| dc.description.abstract | Sandy beaches worldwide often display pronounced morphological features, such as crescentic bars, rip channels and beach cusps. Sometimes also a less well known pattern of transverse finger bars develops. These bars are relatively small, attached to the inner bar or the shoreline, and have in general an oblique orientation. Four years of time-exposure images permit to study the occurrence of transverse finger bars in the surf zone of a beach on the Gold Coast, East Australia. Transverse finger bars are thus observed to occur here on 24% of the study period. The finger bars occur in patches of 3 to 15 bars, with an average transverse finger bar event duration of 5 days. The presence of bars is related to intermediate energy wave conditions. When the offshore wave height is higher than 1.2m., transverse finger bars are never observed. In the majority of the cases, the crests of the transverse finger bars point against the direction of the incoming waves: up-current oriented bars. The hypothesis that, apart from the wave conditions, also the overall morphology of the nearshore zone is of influence to transverse finger bar development, is tested by looking to the shape of the shore parallel bars at the time of transverse finger bar occurrence. It is shown that finger bar occurrence is often coinciding with a state of the shore parallel bars that reflects low to intermediate energy conditions. A morphodynamic model has as well been applied to investigate the Gold Coast transverse finger bars. This model describes self-organization processes in the nearshore zone: the feedback between waves, breakers, depth-averaged currents and bed evolution. The model proved capable of describing the initial growth of up-current transverse finger bars, with characterstics corresponding well to the observations. The model has been used to study the mechansm of transverse finger bar growth. It explains why only waves with a minimal offshore angle of 20 degrees with respect to the shore normal, and an intermediate height promote transverse finger bar growth. The longshore current is found to be the main factor controlling transverse finger bar formation, although also the effect of the turbulent sediment resuspension by the rollers formed on wave breaking plays a role. | |
