ABSTRACT

Wang, J., and Sherman, D.J., 2016. Cusp development on a gravel 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. 937–941. Coconut Creek (Florida), ISSN 0749-0208.

Field experiments were conducted on a gravel foreshore at Portmore Beach, Ireland, to test edge wave, self-organization, and process-based hypotheses. The study site is on a 350 m long, meso-tidal beach, open to waves from the northeast. The gravel foreshore comprises 20–250 mm particles, and cusps are ubiquitous thereon. The foreshore is fronted by a sandy nearshore. Water levels, waves, and currents were measured with an instrument array that comprised seven Marsh-McBirney model 511, bi-directional current meters, and eleven KPSI pressure transducers. Prior to the experiment the study area was graded to remove existing cusps and a video camera installed to monitor morphological change. Beach cusps formed on the morphodynamically reflective foreshore within a few hours of inundation by the rising tide, with Hs = 0.66m, T= 12.2s: breaker heights exceeded 1.1 m and the surf similarity parameter was 0.83 for the foreshore. Mean cusp spacing and width were 7.6 m and 3.6 m. Analysis of phase relationships between waves and currents and the cross-shore structure of spectra indicated the presence of a mode 1, synchronous standing edge-wave that should have generated cusp spacing of 130 m. Analysis of video data indicated a mean swash excursion length of 4.66 m that should have generated cusp spacing of 7.9 m. The results support the self-organization hypothesis and suggest that although edge waves were present, they did not form the beach cusps.

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