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Rapid formation of hyperpycnal sediment gravity currents offshore of a semi-arid California river

Continental Shelf Research

By:
, , ,
DOI: 10.1016/j.csr.2007.11.002

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Abstract

Observations of sediment dispersal from the Santa Clara River of southern California during two moderately sized river discharge events suggest that river sediment rapidly formed a negatively buoyant (hyperpycnal) bottom plume along the seabed within hours of peak discharge. An array of acoustic and optical sensors were placed at three stations 1 km from the Santa Clara River mouth in 10-m water depth during January-February 2004. These combined observations suggest that fluid mud concentrations of suspended sediment (>10 g/l) and across-shore gravity currents (???5 cm/s) were observed in the lower 20-40 cm of the water column 4-6 h after discharge events. Gravity currents were wave dominated, rather than auto-suspending, and appeared to consist of silt-to-clay sized sediment from the river. Sediment mass balances suggest that 25-50% of the discharged river sediment was transported by these hyperpycnal currents. Sediment settling purely by flocs (???1 mm/s) cannot explain the formation of the observed hyperpycnal plumes, therefore we suggest that some enhanced sediment settling from mixing, convective instabilities, or diverging plumes occurred that would explain the formation of the gravity currents. These combined results provide field evidence that high suspended-sediment concentrations from rivers (>1 g/l) may rapidly form hyperpycnal sediment gravity currents immediately offshore of river mouths, and these pathways can explain a significant portion of the river-margin sediment budget. The fate of this sediment will be strongly influenced by bathymetry, whereas the fate of the remaining sediment will be much more influenced by ocean currents.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Rapid formation of hyperpycnal sediment gravity currents offshore of a semi-arid California river
Series title:
Continental Shelf Research
DOI:
10.1016/j.csr.2007.11.002
Volume
28
Issue:
8
Year Published:
2008
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
991
Last page:
1009
Number of Pages:
19