Swash bar stability on a high-energy mesotidal beach

Swash bars are intertidal bar systems characterized by a single, small, low-amplitude bar and trough, often displaying a rhythmic pattern along the shore with troughs converging into shore-normal rip channels. In this study, we assess the stability of swash bars on Spiekeroog North Beach—a high-energy mesotidal beach in the German North Sea—using high-frequency fixed-camera imagery and wave and water level records collected over >100 days. Swash bar size fluctuations and migration rates were calculated and related to water level and wave heights. Three distinct states can be defined: dynamic equilibrium (migration ≤15 m/day, associated with fair-weather conditions), active migration (migration >15 m/day, occurring under moderate to high energy levels), and no-bar states, typically following high-energy events such as storms. Additionally, bar creation and destruction patterns were analysed in relation to local hydrodynamic conditions. These findings were synthesized into a stability diagram, illustrating the likely behaviour of swash bars under specific wave and water level conditions.