Bore collapse and wave run-up on a sandy beach

Wave run-up on beaches and coastal structures is initiated and driven by collapsing incident bores, this process is often considered to define the seaward limit of the swash zone. It is hence a key feature in nearshore wave processes as extreme run-up can lead to structure overtopping and coastal inundation during storm conditions. In addition, the turbulent nature of incident bores and their collapse suspends and advects sediment, resulting in a highly morphologically dynamic swash zone. The cross shore bore collapse location varies from wave to wave and the process is very limited in both spatial and temporal extent, making direct measurement problematic. This paper presents high spatial-temporal resolution LiDAR field measurements of the evolving free-surface in the surf and swash zone which enable the bore collapse detection for 166 waves. These measurements are used to investigate the link between broken wave properties at bore collapse and wave run-up. Incident bores are identified at the seaward boundary of the LiDAR profiles and tracked through the inner surf and swash zones to the run-up limit. It is found that the vertical run-up height exceeds that which would be expected for a perfect conversion of potential to kinetic energy during bore collapse for 24% of the bores measured. By returning to an existing ballistic-type model to describe the run-up of individual waves, we show that wave run-up can be divided into three components: the bore collapse, terminal bore celerity and their non-linear interaction. For the present dataset, the contribution of the bore collapse and terminal bore celerity is 26% and 27% respectively, while non-linear interactions between the two dominates and account for 47% of the measured run-up. By including the terminal bore celerity, the ability to predict run-up is increased by 30% with the determination coefficient r increasing from 0.573 to 0.785. Likewise, the RMS-error for the wave run-up shows an approximately 10% reduction from 0.325 to 0.295 m.