Estimated velocities and inferred cause of overwash that emplaced inland fields of cobbles and boulders at Anegada, British Virgin Islands

A combination of numeric hydrodynamic models, a large-clast sediment-transport model, and extensive field measurements were used to discriminate between a tsunami and a storm striking Anegada, BVI a few centuries ago. In total 161 transported clasts were measured ranging from 1.5 to 830 kg at distances of up to 1 km from the shoreline and 2 km from the crest of the fringing reef. Transported clasts were derived from local limestone outcrops with transport distances from several to 100’s of meters, depending on the inferred source locations. The COULWAVE model was used to simulate overwash by the storm surge and waves of a Category 5 hurricane. The MOST model was used to simulate tsunamis from a Lisbon earthquake of M 9.0, a thrust earthquake of M 8.7 on the northern Antilles subduction zone, and an earthquake of M 8.0 on a normal fault of the outer-rise along the Puerto Rico Trench. A simple sediment-transport model, based on the semi-empirical Morison equations (O’Brien and Morison, 1952) was used to estimate the minimum near-bed velocities necessary to transport the largest boulders. Minimum near-bed flow velocities were estimated by balancing the drag, inertia, and lift forces on the boulders with resistance forces for both sliding and overturning. Because of the uncertainties in the modeling approach, extensive sensitivity analyses are included and limitations of the approach are discussed. Estimates of the near-bed flow velocities required to transport the observed boulders were compared to time-series from simulated storms and tsunamis to determine which events were capable of transporting the observed boulders. The modeled Category 5 hurricane generates the greatest flow depths but some of the lowest velocities. Of the simulated events, only a hypothetical outer-rise tsunami has velocities exceeding the calculated velocities necessary to transport the largest boulders. This study differs from many past studies of extreme-wave deposits because of the inclusion of numeric hydrodynamic modeling, a sediment-transport model, and extensive field measurements. The comprehensive approach implemented here allows for a better understanding of the source event and can be used as a framework for future studies. O'Brien, M.P. and Morison J.R., 1952. The forces exerted by waves on objects, Trans. Am. Geophys. Union 33 (1) (1952), 32-38.