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USGS2006

Cross-shelf subtidal variability in San Pedro Bay during summer, 2001

A total of 16 moorings were deployed across the San Pedro shelf, one of the two wider embayments in the Southern California Bight, from near the surfzone to the upper-slope. On the middle and outer shelf in the summer of 2001, the currents flowed strongly equatorward at the surface and had large vertical shears through the well-stratified water column. This equatorward flow differs from predominantly poleward flow found in previous studies of the coastal margin further west. In deeper water, near the shelf break, the shears were such that near-bottom flows were poleward and incorporated into the upper parts of the Southern California Undercurrent over the slope. Mid-shelf current fluctuations, with periods of 10-25 days, along with upwelling over the shelf, were not related to local winds, but were significantly correlated with the large-scale alongshore pressure gradient. Shorter period (???7-10 days) inner shelf alongshore currents, however, were significantly correlated with the alongshore wind at the shelf break. A CEOF analysis gives two significant modes, with the first mode dominant over the outer and middle shelf. The wind-forced second mode connects the inner shelf to the poleward undercurrent over the slope such that increases in the poleward flow over the slope are correlated with increases in the equatorward current inshore of the 15 m isobath.

Continental Shelf Research↗

USGS2001

Transport processes near coastal ocean outfalls

The central Southern California Bight is an urbanized coastal ocean where complex topography and largescale atmospheric and oceanographic forcing has led to numerous sediment-distribution patterns. Two large embayments, Santa Monica and San Pedro Bays, are connected by the short, very narrow shelf off the Palos Verdes peninsula. Ocean-sewage outfalls are located in the middle of Santa Monica Bay, on the Palos Verdes shelf and at the southeastern edge of San Pedro Bay. In 1992, the US Geological Survey, together with allied agencies, began a series of programs to determine the dominant processes that transport sediment and associated pollutants near the three ocean outfalls. As part of these programs, arrays of instrumented moorings that monitor currents, waves, water clarity, water density and collect resuspended materials were deployed on the continental shelf and slope information was also collected on the sediment and contaminant distributions in the region. The data and models developed for the Palos Verdes shelf suggest that the large reservoir of DDT/DDE in the coastal ocean sediments will continue to be exhumed and transported along the shelf for a long time. On the Santa Monica shelf, very large internal waves, or bores, are generated at the shelf break. The near-bottom currents associated with these waves sweep sediments and the associated contaminants from the shelf onto the continental slope. A new program underway on the San Pedro shelf will determine if water and contaminants from a nearby ocean outfall are transported to the local beaches by coastal ocean processes. The large variety of processes found that transport sediments and contaminants in this small region of the continental margin suggest that in regions with complex topography, local processes change markedly over small spatial scales. One cannot necessarily infer that the dominant transport processes will be similar even in adjacent regions.

Conference Paper↗