Onshore migration of a swash bar during a storm

The morphodynamics of a swash bar was monitored over two tidal cycles during a storm event at Skallingen, Denmark, Higher water levels during the first tidal cycle forced waves to break either landward of, or at, the bar crest. The bar was unable to significantly alter the pattern of wave breaking over the foreshore slope, creating a potential for bar erosion through an offshore transport of sediment. In response, the bar became more symmetric in cross-section, and its height decreased. Lower water levels during the second tidal cycle forced waves to break on the seaward slope, causing the bar to rebuild and its crest to migrate onshore. The onshore migration of the swash bar occurred despite little change in the offshore wave height from one tidal cycle to the next and in conditions that are typically associated with offshore transport and bar migration. The results further illustrate a strong tidal dependency in which water depth relative to bar height is an important control on the morphodynamics of the swash bar, but in a manner that appears contrary to recent evidence from the subtidal zone. Cospectra of velocity and concentration from collocated sensors during the second tidal cycle exhibited a strong infragravity signal, with sediment resuspension largely restricted to the onshore phase of the infragravity oscillations. Suspension during this phase was associated with the passage of individual surf bores and exhibited a depth-dependent phase lag with the velocity accelerations at the leading face of the wave. It is proposed that the primary role of the acceleration is to alter the amount of sediment suspended, with a net onshore transport accomplished by the skewness in the oscillatory velocity field. However, the onshore migration of the swash bar could only be explained by the gradient in the acceleration skewness, which was sufficiently coupled spatially with the morphology