Inter-annual Variations in Wave Characteristics off Ratnagiri, Northeast Arabian Sea

Abstract

The wave data measured using the Datawell directional wave rider buoy at 13 m water depth off Ratnagiri during 2010 to 2013 is used to study the inter-annual variations in the wave characteristics. The percentage occurrence of waves with significant wave height (Hs) less than 1 m is the same in all 3 years, but the occurrence of high waves (Hs > 1 m) is not same in all the years. The extreme wave events occur during the monsoon period and if these wave events persists for longer periods cause coastal erosion and hence we have examined the events lasting more than 5h with different threshold limits of wave height (2.5, 3 and 3.5 m). Average Hs during the monsoon is around 3 times that in non-monsoon period. Average of the maximum wave spectral energy density is 12 times higher in monsoon than in non-monsoon. Inter-annual variations in monthly average Hs is maximum (0.5 m) in June and September due to the change in the monsoon intensity. The inter-annual variation in annual average Hs is around 0.1 m and the annual average peak period is 0.1 to 0.5 s. The monsoon average peak period is slightly higher than the values during the non-monsoon period. The joint distribution of the peak wave period and the mean wave direction indicate three peaks; one corresponding to the southwest swells during the post-monsoon season, the second for the southwest-westerly monsoon waves and the third for the northwest waves during the pre-monsoon season.