Statistical analysis of extreme ocean waves in Galle, Sri Lanka

Study of extreme wave heights is vital for design of coastal structures. The purpose of this study is to estimate the wave heights for several return periods in order to use them in coastal constructions. Wave height measurements collected from Galle, Sri Lanka were analyzed. Analysis was separately performed for sea, swell and overall waves, season wise. Peak Over Threshold method (POT) was used for the sample selection. The possible range of threshold values were identified by using the Mean residual life plot and the specific threshold value was selected using the Generalized Pareto Distribution (GPD). Using the POT method, the GPD was fitted for the sampled data and the special type of GPD was identified statistically. Further diagnostic plots were obtained to ensure the validity of the distribution. Return levels were calculated for several return periods and the confidence intervals were constructed for the return levels.Exponential distributions were the best fitted distributions for south-west (SW) monsoon and October–November (ON) season for sea wave heights, while the Beta distribution was the best fitted distribution for swell wave heights. Pareto distribution fits well with the overall wave heights for south-west monsoon and overall wave heights were fitted well with Beta distribution for October–November (ON) and March–April (MA) seasons. Analysis was omitted for March–April (MA) season and December–February (DF) season for sea and swell waves, while December–February (DF) season was omitted for overall waves as they did not have significant extreme values. It was found that, the return levels for sea wave heights were comparatively higher than the swell wave heights for all the seasons. When designing the coastal constructions, the return levels of extreme wave heights in south-west monsoon should be considered than the other seasons, and also comparatively the return levels of sea waves have significant impact than the swell waves. Moreover return levels of overall wave heights also have to be considered in designing.