Wave-impact characteristics of plunging breakers acting on gravel beaches

This paper presents observations of pressure impulses from wave impacts of breaking waves on steep coarse-grained beaches. It builds upon recent investigations on gravel beaches which suggest the wave impact of a plunging breaker is an important process that shapes the morphology of these beaches and that is not taken into account under many modelling frameworks (Pedrozo-Acuña, A., Simmonds, D.J., Chadwick, A.J. and Silva, R., 2007. A numerical–empirical approach for evaluating morphodynamic processes on mixed and gravel beaches. Mar. Geol., 241(1–2): 1–18). Full scale laboratory measurements of induced pressures by plunging breakers acting on gravel beaches are reported showing that these are more significant than those reported for sandy beaches under the forcing of similar wave conditions. Additionally, four different types of pressure-curves are identified and classified in terms of the type of forcing behind its generation. It is shown that measured impact pressures within the gravel beach are a consequence of plunging breakers propagating over the beach profile, with maximum pressure values being up to 4.5 times the quasi-hydrostatic induced pressure for a given wave height. It is demonstrated that the largest measured impacts are not only related to larger values of wave height and vertical displacement of the free surface but also to the wave steepness of the incident wave. Results suggest that between two plunging breakers of the same height, the steeper will produce a stronger wave impact. The plunging of breakers onto a gravel beachface and the ensuing pressure gradients are suggested as a key mechanism for understanding sediment transport and the resulting morphology of gravel beaches.