Scripps researchers rate as a success a recent test of a prediction model that could create more rapid assessments of the danger posed by sewage spills and other contamination events to beachgoers.
When land-based contaminants invade the coastline, local officials typically must wait a day or two for water test results before they can determine which specific beaches to close, potentially exposing the public to infection and disease. Gauging the spread of coastal water pollution is also complicated by the fact that currents and waves within a few hundred meters of the shoreline cannot be observed continuously over large areas. This nearshore surf zone is not only extremely turbulent, but also too close to the shoreline to be observed by the numerous newly established high-frequency radar networks that supply coastal wave and current data.
But following an experiment in the ocean waters off Huntington Beach, Bob Guza and Falk Feddersen, scientists at Scripps’s Coastal Data Information Program (CDIP), say they may have successfully developed a new model for predicting where nearshore contaminants will end up.
The method combines measurements of offshore conditions with a model describing how waves break across the surf zone and create currents that move parallel to the shoreline. The technique was validated last October during a month-long field experiment tracking the transport and mixing of water near the border of Huntington Beach and Newport Beach, an area of chronic water quality problems, especially storm water runoff.
A CDIP research team set up operations at “surf city” south of the Huntington Beach Pier along one of Southern California’s most popular surfing beaches. Researchers blasted in and out of the waves riding on a souped-up jet ski outfitted with GPS to fix its location, various sensors, and underway water sampling systems.
Seven tripod frames instrumented with pressure and temperature sensors and current meters were anchored to the seafloor in a line running from the beach to a depth of about 4 meters (13.1 feet), forming a cross-shore transect. Additional sea-bottom instrument packages were deployed alongshore to make measurements away from the main transect. These fixed stations supplied real-time data on wave heights and currents.
To mimic a contamination event, the scientists released a fluorescent, EPA-approved dye from a pump system set up in the transect line for several hours at a time. The bright pink dye started out as a small spot that quickly spread out and along the shore as each successive wave hit the beach. The jet ski repeatedly sampled from offshore through the surf zone while other team members took water bottle samples nearer the beach at various locations away from the release site. Thousands of measurements were collated to produce detailed maps of dye concentration and dispersal in the study area.
The new data will allow researchers to understand not only where pollutants travel in the surf zone, but also how and where the waves and currents disperse the heaviest concentrations.
“This was the most comprehensive experiment of its kind, and the model passed with flying colors,” Guza said. “Scripps will be making these kinds of predictions at many coastal locations in Southern California, and this was the first time we were able to confirm the model predictions with results from the field. We plan to extend this valuable information to many other locations and decisionmakers.”
The beach pollution-tracking project is part of the Southern California Coastal Ocean Observing System and is supported by the California Coastal Conservancy, the National Oceanic and Atmospheric Administration, the Office of Naval Research and Sea Grant.
Real-time estimates of waves and alongshore currents for beaches in San Pedro Bay (Ventura County Line to Palos Verdes Point), and Santa Monica Bay (Palos Verdes Point to Dana Point) are available at http://www.sccoos.org/projects/hb06/.
— Chuck Colgan