Searching for Factors that Limit Observed Extreme Maximum Wave Height Distributions in the North Sea

The probability that individual waves are much larger than the significant wave height is studied in a large set of observations. It is investigated whether steepness and shallow water effects are limiting factors for extreme wave heights. The relation between observations and a model freak wave index is examined.

Measurements from two locations in the North Sea are used, one with a depth of 80m, and one with a depth of 20m. The data consist of the significant wave height, wave period and maximum wave height of 20 minute records. The total amount of the records covers several years. The freak wave model index from the European Centre for Medium-Range Weather Forecasts (ECMWF) wave model is collocated with the observations.

The instrumental data show Rayleigh like distributions for the ratio of maximum wave height to significant wave height. Our analysis is limited by uncertainties in the instrumental response in measuring maximum wave height. The data indicate that steepness is a limiting factor for extremewave height. At the shallow water location, extreme waves are not more frequently observed than at the deep water location. The relation between
the freak wave index of the ECMWF wave model and enhanced extreme wave probability is studied.