COASTAL SEDIMENT BUDGETS AND THE ROLE OF THE INNER CONTINENTAL SHELF

Coastal sediment budgets, whether produced using empirical data or a numerical model, are imponderable. Most sediment budgets attempt to quantify sediment flux only for the beach and surf zone, and simply use the adjacent continental shelf as a source or sink term in order to balance the nearshore budget. Recent studies of regional coastal geology and shoreline behavior, however, demonstrate that sediment exchange between the beach and the shelf is a significant component of the nearshore sediment budget. The relationships between the shallow geology of the inner shelf and adjacent coastal sediment budgets are described for four locales. At Fire Island, NY, decade-scale beach monitoring data, wave modeling, and sediment budget calculations suggest that shoreward sediment flux from shoreface-attached sand ridges is equal to the shore-parallel sediment flux. Data from the northern Outer Banks, NC suggest a direct relationship between the spatial geometry and composition of underlying geologic units and the sediment supply rate to adjacent beaches. At Myrtle Beach, SC, a minimum net onshore sediment flux of ~85,000 cubic meters per year is required to explain the observed coastal evolution. At Wrightsville Beach, NC, over 35 years of large-scale beach nourishment (~8 million cubic meters) has resulted in the accumulation of nearly 2 million cubic meters of nourishment sediment on the lower shoreface and inner shelf in water depths >9 m. The rate of offshore transport is an order of magnitude greater than the offshore transport predicted by the prevailing sediment budget for this area. An increased understanding and ability to quantify beach-shelf sediment flux should result in improved sediment budget calculations, but existing models need to be improved significantly to be of practical use.