ABSTRACT

Stokes, C.; Russell, P., and Davidson, M., 2016. Subtidal and Intertidal Three-Dimensionality at a High Energy Macrotidal Beach. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 472–476. Coconut Creek (Florida), ISSN 0749-0208.

Three-dimensional beach features such as crescentic sandbars and rip channels influence a beach's response to storm waves, as well as significantly affecting the safety and amenity provided by the surf-zone for beach water-users. A dataset of 5.5 years of quasi-weekly bar measurements, and quasi-monthly intertidal surveys from Perranporth beach (Cornwall, UK) were used to quantify seasonal to inter-annual changes in three-dimensionality. These changes were seen to be dictated by the cumulative effects of antecedent wave and tide conditions. Although tide range was seen to be important, particularly at the outer bar, a disequilibrium stress term was well correlated with three-dimensionality at the inner and outer bars and lower intertidal beach. This term examines the disparity between the instantaneous dimensionless fall velocity parameter (Ω), and the overall mean dimensionless fall velocity (Ω̄), and is moderated by the instantaneous wave power (P). Low values of disequilibrium stress correspond to Ω that are cumulatively smaller than previous values, indicating the waves have moved from high steepness to low steepness, as normally occurs in the months following winter. It is under such circumstances, as the beach begins to recover, that maximum values of beach three-dimensionality and associated rip channel development were seen to occur.

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