ABSTRACT

Low energy beaches are often characterized by a particular series of offshore sand bars which differ markedly, both in shape and sediment characteristics, from the classic offshore bar of high and moderate energy environments. This study reports on the physical interactions of waves and currents with low energy bathymetric features that result in producing a distinct areal distribution of sediment parameters are well correlated with the subtle bathymetric features of the offshore area and thereby may provide a method of determining low energy environments in ancient deposits.

Prominent features in low energy environments are subtle digital sand bars which trend perpendicular or at a high angle to the beach face. These bars are called transverse bars. These bars have maximum relief of approximately 0.5 meters, lengths varying between 100 meters and 3,350 meters, and a quasi-regular spacing varying between 300 and 2000 meters.

Investigations by several authors indicate that these bars act as avenues for onshore or offshore sediment transport. Recent study by the present author has determined that the sediment is transported as a result of a nearshore current component caused by the transfer of momentum from unstable nearshore waves to the currents. The current component is characterized by a narrow relatively high velocity current directed along the axis of the bars with sluggish return currents between the bars. This results in producing relatively coarse, well sorted sediments along the crest of the bar and finer, less sorted sediments between the bars.

The scale of these features as well as the obvious differences in sediment parameters should permit their recognition in ancient deposits. Their presence allows inference to be made about ancient wave energy levels and average wave approach directions.