Assessment of extreme wave flooding from geomorphologic evidence in Bonaire (Netherlands Antilles)

Along the rocky coasts of Bonaire boulders are arranged in clusters and/or ramparts; they represent the imprints of the impact of catastrophic waves induced by storms, hurricanes and tsunamis. The coast of Bonaire was surveyed in order to identify boulders that show unambiguous evidence of either a tsunami or a hurricane impact. In particular, the detailed surveys of two localities - Boka Washikemba and Spelonk Lighthouse - showed boulders broken into several pieces; this singularity is indicative of a unique depositional event. Moreover, detailed measures on the 15 largest boulders were taken in order to obtain their a-, b-, c-axes, their landward position from the coastline and altitude above mean sea level. These data were needed to evaluate the pre-transport setting and to estimate the paleo-wave energy responsible for their deposition. Applying the most recent equations derived by a morphological and hydrodynamic model, the minimum wave height - either induced by tsunami or storm - were able to detach and transport the boulders and these have been calculated. In case of storm waves, the minimum calculated height for transporting blocks was 13 - 28 m; this wave height is very improbable even in the case of the strongest hurricane. A tsunami whose minimum calculated wave height capable of moving the boulders ranges from 3 and 7 m would be more probable. With magnitudes of impacting waves an inundation model was developed; the minimum flooding of past tsunami is assessed to be around 100-140 m according to geomorphological evidence and the main coastal parameters influencing wave inundation (such as coastal slope, surface roughness and bathymetry).