Comparison of modelled nearshore transverse sandbars with field observations

Patches of several transverse sand bars have been observed in the surf zone of open beaches spaced with a remarkable alongshore periodicity (from 20 to 200 m). A transverse bar is an elongated accumulation of sand attached to the low-tide shoreline that extends inside the surf zone with an oblique orientation up to 1 m depth. Due to the preferred breaking of waves at shallow areas, the white surface rollers over the bars are the clearest signal in the video-images that are collected hourly at different sites in the world. More than 40 events of formation and evolution of bar patches were recently described at Noordwijk beach, the Netherlands, using this technique. Bar migration and shape, with the crests deviating from the shore-normal against the alongshore current flow (up-current orientation), were evaluated. Wave conditions during bar presence were also determined.