The statistics and kinematics of transverse sand bars on an open coast

Abstract

Abstract Ten years (1987–1996) of time exposure video images of the nearshore region at Duck, NC were used to study transverse sand bars, bathymetric features of intermediate length scales (10–200 m) oriented oblique or perpendicular to the shoreline. These transverse sand bars extend seaward from both the shoreline (trough transverse bars) and the shore-parallel sand bar (offshore transverse bars). Transverse bars had not previously been observed in an energetic coastal environment such as that at Duck, and their dynamics and role in nearshore processes is unknown. Frequency of occurrence statistics and length scales of the transverse bars were calculated using the video images. Trough and offshore transverse bars appeared a mean of 39 and 73 days per year, respectively. The offshore bars were found to be much larger features than the trough bars, with mean wavelengths (alongshore spacing between consecutive crests) of 79 and 172 m for trough and offshore bars, respectively. Both the trough and offshore bars were found to persist for periods of days to months. The alongshore movement of the bars was measured and compared to estimates of surf zone longshore currents which were calculated from wave height and wave angle data. Both sets of bars were observed to move at rates up to 40 m/day. At times, both trough and offshore bars were observed shifting in the same direction as the current was flowing, and at other times, both sets of bars remained stationary, even under relatively strong longshore currents. Trough bars were also observed moving against the current. An hypothesis, proposed by Barcilon and Lau (1973) [J. Geophys. Res. 78(15): 2656–2664], that the transverse bars were created as a sea bed instability under longshore currents, was tested by comparing the magnitude of estimated surf zone longshore currents with times of formation or presence of transverse bars. There was no evidence to suggest that the bars were formed by this simple longshore current instability mechanism. Instead, it is plausible that the combined effects of waves and currents may drive the formation of these features.