New large-scale laboratory experiments into the beach slope effect on intra-swash sand transport processes

In this paper, new large-scale experimental data are presented showing the beach profile evolution, sediment transport and intra-swash hydrodynamics of two different beach slopes (1:15 and 1:25). In these experiments an erosive bichromatic wave series with groups of 4 waves (equivalent to random time series with Hs=0.65 m and Tp=3.5s) were produced in consecutive runs with each wave group repeating every two group periods (TR=2Tg=28.6s) in order to produce distinct types of swash interactions. Results show that the swash at the 1:15 slope developed a concave morphology with offshore directed transport, as typically seen for erosive conditions. Conversely, the swash at the 1:25 slope developed a beach step with overall onshore-directed transport typical of an accretive beach, even though the surf zone showed characteristics of an erosive regime. The differences in the developing morphologies are associated with the differences in wave-swash interactions observed at the different slopes. Following the observed intra-swash hydrodynamics, we hypothesize that the stronger bores on the steeper slope result in shorter swash periods, leading to variations of the swash interactions’ cross-shore location and to offshore-directed transport in the lower and middle swash. On the gentle slope, the backwash vortexes that generate the beach step stir up the sediment that is transported and deposited onshore by the following longer up-rushes. Further analysis of intra-swash sediment concentrations, both suspended and in the sheet flow layer, and hydrodynamics will be done to investigate the slope effect on the sediment transport processes.