Beach Morphological Predictions: The Impact of a Temporally Varying Sediment Fall Velocity

Prodger, S.; Russell, P.; Davidson, M., and Miles, J., 2016. Beach Morphological Predictions: The Impact of a Temporally Varying Sediment Fall Velocity 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. 447–451. Coconut Creek (Florida), ISSN 0749-0208.

This paper introduces new field measurements that allow quantification of the relative importance of temporal variations in grain size to beach morphology classification. A dataset of 7 years of daily remotely sensed beach morphological measurements and monthly intertidal topographic surveys with surface sediment sampling were used to assess how observed temporal variations in sediment size (or fall velocity) influence morphological predictions at two energetic, sandy macro-tidal beach sites. Beach morphological predictions were obtained via the widely used sequential classification scheme of Masselink and Short (1993), where time varying wave height and wave period are usually used to drive changes in beach state. Beach state was found to be highly seasonal, with an evolution from more dissipative states in the winter to lower intermediate states in the summer. Beach state predictions were made using both a constant and time-varying sediment fall velocity and then compared to visual observations of morphological state. Predictions using a constant fall velocity correlated poorly (r² = 0.32) with observations, whereas predictions made using a time-varying fall velocity, correlated better (r² = 0.79). A feedback loop was also evident in the system, with energetic waves promoting a migration towards a dissipative state, and a coarsening shoreface grain size.