Daily to interannual cross-shore sandbar migration: Observations from a multiple sandbar system

The most conspicuous characteristic of the cross-shore behaviour of nearshore multiple sandbar systems in sea-dominated settings is interannual net offshore migration (NOM), while that of similar systems in swell-dominated environments appears to be the on/offshore migration in response to individual wave events. Here we analyse an approximately 7.5-year data set (July 1999–February 2007) of daily 10-min time-exposure video images of the swell-dominated double-barred northern Gold Coast, Australia and show that events with wave heights exceeding three to four times the annual average can trigger NOM behaviour similar to that observed elsewhere. During four such events, we observed how the outer sandbar migrated some 75–100 m further offshore than usual and then decayed during slow onshore migration in the subsequent low-energy weeks. The next, moderately high wave event caused the (≈20–30m/day) offshore migration of the inner bar to become the new outer bar and the onshore birth of a new bar. Between the large wave events both the outer and the inner bars responded primarily to the seasonal signal in wave height, whereby both bars were located further offshore during the higher (lower) energy winter (summer) months. We suspect that bar size (height and width) may control whether a multiple sandbar system exhibits episodic NOM (as observed here) or interannual NOM. In contrast to interannual NOM, episodic NOM is steered by the order of storm events.