Sediment transport on the Palos Verdes shelf, California

Abstract

a b s t r a c t Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature-salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35-and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16 Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8 metric tons m À 1 yr À 1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of $ 0.2 mm yr À 1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates ( $ 5 mm yr À 1 ), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit.Published by Elsevier Ltd.

Suspended particulate layers and internal waves over the southern Monterey Bay continental shelf: An important control on shelf mud belts?

“…Consequently, this roughness parameter is seldom known precisely [e.g., Cheng et al ., ]. We did not directly measure z o , so we used a hydraulic roughness value of z o = 9 × 10 −5 m, which was a model‐tuned estimate used for the soft, muddy portion of the Palos Verdes shelf in southern California [ Ferré et al ., ]. Ferré et al .…”

“…Ferré et al . [] note that this roughness value is lower than those measured for other shelf environments, but is consistent with a seafloor characterized by soft, smooth mud. The critical shear stress threshold, τ cr , for mobilizing noncohesive sediment was calculated using the formulation described by Soulsby [].…”

Evaluation of sediment properties using wind and turbidity observations in the shallow tidal areas of the Venice Lagoon

“…In many cases these applications are typically limited to data collected during settling periods, in the absence of wave‐induced sediment resuspension (as is usually the case in marsh areas), which is the most common initiator of erosion processes in tidal flats and subtidal areas [ Carniello et al , ; Fagherazzi et al , ; Lawson et al , ; Fagherazzi and Wiberg , ; Carniello et al , ]. In a few cases [ Shi et al , ; Lumborg and Windelin , ; Ferrè et al , ] local‐scale observations of waves, currents, and sediment properties have been used in sediment transport models to infer rates of sediment erosion, transport, and deposition within limited study areas.…”

“…In many cases these applications are typically limited to data collected during settling periods, in the absence of wave-induced sediment resuspension (as is usually the case in marsh areas), which is the most common initiator of erosion processes in tidal flats and subtidal areas [Carniello et al, 2005;Fagherazzi et al, 2006;Lawson et al, 2007;Fagherazzi and Wiberg, 2009;Carniello et al, 2011]. In a few cases [Shi et al, 2000;Lumborg and Windelin, 2003;Ferrè et al, 2010] local-scale VENIER ET AL.…”

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“…The mean circulation (i.e., low‐frequency, quasi‐steady circulation) over the inner shelf may be governed by tidal processes, baroclinic circulation, wind, waves, or some combination of these. Quantification and numerical modeling of the mean inner‐shelf circulation is relevant to studies of heat budgets [ Wilkin , 2006], water quality [ Rabalais et al , 1996], sediment transport [ Ferré et al , 2010], and larval transport [ Garland et al , 2002]. Limited observations, combined with assumptions of alongshore uniformity, are often interpreted to yield a comprehensive description of the circulation.…”