Wave-swash interactions: Collisions of breaking waves in shallow water

On beaches, surface waves break and create a fast and shallow flow that climbs the sloping beach surface. This 'swash zone' flow, which controls the net import and export of sediment on the beach, is known to be well described by solutions to the non-linear shallow water equations. However, observations suggest that collisions of consecutive waves, known as wave-swash interactions, play an important role in suspending sediment in ways that are not well understood or modelled. We present results from a laboratory study on wave-swash interactions to improve upon existing knowledge of this phenomenon, which has only been described qualitatively in the past. We use consecutive solitary waves with different wave heights and separations to generate various wave-swash interactions and find that they can be quantitatively characterized in terms of two dimensionless parameters: (1) the ratio of wave heights and (2) the dimensionless time separation between consecutive wave crests. Using measurements of wall pressure and free-surface displacement, we also infer the total vertical acceleration. The peak vertical acceleration is upward-directed and can easily exceed gravity, even in very shallow water depths. These findings suggest that wave-swash interactions are capable of inducing large material suspension events of sediment or solutes in sediment pores.