Horizontal asymmetry and steepness distributions for wind-driven ocean waves from severe storms

Results on horizontal asymmetry and steepness distributions from analyses of ocean wave data collected during 10 severe storms in the northern North Sea are presented. The data have been collected at a sampling rate of 5 Hz using laser altimeters mounted on a fixed platform permitting the shapes of individual waves to be quite closely defined. This has allowed the steepness of the fronts and backs of wave crests and troughs to be examined. It is found that, on an average, as the non-dimensionalised wave height increases, the horizontal asymmetry becomes more pronounced. That is, the fronts of large wave crests tend to be steeper than their backs. Regression has been used to establish relationships between individual non-dimensionalised wave heights and steepness measures. The generalised Pareto distribution has then been used to establish a simple model for predicting the probability of extreme wave steepness conditional on the non-dimensionalised wave height. q 2003 Elsevier Ltd. All rights reserved.