Palos Verdes

Circulation patterns on the Palos Verdes Shelf

A year-long study of the processes that resuspend and transport sediment on the Palos Verdes shelf was begun in May, 1992. A detailed description of current speed and direction in the region and how these current patterns change with location and time was needed as part of this study. For this program, four moorings that measure current, temperature, salinity and water clarity were deployed on the shelf and upper slope for an extended period of time. The information gathered by this array of moorings was designed to provide both a basic description of the circulation patterns in the region and an understanding of important physical processes, especially those processes that control the resuspension and transport of sediment and associated pollutants.

The exact design of each mooring was specific to each measurement site, although there were many features common to each:

• All instruments at a site were attached to a single mooring.
• Either a single or dual acoustic release package was used to connect the mooring to the anchor.
• Each had a surface buoy to mark the measurement site.
• Near-surface instrumentation was attached 4 m below the buoy and elastic tethers below the surface instruments were used to stop the mid-depth and near-bottom instruments from being moved around the by surface waves.

The types of current meters used were vector-averaging current meters (VACM) and vector-averaging current meters with attached transmissometer (VTCT) to record water clarity data. The meters measured the average current direction and speed every 3.75 minutes.

These instruments not only measured currents, but the individual meters were outfitted with different sensors to measure several variables at each site. These additional data types were collected:

• Water clarity (transmission)was measured using transmissometers with a 25-cm path length. The clear water reading for each transmissometer was recorded before each deployment. The sample rate was the same as for the associated current meter.
• Conductivity, which tells us how salty the water is, was measured using conductivity cells. Each conductivity cell was calibrated before each deployment. The sample rate was the same as for the associated current meter.
• Water temperature was measured with temperature sensors installed in the VACM's and VTCT's or data loggers. All temperature sensors were calibrated before deployment. The sample rate was the same as for the associated current meter.

An Anderson sediment trap 0.5-m in diameter was used to collect the resuspended sediment at each site. Intervalometers that dispense discrete layers of Teflon beads at a specified interval were used in many of the sediment traps to generate a time base for when a particular layer of sediment was collected in the trap. The traps were poisoned to inhibit biological mixing and modification of the successive layers in the captured sediment.

All the moorings for this study were designed and prepared by the Sediment Transport Support Team at the Woods Hole Field Center, a part of the USGS Marine and Coastal Geology Program.

The NOAA research vessel Vickers was one of the vessels used for deployment/recovery of the moorings.

For additional information, contact Marlene Noble
Phone: (650) 329-5486