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Web Wave Watch

May 16, 1997

Sensors attached to an oil platform off Point Conception gather data for a wave forecasting program run by UC San Diego's Scripps Institution of Oceanography. The information helps produce a Southern California swell model that is updated hourly. This and other results of the Coastal Data Information Program are available on the Scripp's Web site on the Internet at: http://cdlp.ucsd.educ

Sensing Wave Movement

Height and period (side view)

Sensors can tell the height of a wave by measuring the pressure in the water column above them. The more pressure, the bigger the wave. The period, or frequency, is known by the time difference between peaks.

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Swell direction (overhead view)

Having an array of sensors on the platform allows the swell direction to be known. A wave reaching two sensors at the same time (left) comes from a different direction than a wave reaching the same two sensors at different times (right). Eight sensors allow for more precise data.

Monitoring the Southern California Coast

Chevron's Harvest Platform (1) northwest of Point Conception is an ideal location because it is open to swells from the north, west and south. Information collected there allows anyone with an interest in the ocean to stay informed. Every second, data from sensors is stored in a computer (2) on the platform. Once an hour, computers at Scripp's (3) call to download the most recent data, which prompts the Harvest computer to transfer data by microwave to an on-land receiver (4) and on to Scripp's by telephone lines. (5)

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Pressure sensor

Water exerts pressure on a rubber diaphragm covering an oil-filled cylinder linked by wire cable to a computer on the platform.

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Beachgoers

Surfers, swimmers and lifeguards have keen interest in waves.

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Real Estate

Large surf can wreak havoc on beachside homes.

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Marine Traffic

Ship captains, fishermen and the Coast Guard all rely on accurate wave data.

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Development

Jetties, harbors and other projects affect, and are affected by, wave movement.

West Coast Network

The Coastal Data Information Program is a cooperative project of the Army Corps of Engineers and the California Department of Boating and Waterways, with an annual budget of $650,000. These agencies contract with Scripps to oversee wave measuring networks on the East and West coasts and Hawaii. The networks-bouys and offshore and near-shore sensors-also feed information to the National Weather Service.

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Buoys

These report wave height, period and direction by measuring the displacement (up and down movement) from mean sea level every second and transmit the data to onshore receivers. Buoys cost about $45,000 each.

From The Web Site: Two Swells

West Swell

Most winter swells come from storms generated far north and west of California. At right, a large west swell brings 4- to 6-foot waves to most beaches, with bigger ones sneaking between the islands and hitting west-facing beaches in Oxnard, the South Bay and the San Diego area.

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Wave Height Scale

Coastal waters are color coded according to the height (in feet) of the waves passing through them. Reds and yellows are areas of biggest swells, blues and greens the smallest.

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South Swell

In the spring and summer, swells are usually generated to the south, often in the Southern Hemisphere. Forecasting south swells is done after the fact because swells at the Harvest platform have already hit most area beaches. At right, a small south swell is mixing with a weakening northwest swell. Waves are mostly in the 2-to 3-foot range, but larger in the open ocean from a northerly direction.

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Directional Spectrum

South swell from about 180 degrees, northwest swell from about 300 degrees.

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Island Shadow Effects

Local coastal forecasting is affected by the large number of islands. These block the swell at beaches directly behind them (in line with the swell direction), causing waves to vary greatly within a few miles. Waves in shadow areas, at left, are about half as big as those in areas that receive the swell more directly.

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Directional Spectrum

This compass, with the Harvest platform as its center point, shows the direction of swell propagation (here about 275 degrees), intensity (bright colors have the most energy and, therefore, size) and period (time between swells; here about 13 seconds). The closer to center, the greater the period, meaning that the swell comes from a greater distance. Brightest blotch shows mains swell.

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Island Shadow Effects

Even this small swell shows shadow effects, with waves along the Santa Barbara- Ventura coast and on the south side of the Palos Verdes Peninsula half the size of nearby areas.

Source: Scripps Institution of Oceanography; UC San Diego; Chevron; Texaco; Army Corps. of Engineers; California Department of Boating and Waterways

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