Sandbars are dynamic morphological features commonly observed on sandy beaches. They protect the adjacent beach from direct wave attack and are important expressions of sediment transport patterns in the surf zone, migrating across the shore in response to varying wave conditions. One of our research goals is to understand and model the coupling between waves, currents, sediment transport, and morphological change that results in nearshore evolution, especially sandbar migration.
During the Duck94 – CoOP experiment, fluid velocities, pressure, and seafloor location were measured at 12 locations along a cross-shore transect spaning the inner and outer surf zones, to a depth of 5 m. These observations were used to investigate cross-shore bar migration mechanisms.


During storms, large waves break on the sandbar driving strong offshore flowing currents that move sediment and the bar offshore (A in the figure below). The offshore bar migration is predicted accurately by a commonly-used model known as an 'energetics-based sediment transport model', which relates sediment transport to mean currents and wave velocities.

Schematics of the feedbacks between waves, currents, and morphology that drive sandbar migration. (A) During storms, large waves break on the sandbar, producing currents that transport sediment and the sandbar offshore. The location of wave breaking moves offshore with the sandbar until conditions change. (B) Between storms, small unbroken waves pitch forward on the sandbar, producing strong onshore-directed fluid accelerations (rectangular insert in panel B) that transport sediment and the sandbar shoreward. The location of the peak in acceleration-induced transport moves onshore with the sandbar until conditions change.


Between storms, when waves are small and mean currents are weak, the sandbar migrates towards the shore (B in the figure above), but the energetics model fails to predict the observed onshore bar migration. Field observations (see also: Elgar et al. 2001.pdf) suggest that onshore sediment transport and bar migration might be related to wave-induced fluid accelerations, which are not included in the energetics model.

In the surfzone, near-breaking waves pitch forward, resulting in abrupt accelerations during the passage of the steep fronts of the waves, followed by gradual decelerations during the passage of the gentle rear of the waves. A model that relates sediment transport to such skewed accelerations predicts the observed onshore bar migration (see also: Hoefel and Elgar, 2003). A combined model that includes the effects of transport by mean currents, wave velocities, and wave-induced fluid accelerations simulates both onshore and offhore sandbar migration observed over a 45-day period.