Dynamic equilibrium of sandbar position and height along a low wave energy micro-tidal coast

Nearshore sandbars play an essential role in dissipating incident wave energy and protecting the beach landward. Thus, understanding the dynamic equilibrium of nearshore bars is valuable to beach management and shore protection. This study examines the sandbar equilibrium in terms of bar height and cross-shore bar location, in order to assess how the dynamic equilibrium is maintained and influenced by storms along a low wave energy micro-tidal coast. The bar height and bar position were extracted from 51 beach profiles surveyed every two months, spaced at 300 m along a 15-km stretch of beach from October 2010 to August 2014. For the studied coast, alongshore variation in equilibrium bar position measured from the shoreline ranges between 40 and 80 m and equilibrium bar height between 0.20 and 0.70 m. Greater equilibrium sandbar height tends to occur around a headland, where waves are higher. Alongshore variations of bar behavior were observed during storms, with both onshore and offshore bar migration observed during one storm. Water depth over the pre-storm sandbar crest is a major factor controlling the storm-induced onshore or offshore bar migration. On average, the depth over the onshore migrating sandbar is found to be 0.20 m deeper than that over the offshore migrating bar during both summer and winter storms. There is no significant correlation between incident wave angle and sandbar height changes, while significant correlation exists between wave angle and sandbar movement under certain wave conditions, with more oblique waves being associated with further offshore movement of the sandbar. Energetic storm conditions tend to make the bar higher than the equilibrium height, while post-storm adjustment would restore the equilibrium height within 4-6 months. Although the exact values may vary at different locations, the concept of dynamic equilibrium of bar height and distance to shoreline could apply at many locations.