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Hurricane-Induced Sediment Transport in Open-Shelf Tropical Systems--An Example from St. Croix, U.S. Virgin Islands
Dennis K. Hubbard (2)
Hurricane Hugo passed directly over St. Croix on 17 September 1989. Sustained winds in excess of 110 knots (gusts to 165 knots) and waves 6-7 m in height accompanied the storm. Along the north coast, wave height was lower (ca. 3-4 m) due to the leeward position of the shelf. In the deeper reefs at Cane Bay and Salt River, damage was confined primarily to the soft-bodied benthic community (e.g., sponges, gorgonians); coral damage was much less severe, largely because of the buffering effects of the water column.
The greatest change observed after the storm was wholesale flushing of sand from shelf-edge areas. In Salt River submarine canyon, a minimum of 2 million kg of sediment were flushed into deeper water. The transport rate associated with the storm was eleven orders of magnitude above that measured during fair-weather, and the volume of sediment that was removed from the canyon equalled roughly a century of normal sediment accumulation. At Cane Bay, 336,000 kg of sediment were flushed from a single channel, with similar amounts removed from adjacent breaks in the shelf-edge reef.
A current meter in Salt River submarine canyon provided information on the timing and intensity of the oceanographic processes related to Hurricane Hugo. As the storm approached, waves piled water against the shoreface and in Salt River Bay. As the storm passed over St. Croix, the change in wind direction, followed by a decrease in wave height, triggered a release of water trapped in the bay and along the adjacent shoreline by waves earlier in the storm. For a period of 4-6 hours, net down canyon currents reaching 2 m/s and oscillatory flows up to 4 m/s occurred along the base of the western canyon wall, removing up to 2 m of sand. Similar events were likely responsible for the wholesale removal of sand in eastern Cane Bay. The paradoxical concurrence of wholesale sediment transport a d low-level reef damage is related to the protection from waves but not wind afforded by the north coast of St. Croix, facing away from the direction of storm approach.
These observations and measurements provide our first opportunity to relate sediment export in such a high-energy event to the physical processes that were responsible. Calculations based on post-Hugo measurements are in agreement with an earlier sediment budget for Salt River canyon. Sediment export in Cane Bay exceeded the volume similarly predicted.
Because such events are probably common on all exposed carbonate shelf margins, storms like Hurricane Hugo are among the most important factors in the cycling of sediment through exposed, open-marine environments both now and in the geologic past. The patterns of reef damage and sediment transport are much more complicated than previously envisioned, and more thoughtful consideration of their variability and the processes responsible is essential to an understanding of the signature that will be left by major storms.
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