SAN PEDRO BAY OBSERVATIONS
San Pedro Bay/Orange County Focus Region - Nearshore and Surfzone Observations
The nearshore, defined as the region extending from the shoreline to approximately 2km offshore (roughly 30m water depth), consists of the surfzone (within a few ~100m of the shoreline) and the transition zone (seaward of the surfzone). The nearshore is the most heavily used part of the coastal ocean, and is also the region where water quality is most seriously impacted by pollutants.
Although nearshore currents are critical to prediction of the fate and origin of point and non-point pollutants, they cannot be observed continuously in time over large areas because they are inshore of HF radar coverage. To improve prediction of nearshore currents, in-situ observations spanning relatively small regions for limited time periods will be used to validate and calibrate nearshore models that can be applied continuously over larger areas. The sites selected for intensive observations have persistent, serious water quality problems so the resulting current maps will also be useful, site-specific near-realtime products.
The region of focus observation for the nearshore and surfzone components will be in the San Pedro Bay/Orange County region. San Pedro Bay impacts a large population base where there is great public interest in beach water quality. Contrary to public opinion, the outfall has not been proven to be the major source of contamination. Land-based sources are known to be bacterial contributors. The Santa Ana River is a known source of bacteria that is readily advected alongshore by the wave-driven surf zone transport. The alongshore propagation is easily observed in hourly bacterial data during spring tide events. San Pedro Bay has a good historical data set. The Orange County Sanitation District has maintained a mooring (currents and temperature) near their outfall for several years. There was an intensive field effort in 2001 that included 12-13 moorings, effluent plume mapping, and shoreline bacteria sampling at hourly intervals during 6 spring tide events. (see http://pubs.usgs.gov/of/2003/of03-62/ ) The Orange County Sanitation District (OCSD) has already identified itself as a partner for this focused effort. They have several moorings, in addition to their permanent outfall mooring, which will be made available as outside resources to the SCCOOS focused study site.
Complementary observations of nearshore currents will be made with drifters, bottom mounted surfzone sensors, and ADCPs mounted on moorings and on autonomous underwater vehicles (AUVs). The moorings will include thermistor chains and a bottom pressure sensor. The drifters provide trajectories of passive, near-surface pollutants. The continuous velocities over the water column provided at a few locations by moored ADCPs are complemented by the spatially extensive observations of AUV-mounted ADCPs. ADCPs perform poorly in bubbly surfzone waters, so single-point flowmeters (ADVs) will be used.
There are several key factors which affect nearshore transport of pollutants in this region that will be addressed by SCCOOS during this effort. These factors are:
Rendering of the observational tools being used during HB06.
- Alongshore advection driven by both wave forcing and offshore currents, and pressure gradients
- Surface-wave driven dispersion near the surfzone
- Exchange between the surfzone and the region outside the surfzone by eddies, wind driven overturning, or internal waves carrying to shore sub-thermocline water
- Alongshore transport beyond the surfzone in the transition zone
- Other forcings on near surface flow - e.g. sea breeze pattern driving afternoon near surface shoreward flow.