Analysis of Extreme Wave Climates in Rhode Island Waters South of Block Island

This project is one component of comprehensive multidisciplinary studies performed at the University of Rhode Island 2008 to 2010, to develop a Special Area Management Plan (SAMP) for siting offshore wind farms in Rhode Island waters. A characterization of extreme wave climates is required when considering such ocean structures, e.g. for designing wind turbine support structures and foundations or for verifying the long-term stability of their foundations against scour. An analysis of the extreme wave climate off the southern coast of Block Island was performed, from which significant wave heights and peak spectral periods were derived, for sea states corresponding to long return period events (i.e., 20, 50, 75 and 100 years) from a variety of directions. Wave propagation simulation analyses using STWAVE, a steady-state spectral wave model and forced by wave hindcast data from the US Army Corp of Engineers, Wave Information Studies (WIS) were performed to estimate these extreme events. Various model grids were used, which represented rectangular areas, from 616 to 978 km 2 , surrounding Block Island and the regions where potential wind farms might be sited to the southwest, south, and southeast of the island. Simulation results predicted the occurrence, south and southeast of the island, of significant wave heights of at least 8 m for all return periods (upper 95% confidence limit), with wave heights exceeding 10 m in the 100 year case. Southwest and west of Block Island, significant wave heights were significantly reduced in all cases, by about 2 meters, due to intense breaking and dissipation over the shallower waters (15 m deep) between Block Island and Montauk Point (eastern end of Long Island) associated with large boulders and quaternary glacial deposits.