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CORVALLIS - Oregon beaches are just now starting to recover from an unusual double whammy of erosion - a strong La Nina episode on the heels of one of the most intense El Ninos of the entire century.

The recovery, though, may take years.

Paul Komar, professor emeritus in the College of Oceanographic and Atmospheric Sciences at Oregon State University, says the processes of the two phenomena are different. But one result is the same: Both tend to strip sand - and lots of it - off of Oregon beaches.

"With El Nino, you get hot spots of erosion," Komar pointed out. "Usually, it takes away the sand north of the headlands, and shifts inlets to the north. The sand also piles up in the entrances to bays. With La Nina, it's the effect of powerful storms with big waves hitting all of the Oregon coast. The waves just take sand off the beach and out to sea."

Komar will speak on the topic this Saturday, July 17, at the da Vinci Days Festival in Corvallis. His talk, "El Nino and La Nina: Sibling Rivals in Oregon Coast Erosion," will begin at 12:30 p.m. at LaSells Stewart Center on the OSU campus. It is free and open to the public.

An expert on coastal erosion, Komar is leading a small research team that plans to measure the effects of El Ninos and La Ninas. His hope is to develop a model that will enable land use managers to better predict how Oregon beaches might be affected by changing conditions.

The intense El Nino episode of 1982-83 first brought attention to the erosion problems caused by the phenomenon, which previously had been blamed primarily for fish kills. By the spring of 1983, however, the spit at Alsea Bay had almost completely washed away, jeopardizing dozens of houses in the Bayshore area of Waldport.

A revetment was built to slow the erosion and, when the effects of El Nino subsided, the sand slowly began to come back. The lesson, however, was quickly forgotten.

Within a dozen years, Komar said, more houses were built on the dunes - some of them seaward of the revetment that was buried by returning sand. When another strong El Nino struck in 1997-98, it happened again, threatening those new homes.

Similar erosion problems occurred at Netarts Spit and Cape Lookout State Park.

"When El Nino finally subsided, everyone breathed a sigh of relief," Komar said. "Then along comes La Nina, about which we know very little, and a new set of problems took over."

There are some fundamental differences between El Ninos and La Ninas, he said.

In a "normal" year, trade winds blow from South America to Asia and pile up water in the far western Pacific. When an El Nino hits, those winds subside, and the piled-up water moves slowly back along the equator to South America. Those warm waters, blamed for fish kills off Peru and other South American countries, then travel north along the shoreline, eventually to Oregon.

The higher water levels erode sand off the beaches and push it northward. It's a constant give-and-take situation, Komar said. As sand is stripped off beaches, other sand moving up from the south can replace it. Areas north of headlands and other "hot spots," don't get that replacement sand, he added.

During a La Nina, it is the series of intense storms that cause the damage. Strong winds push huge waves high up onto the beach and literally suck the sand out to the ocean. Unlike El Nino, the effect of La Nina on erosion is everywhere, Komar said.

"Scientists always thought that El Nino was the chief threat to Oregon beaches and it may still be," Komar said. "But we're starting to learn more and more about La Ninas. We don't have as much data on them, so we're a little earlier in the process."

What scientists do know, Komar said, is that the storm "surge" can be intense. Last winter, for example, La Nina had three "100-year storms." In the same way researchers measure river floods, they can estimate deep-water wave heights and predict the highest level likely to hit in a century. Off Oregon, the 100-year projection is an average wave height of about 10 meters, or 33 feet.

Of the three storms that surpassed that projection, one that hit March 2-4 of this year took the prize, Komar said. The average wave height was 14 meters, or 46 feet, with the highest waves in a 20-minute stretch hitting 25 meters, or 80 feet. That's equivalent to a 10-story building, he pointed out.

"The largest wave ever reliably measured was in 1933 by the U.S.S. Ramapo, which logged a 112-foot wave off Antarctica," Komar said. "So we're in the ballpark."

So little historical data exists on La Ninas, that researchers aren't sure if the March 1999 storm was a 300-year event, or a 500-year event.

"In reality," Komar said, "we probably will have to re-evaluate our 100-year projection."

The back-to-back double whammy of La Nina on top of El Nino was a rare opportunity for researchers to compare the different effects of the phenomena. Together, they combined to completely change the way Oregon beaches look. It is an effect that may take a while to overcome, Komar said.

"The sand is starting to come back, but it can be a slow process," he said. "Strong storms can strip a beach in a matter of days, but sometimes it will take years for it to rebuild."