Observation and prediction of a coastal rogue wave

Direct observations of rogue waves in high sea states are rare. However, rogue waves can pose a danger to marine operations, onshore and offshore structures, and beachgoers. Here we report on a 17.6m rogue wave in coastal waters with crest height about twice, and wave height almost three times the significant wave height. These are amongst the largest normalized heights ever recorded.

Simulations of random superposition of Stokes waves in intermediate water depth show good agreement with the observation, whereas non-linear wave modulational instability did not contribute significantly to the rogue wave generation. We present a spectral parameter that can easily be derived from a routine wave forecast as an indicator of rogue wave risk. These results confirm that probabilistic prediction of oceanic rogue waves based on random superposition of steep waves are possible and should replace predictions based on modulational instability. Furthermore, large individual waves offshore do not necessarily result in extreme runup on the beach.