OFFSHORE SAND SOURCES FOR BEACH REPLENISHMENT - POTENTIAL BORROWS ON THE CONTINENTAL-SHELF OF THE EASTERN GULF-OF-MEXICO

Erosion of sandy beaches is a worldwide problem that elicits innovativ e geoengineering techniques to reduce adverse impacts of shoreline ret reat. Beach replenishment has emerged as the ''soft'' shore-stabilizat ion technique of choice for mitigating beach erosion. This method of s hore protection involves the addition of sand to the littoral sediment budget for sacrificial purposes. Because inland sand sources are ofte n uneconomical or impractical to use, and known nearshore sources are limited finding adequate quantities of suitable sand on the inner cont inental shelf is often vital to beach replenishment projects. The tech nical studies of survey and materials analysis that identify, and deli neate usable sand sources are sometimes almost as expensive as small-p roject dredging, pumping, and placing the sand on the beach as fill. I nadequate quantity or substandard quality of shelf sand, as well as of ten-prohibitive overhead expenses, thus compel shoreline managers to s eek suitable sand sources offshore. In the study area off the central- west coast of Florida, offshore potential borrow areas (PBAs) were ide ntified on the basis of studies conducted in reconnoitory and detailed phases. Sophisticated state-of-the-art equipment used in this investi gation provided more detailed subbottom mapping information than is no rmally obtained with conventional seismic equipment. An example of san d exploration studies was incorporated in a 215-km(2) survey of offsho re areas by conducting bathymetric surveys and subbottom seismic profi ling, collecting jet probes, grab samples, and vibrocores, and analyzi ng sediment grading in. subsamples from vibrocores. These combined ana lyses indicated that at least 8.8 x 10(6) m(3) of sand is available in potential borrow areas from 7.0 to 12 km offshore in water depths of 8.0 to 11.5 m. In the PBAs, mean grain size of sand falls into the ran ge 0.13-0.53 mm, sorting averages 0.65-1.31 phi, and the overall silt content varies from 3.9-8.5%. High silt contents (13-19%) mapped in so me areas make these sedimentary deposits unsuitable as fill for artifi cial beach renourishment.