Berkeley - Swimmers, waders, dog-walkers and beachcombers at San Francisco's Ocean Beach, beware.

Francis James Smith at Ocean Beach  Robert Sanders, photo

The beach is notorious for its rip currents, and the fast-moving rips that have formed already this season represent a hazard to the unwary, warns Francis Smith, a University of California, Berkeley, graduate student in geography who has studied local rip currents for the past five years. Smith also is a swimmer, diver and former National Park Service lifeguard at China Beach.

"Ocean Beach is the most hazardous and dangerous piece of shoreline associated with an urban environment in the whole United States," he said today (Thursday, May 23) at a media briefing in the parking lot at Ocean Beach in an effort to publicize the danger.

Last month, three swimmers were swept out from shore by rip currents at Ocean Beach, and one had to be rescued by firefighters and the Coast Guard, with the help of a nearby surfer. Those unfamiliar with the ocean, as well as non-swimmers swept off their feet by waves, are carried to sea and drowned each year.

"In 1998 more than a dozen people drowned there," Smith said. "I want to help prevent drownings, so I'm warning people that these rip currents are already in place and will persist through the summer."

Rip currents are fast, outgoing streams of water that can quickly sweep swimmers far from shore. They are distinguished from rip tides, which are bores that travel up and down river mouths and estuaries with the tide.

Click here to view diagram showing three types of rip currents

According to the United States Lifesaving Association, in 2000 more than 22,000 people were rescued from rip currents in the United States, out of a total of nearly 71,000 rescues. About 80 percent of lifeguard rescues at the country's surf beaches involve people caught in rip currents, the association claims.

Knowledgeable swimmers can escape rip currents by paddling parallel to the shore until free of the current, then heading toward the beach. Surfers love them, however, as a freeway through the surf zone to get set for the next wave.

"To surfers, these fast currents are like streetcars, but the currents are a danger to those who don't understand them," said Robert Wiegel, professor emeritus of coastal engineering and Smith's advisor. "There are dangerous rip currents at beaches around the world, and most heavily used beaches in urban environments have lifeguards. Ocean Beach's beach patrol, with trained lifeguards, has done an excellent job preventing drownings."

"Usually rip currents don't go out much beyond the surf zone, but at Ocean Beach the surf zone is big," Smith said. One satellite image of the San Francisco area shows a rip current that seems to stretch a quarter mile into the ocean.

Smith has been fascinated by the currents at the beach for years, and four years ago convinced the city of San Francisco to post warnings about rip currents along the Great Highway, which borders Ocean Beach. He designed a series of four beach hazard symbols and beach hazard signs that earned him a certificate of appreciation from the United States Lifesaving Association in 1999.

Working with the late geographer Bernard Nietschmann at UC Berkeley, he wrote a master's thesis on Ocean Beach rip currents, and currently is working on a PhD thesis delving into why rip currents occur where they do.

Typically, the currents at Ocean Beach dissipate during the winter and reappear in spring, Smith said. He has tried to capture on video the appearance and disappearance of the rip currents, and first observed them forming this year in April. He also recently obtained spectacular footage of a lineup of rip currents at Baker Beach, north of Ocean Beach and within sight of the Golden Gate Bridge.

He hopes eventually to put together a packet with video to educate people about where and how the currents form at Ocean Beach.

Rip currents are generated, Smith said, when waves hit a section of beach that deflects the water into a fast, seaward trough, which funnels through a channel in an underwater sand bar that prevents the water from retreating back the way it came. Rip currents are often associated with a small underwater sand bar, frequently visible at extremely low (negative) tide, and they probably reinforce one another, Wiegel said.

The outgoing current can reach speeds of up to six miles per hour - nine feet per second - which is faster than an Olympic swimmer, Smith said. Typically 25 to 150 feet wide, they dissipate only after passing through the surf zone, perhaps hundreds of yards offshore.

"Rip currents are caused by different things, and Francis is trying to sort out the causes at Ocean Beach," Wiegel said. "But he primarily wants to help minimize drownings. If alerting people can save just one life, he's doing a good job."

More information on rip currents can be found on the Web site of the United States Lifesaving Association, http://www.usla.org.