Pickup of Negatively Buoyant Particles in Wave–Swash Interactions

Abstract The transport of sand, cobbles, and negatively buoyant debris in the swash zone governs their mass exchange between land and sea. To explore a candidate mechanism for the suspension of such particles on natural beaches, we conducted an idealized laboratory investigation of particle pickup on an impermeable sloping beach in a wave flume. We generated wave–swash interactions using consecutive solitary waves with varying amplitude and separation times and observed the pickup of a hollow aluminum cylinder in the swash zone constrained to vertical motion by a wire. Pickup occurred at the arrival of the second wave, and not the first, indicating that in our experiments the wave–swash interaction produced the vertical forces required for pickup. Moreover, pickup did not occur in many interactions where previous measurements had indicated large vertical flow accelerations. When pickup did occur, the separation time between waves was slightly smaller than the duration of the first wave’s swash flow. A model of particle pickup based on vertical flow accelerations over-predicted the maximum particle velocity, suggesting that large, short-lived vertical flow accelerations in wave–swash interactions can potentially be sufficient to cause pickup. Variability in the particle trajectories suggests that fluctuating (turbulent) vertical velocities at the time of pickup can enhance or dampen the pickup. Taken together, and acknowledging the idealized setup, our results are consistent with the intermittency of sediment suspension and debris pickup observed in the field being associated with the highly variable vertical flow accelerations and velocities across the zone of wave–swash interactions.