Sediment Transport and CaCO3 Budget on a Fringing Reef, Cane Bay, St. Croix, U.S. Virgin Islands
Abstract:Continuous (1 year) study of the rates and mechanisms of sediment transport on the fringing reef indicates major volumetric transport in a seaward direction during storms by wave-generated oscillatory currents. The narrow (300 m) shelf in this semi-protected bay is nearly flat in its landward portion (forereef terrace) and slopes increasingly seaward (forereef slope) before terminating in a steep reef wall at −32 m. A marine hardground separates the active reef from the shoreface and acts as a barrier to transport under all but storm conditions.
Sediment accumulates on the shelf throughout most of the year, and is moved seaward during winter storms across the sandy forereef terrace and through sand-filled channels at the shelf edge. During calm sea conditions transport rates average about 4 × 10−5 m3/m day−1 while during storm seas, transport rates vary from about 4 × 10−4 (forereef terrace) to 4 × 10−3 m3/m day−1 (forereef slope). Absence of transport rates intermediate between storm and calm sea values is due to subtidal algal mat which binds the upper 0.5 cm of sediment and inhibits movement. The critical energy threshold required to destroy the mat is only attained by currents generated during times of long-period swell or storms. Biologically-induced transport is active year round on the forereef slope but is volumetrically subordinate to physical processes. Transport by gravity is relatively unimportant shallower than −32 m and contributes mainly by accentuating the effects of physical and biological transport.
Mass balance calculations of CaCO3 production and transport indicate that the observed sediment transport processes cannot remove all sediment produced annually by reef organisms. The reef depends on occasional extreme events, such as hurricanes and tropical storms, to balance sediment transport with production so as not to drown in its own accumulated sediments.
Document Type: Research Article
Publication date: September 1, 1984
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