Scientists map miles of underwater dunes

http://news.yahoo.com/s/ap/20060721/ap_on_sc/submerged_sand_dunes
Scientists map miles of underwater dunes

50 minutes ago

SAN FRANCISCO - More than 2 square miles of some of the world's largest underwater sand dunes were mapped in the Pacific Ocean off the San Francisco coast as part of a study to help researchers better understand beach erosion.

The submerged dunes, called sand waves, are located just west of the Golden Gate Bridge, measuring more than 30 feet high and 700 feet long, according to researchers with the
U.S. Geological Survey.

The sand waves were created by the displacement of sediment by powerful currents rushing through the mile-wide opening between the ocean and San Francisco Bay.

About 500 billion gallons of water move through the opening every six hours, said Patrick Barnard, a coastal geologist with the U.S. Geological Survey, who authored the study released this week with oceanographer Dan Hanes.

"Part of the purpose of our (sand wave) survey was to assess changes in San Francisco Bay in the last 50 years," Barnard said. "We've found the bay has lost a huge amount of sediment since 1956 -- approximately 137 million cubic yards."

Researchers first mapped the sand waves with sonar devices two years ago, but new three-dimensional technology has helped them measure their size.
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