Wave overtopping hazard to pedestrians: video evidence from real accidents

Admissible wave overtopping is a key design parameter for coastal defences. For defences accessible to the public, it is likely that the admissible overtopping will be determined according to that which does not expose the public to direct hazard. Established methods base this admissible overtopping upon a combination of factors: mean overtopping discharge; individual maximum wave event volume, and the type of person exposed to the hazard, from unaware, easily frightened pedestrian through to trained and safety-aware personnel. The actual mechanism for the hazard – the actual flow causing a person to lose stability and fall – is less well explored. Literature studies, largely from the fluvial flooding community, are revisited and appraised against a simple analytical model for a person’s stability in a flow. The recent ‘resource’ that are ‘youtube’ videos is then utilised, with three videos of actual overtopping accidents being identified and analysed quantitatively. Results are in broad agreement with the trends from the controlled studies. Significantly, however, the conditions found to be hazardous are under significantly shallower, faster flows than previously reported, with flows as shallow as 0.1 m shown to result in a person’s fall. This work adds confidence to the use of the analytical model as an appropriate tool reflecting the actual physical process linking overtopped flow to direct pedestrian hazard.