Seasonal rotation of a mixed sand-gravel beach

Abstract

Beach rotation is the result of alternation between two prevailing wave directions in which sufficient time elapses to allow longshore sediment transport to drive sediment to one end or other of a headland enclosed bay. The key features of beach rotation are usually a bi-directional wave climate with sufficiently persistent episodes of each wave direction to alter the beach orientation, and headlands to trap the sediments transported along the shore. In this paper we examine shorelines derived from digital cameras that show a seasonal beach rotation due to changes in wave direction, but in the absence of any headlands. We speculate that the driving forces behind the beach rotation are the winter and spring wave regimes, and the presence of the adjacent sand bank. Beach rotation in the absence of headlands could conceivably result from longshore transport gradients. Coughlan et al. (2007) showed that there are steep gradients in longshore transport in the lee of the sand bank partly as a result of significant alongshore variability in wave height. The shoreline data also show longer-term responses that are likely to be connected to changes in bank position.