Geomorphological signatures of known hurricanes and validation of theoretical emplacement formulations: Coastal boulder deposits on Cuban low-lying marine terraces

Abstract

Coastal boulder deposits, observed worldwide, provide geomorphological evidence of extreme wave events such as storms, hurricanes and tsunamis. Theoretical formulations have been developed for determining hydrodynamic conditions responsible for boulder emplacement on the shore, which increasingly make use of boulder geometry and associated site geomorphology. Nevertheless, information on extreme events responsible for the emplacement of coastal boulders is rarely available, meaning there has been limited opportunity to test existing formulations in the light of real hydrodynamic and geomorphic data. In this study, we take advantage of the important record of coastal boulder deposits on Cuba Island to compare the hydrodynamic parameters (minimum flow velocity) deduced from the boulders' morphology and emplacement characteristics, with hydrodynamic conditions (maximum wave height and orbital velocity) that occurred during the tropical cyclones responsible for the boulders' actual emplacement. We selected four sites where three hurricanes have emplaced five boulders on low-lying coral reef terraces over the last 50 years. Using terrestrial Structure-from-Motion photogrammetry, we determined with precision the boulders' shape and volume, which in combination with density, mode of emplacement and distance from the shore, were used to calculate the minimum flow velocity responsible for dislocation of the coral reef terrace and inland transport. To serve as comparisons, available modelled data of wave height and period were used to estimate the maximum orbital velocity that possibly occurred during the weather event using linear wave theory. Our results show that for all boulders studied except one, there is a good agreement between the values of minimum flow and maximal orbital velocities, with minimum flow velocities for boulder emplacement consistently smaller than the maximum wave orbital velocity during the weather event. The difference observed for one boulder is attributed to specific site effects, highlighting in this case the limitation of using distant hydrometeorological data for characterizing wave processes responsible for coastal boulder deposits. Helped by detailed data collected on boulders with large differences in morphology including size, and mode of emplacement, this study confirms the pertinence of using formulations relating boulder morphology and site characteristics to the minimum flow velocity that detached and transported the boulder. It further emphasizes the importance of obtaining adequate boulder and geomorphic setting characterizations to link geomorphological proxies and extreme wave events.