Wave zonation for the Maltese islands

Abstract

Malta is a small island in the middle of the Mediterranean. The sea is an integral part of our daily life and also an important economic sector. Notwithstanding all the advances in technology, the sea also poses its hazards. Studies have shown that worldwide 0.3% of all commercial vessels are lost at sea each year claiming the lives of an average of 1,000 people per year. Though not the only cause of the incidents, waves and bad weather could be the instigators for the incidents to occur. It was also found that in most cases (approximately 66%) the incidents occurred in less than extreme conditions. The weather forecasts for mariners are mainly based on Significant Wave Height Hs and the Mean Wave Period Tz. Based on this, the study focused on identifying other wave parameters that adversely affect the vessel stability. Besides Hs, wave length, wave steepness and individual wave height were identified to impact vessel stability. The Linear Wave Theory was subsequently applied to the data generated by the SWAN wave model, to provide operational and forecasted data for these parameters around the Maltese territorial waters. These two dimensional plots are easy to interpret and provide the vessel master with added information to enable calculated decisions to be taken based on forecasted operational data. To guarantee the long-term sustainability of coastal and off-shore projects, structures should be designed to withstand the most extreme wave conditions during their life time. This study carried out an Extreme Value Analyses (EVA) to determine the highest Significant Wave Height Hs that can hit the Maltese islands with a return period of N years, N being 50, 100 or more years depending on the end application. For this purpose, various distributions and extreme parameter analysis were used. Results were compared to determine the most accurate estimate of the 100-year return period significant wave height, Hs100. The Hs100 was ultimately calculated to be 8.8 m. This value compares very favourably to other estimates obtained for other areas in the Mediterranean Sea. Besides the Hs100, two methods were used to determine also the highest individual wave height h that has an annual exceedance probability of 0.01. The two methods gave different values, 15 m and 13 m, with the former estimate considered to be more reliable. Through the use of wave data, this project addressed the important issue of safety at sea. This project also carried out an assessment of the extreme wave conditions. By means of forecasts in the form of 2-D plots of significant wave height, wave length, wave steepness and individual wave height, together with the EVA, this study opens the way to new products that can be offered operationally at the service of mariners and structural engineers.