What causes sneaker waves along Sonoma County's coast?

Amanda Viola was taking family pictures at a popular Sonoma County beach in February when an unusually large wave surged ashore, swept past four family members and knocked them down, then dragged her 7-year-old daughter out to sea.

They never saw it coming. As her brother and boyfriend struggled in chest-deep water, Viola launched herself into the incoming 12-foot waves in an effort to reach her daughter. In one terrifying moment, they had gone from enjoying a day at the ocean to fighting for their lives in cold violent surf.

That's not an unfamiliar scenario to Tim Murphy and Joe Stoffers, the two permanent State Park Peace Officer Lifeguards entrusted with protecting, and sometimes rescuing, the public along 35 miles of Sonoma County coastline, from Bodega Head to Salt Point State Park.

Stoffers, a lifeguard since 2004, describes his work in and out of the ocean - watching people, calculating who needs to be talked to, reaching abalone divers in trouble - with confidence and infectious enthusiasm. Murphy grew up near the Atlantic Ocean and has been a lifeguard here since 1995. He has been recognized with many state and national honors for tough rescues.

The area's treacherous waves, unwary visitors and steep beaches keep their small team busy. In fact, just minutes before Viola's daughter was swept off Goat Rock Beach, lifeguard Aaron Pendergraft had warned the group about getting too close to the surf.

He was driving back from a beach check further south when he spotted Viola and her daughter tossing in the heavy shore pound. Without pausing to put on his wetsuit, he went in after them.

After 27 minutes of clinging to his flotation device in bone chilling 53-degree water, the three were plucked by airborne harness to safety by Henry 1, the Sonoma County Sheriff's helicopter, with mother and daughter intensely grateful to be alive.

Not everyone is as lucky. Murphy notes that since the 1930s, there have been at least 126 deaths at Wright's Beach alone, sometimes with four or five victims at a time, as would-be rescuers dash in only to drown themselves.

Unlike iconic Southern California beaches, where the ocean is more forgiving and beachgoers flock to the water, Northern California's coastline demands greater respect and poses a higher risk, said Troy Nicolini, the meteorologist in charge for the National Weather Service in Eureka and an energetic oceanographer and the tsunami program manager for northwestern California.

Nicolini has made a study of so-called sleeper or sneaker waves, the deadly, larger-than-average swells that can surge dozens of feet higher up the beach than expected, overtaking the unwary. He's motivated by a grim statistic: sneaker waves kill more people each year on the North Coast than all other weather hazards combined.

The problem with sneaker waves comes when they meet people who have seriously underestimated the risk they're in, Nicolini said. They're called sneaker waves because they often appear with no warning after long periods of quiet surf and much smaller waves, lulls that can last for 10 to 20 minutes. People arriving on the beach see the smaller waves and assume they aren't going to reach any higher, then just stop paying attention.

“Survivors I've interviewed all say the same thing,” Nicolini said. “They thought they were far enough from the surf to be safe. And they never saw the wave coming.”

Nicolini suspects the lulls are actually part of the sneaker wave phenomenon, based on computer models he has developed to predict how they occur. The waves are initially generated by low pressure storms in the Gulf of Alaska. Powerful winds transfer their energy to the ocean surface, building swells.

Those swells are actually bands of energy moving through the water, spaced a certain distance apart. Some swells travel faster than others, and that's where trouble starts. If one swell catches up with another swell and they overlap, their energies are combined, and the merged swell grows in height and strength.

On the other hand, when two swells don't overlap but just follow each other, they tend to cancel each other out, and the sea becomes flatter. Over time and distance, this process creates patterns of swells, called sets, where the largest swells end up buried between periods of relatively smaller swells. And if conditions are right, these sets eventually reach northern California shores intact.

To a family on the beach playing tag with the small waves, everything seems safe. But when the big energetic swell hits the shallow steep beaches of Sonoma County, they can rise and run with amazing speed and power.

Experienced surfers know this, which is why they'll stand on the bluffs and read the ocean for 20 minutes or so before heading for the water. They're observing the sets coming in and gauging the pattern: the swells in a set, the height of the biggest swells, timing between swells, and how the swells break as they approach the shore.

North Coast beach goers would do well to follow the surfers' example, Nicolini said. Instead of rushing to the water's edge, he advises first spending some time watching the ocean. Sneaker waves are only dangerous if you're in their way.

Murphy and Stafford agree. The problem isn't really sneaky waves, it's complacent people. The lifeguards start each shift by studying NOAA reports on ocean conditions, tide tables and other information to see what's out there and what's coming, then decide which beaches will have the biggest risk that day.

Along the Sonoma Coast, most of their time is spent observing and communicating with beachgoers, making what they call safety contacts. They approach people to warn them about surf conditions and riptides, and advise people to stay up on dry sand, above the highest marked surf line. They try to keep parents carrying small children away from the water; they've seen them knocked down, losing hold of their tots in rough surf.

They also suggest when people should move to a safer beach and are especially watchful for unattended children playing too close to the water's edge.

Last year their team made 22,000 safety contacts. Not everyone appreciates their concern.

“About 90 percent of folks listen. But then there's some with false bravado or who say, ‘We've been coming out here for 20 years,'” Murphy said with a sigh.

Murphy and Stafford like the public to come and enjoy the beauty and fun of a day at the coast. Last year nearly 3.15 million visitors did, and almost all of them went home safely. But that's a lot of people for just two full-time, and seven to 10 seasonal lifeguards to protect. They're hoping to recruit new aquatic staff and are waiting for state budgets to allow them some new equipment and warning signs in English and Spanish.

In the meantime, lifeguards are counting on beach goers, tide poolers and surf anglers to be aware, understand and respect the ocean and changing conditions, and stay safe.

They made 54 rescues in 2015, and another four people drowned.

“But our main role isn't rescue,” Murphy said. “It's to prevent another tragedy from ever happening in the first place.”

Stephen Nett is a Bodega Bay-based certified naturalist, writer and speaker. Contact him at steve@californiasparks.com.