Modeling boulder transport by storms and tsunamis: Application to Caribbean hazard assessment

Mark L Buckley, University of Rhode Island

Abstract

A combination of numerical hydrodynamic models, a large-clast inverse 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 cobbles and boulders 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 a fringing coral reef. Transported limestone clasts were derived from outcrops in the low lying interior of Anegada. Estimates of the near-bed flow velocities required to transport the observed boulders were calculated using a simple sediment-transport model, which accounts for fluid drag, inertia, buoyancy and lift forces on boulders and includes both sliding and overturning transport mechanisms. Estimated near-bed flow velocities are converted to depth-averaged velocities using a linear eddy viscosity model and compared with flow depth, acceleration, and depth-averaged velocity time series from high resolution coastal inundation models. Such models simulate overwash by the storm surge and waves of a category 5 hurricane and tsunami inundation from a Lisbon earthquake of M 9.0 and two hypothetical earthquakes along the North America Caribbean Plate boundary. A modeled category 5 hurricane and three simulated tsunamis were all capable of inundating the boulder fields and transporting a portion of the observed clasts, but only an earthquake of M 8.0 on a normal fault of the outer-rise along the Puerto Rico Trench was found to be capable of transporting the largest clasts at their current locations. Model results show that, while both storm waves and tsunamis are capable of generating velocities and accelerations necessary to transport large boulders near the reef crest, attenuation of wave energy due to wave breaking and bottom friction limits the capacity of storm waves to transport large clasts at great inland distances. Through sensitivity analysis, it is shown that even when using coefficients in the sediment-transport model, which yield the lowest estimated minimum velocities for boulder transport, storm waves from a category 5 hurricane are not capable of transporting the largest boulders in the interior of Anegada. Because of the uncertainties in the modeling approach, extensive sensitivity analysis is included and limitations are discussed. ^

Subject Area

Geology|Physical Oceanography|Engineering, Marine and Ocean

Recommended Citation

Mark L Buckley, "Modeling boulder transport by storms and tsunamis: Application to Caribbean hazard assessment" (2011). Dissertations and Master's Theses (Campus Access). Paper AAI1501407.
http://digitalcommons.uri.edu/dissertations/AAI1501407

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