Sediment suspension events in the inner surf and swash zone. Measurements in large-scale and high-energy wave conditions

The present study presents a database of hydrodynamic properties and suspended sediment concentration collected within the inner surf and swash zones aiming to improve the current understanding of the sediment dynamics occurring within the beach area closest to the shoreline.Experimental measurements were conducted in a large-scale wave flume under high-energy wave conditions at three cross-shore locations, representing inner surf and swash zone conditions. 47 tests, each one comprising 500 wave trains with identical wave conditions were measured.Obtained hydrodynamic properties and suspended sediment concentrations were observed to be highly repeatable between successive tests despite the sediment suspension event-like pattern, the beach evolution between tests, and the apparent randomness of the sediment suspension phenomenon.The hydrodynamics close to the shoreline (inner surf and swash zone) is dominated by short incident broken waves and long-wave water level oscillations induced by wave grouping. The analyzed time series of measured water surface elevation, horizontal velocity, computed Turbulent Kinetic Energy (TKE), and sediment concentration revealed that the suspended sediment concentration in this coastal zone does not correlate strongly with either the incident bore height, or the short-wave horizontal velocity or the TKE; in other words high/low values of these variables do not always promote high/low values of sediment suspension. In contrast, the highest suspended sediment concentrations were observed to occur by the combined action of incident bores and the trough of long-period water level oscillations. This pattern was more apparent in the inner surf than in the swash zone. High suspended sediment concentrations were also observed to coincide with negative peaks in long-wave horizontal velocity modulation resulting in enhanced negative sediment transport rates and beach erosion close to the shoreline.

Research highlights
► Hydrodynamics and suspended sediment concentration (SSC) measured close to the shoreline.
► SSC events are highly repeatable.
► SSC does not correlate with incident or long wave velocity nor with turbulent kinetic energy.
► SSC occurs because of incident bores, during the trough of long-wave water level fluctuations.
► Enhanced beach-face erosion due to the coincidence of high SSC and negative long-wave velocities.