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ABSTRACT: The sand-covered floor of Central San Francisco Bay is molded by tidal currents into a series of bedforms, each of which is stable through a discrete range of tidal velocity, grain size, and water depth. Many of the bedforms move during average tide cycles, and do not require storms, floods or abnormal flow conditions to be active. The net direction of bottom sediment transport has been deduced from bedform asymmetry. The geometry of Central Bay exerts considerable control on the sediment transport pattern. Tidal flows accelerate as they pass through the narrow Golden Gate and produce ebb and flood jets that transport sediment away from the Gate. Lower velocity flows that occur between the shoreline and the jets are ebb dominant within the Bay, and flood dominant outside the Gate, and these flows transport sediment toward the Gate.
In Central Bay, where many of the bedforms are active during average tide cycles, sediment turnover, which is important in organic and inorganic exchange between the sediment and the water column, results largely from bedform migration. This rigorous hydraulic regime also acts to reduce biological turnover by benthic organisms by producing an environment more suited to animals that extract nutrients from the water column and surface and suspended sediment, rather than from buried sediment.
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