If you happened to be very close to the Mammoth Mountain ski resort in Mammoth Lakes, California, early on a clear morning following a recent series of winter storms, you might have heard the sound of Nat Heit and his crew firing 105-mm howitzers to try to cause avalanches.
But Heit, a ski patrol manager at Mammoth Mountain, wasn't being malicious or endangering anyone. He and as many as 30 colleagues were doing what they frequently do at five or six in the morning after a heavy snow: bring the avalanches down before the mountain's slopes are covered with skiers.
And while using military equipment to create shockwaves might seem like an extreme way to prevent the kind of tragedies that claimed the lives of at least seven people in Utah in the last two weeks, it's common throughout the United States.
"The object of all of this is to set up frequent small avalanches while it's building up," Heit said, "rather than it building up to an avalanche that's huge or catastrophic or damaging."
He also said many other ski resorts in the United States use explosives as a way of heading off disaster.
Daniel Journeaux, the president of Janod, a Swanton, Vermont, contracting firm that specializes in avalanche, landslide and rockfall prevention, said other kinds of explosives also work well to keep heavy snowfall from turning deadly.
One system, known as Avalhex -- a balloon filled with hydrogen and ambient air mounted on a tower above a snowy slope -- can be exploded remotely to set off a shockwave that can trigger avalanches anywhere within a 120-foot radius. These devices are set up in groups in order to blanket a mountainside.
Jorneaux said non-explosive ways can also stave off avalanches. His company installs one such system, a series of flexible barriers, or snow fences, are put in at the top of a slope where engineers determine an avalanche might start. The idea is to hold the snow in place so it can't release.
An Austrian system, known as Snowgripper, works by installing a series of cone-shaped devices up and down a slope, Jorneaux said. The cones keep the snow in place by collectively changing the dynamic that otherwise pulls the snow downward.
But Jorneaux said though avalanches in many places are easy to predict because they occur regularly or at least semi-regularly, other locations can go in 50-, 100- or even 200-year cycles.
Still, he said, even though dozens of people die in avalanches every year, the government -- currently considering a Senate bill that would devote $90 million to avalanche-prevention systems and awareness and research -- should be able to keep it from happening.
"There's no reason that anybody should be killed by an avalanche," he said. That's "the bottom line, because the technology exists" to stop them.
Meanwhile, the recent winter storms that battered Southern California were deadly even where there was no snow. A giant landslide overcame the town of La Conchita, California, killing 10 people on Jan. 10.
While the slide that hit La Conchita was extraordinary and happened so quickly nothing could be done to stop it, experts say the technology exists to prevent many landslides.
According to Roy Bibbens, a supervising transportation engineer at the California Department of Transportation, a series of things can be done to reduce the "driving forces" that cause landslides.
The first thing authorities can do, he said, is excavate the top of a saturated slope, replacing heavy, water-laden soil with lightweight fill that reduces the force pushing downward.
Another method is to de-saturate a slide, either by surface drainage or by a subsurface process that pulls the water out.
Bibbens said authorities sometimes choose to attack a potential slide from the bottom instead, using some sort of buttress or retaining wall to hold a hillside in place. And while that method is best before a slide starts to move, Bibbens explained that if movement is slow -- inches or feet an hour -- a buttress can still be used.
"It depends on how fast it's moving and also the size of the slide," he said. "The strategies have to be appropriate to the size of the slide."
Jorneaux said his firm sometimes drains slopes of water to keep them from sliding. But he said another, albeit expensive, method is to use anchors, called soil nails. The principle for using soil nails is similar to the snow fences his firm employs to stop avalanches. That is, the anchors are placed along a slope where a slide seems most likely to start.
"They knit it all together," Jorneaux said. "You use several soil nails to make a more homogeneous unit, tying it all together."
