicator of overwash occurrence. Findings show that threshold values between collision and overwash are scale-dependent, requiring calibration to reflect dune freeboard effects accurately. Additionally, OP estimation is highly sensitive to the beach slope definition; using a non-representative slope can underestimate OP exceedance, overwash frequency, and severity.

Future laboratory studies should treat water level as a key design parameter and incorporate long-term morphological feedback and field-scale validation. These steps will improve predictive model accuracy and inform the development of effective coastal resilience strategies under extreme conditions.