CalTrans Avalanche Control - Jan 19, 2006
Avalanche control busy as snow piles up: Danger is high for backcountry
Caltrans crews were busy blasting to reduce avalanche danger on highways 50 and 88 after another foot of snow fell at higher elevations in Tahoe on Tuesday and Wednesday. That's on top of 2 feet of powder which fell last weekend.
The Sierra Avalanche Center in Truckee issued a HIGH avalanche danger advisory Wednesday for backcountry travelers in Tahoe. The center warned that recent storms have created unstable layers in the snowpack. Avalanche danger is greatest in areas where the wind has blown the snow so that it accumulates deeper in one area, a phenomenon called wind loading.
Caltrans crews closed both highways for several hours Wednesday to perform avalanche control. Bombs are not used on Echo Summit. Rather, an air cannon creates a loud noise that jars loose the unstable snow.
"The concussion is what causes the snow to come down," said Caltrans spokesman Mark Dinger. The transportation agency has automatic sensors along the Echo Summit crest which transmit conditions to a computer. Crews also check in person on a Sno-Cat before determining whether controls are needed.
Avalanche control is usually required every time it snows more than 12 to 18 inches, Dinger said. "Our consideration is the safety of the public," Dinger said. "We don't want to arbitrarily close the highway if we don't have to."
Avalanche control closures are more common on the way to Kirkwood on Highway 88 at the Carson Spur and Carson Pass. "If conditions warrant that we can wait, we usually try to cooperate with Kirkwood's incoming times and outgoing times," said Caltrans spokeswoman Zelie Nogueira. On Highway 88, Caltrans uses special wind obstacles called jet roofs to prevent dangerous cornice loading. Air cannons and explosive devices are used to urge unstable snow to come lose.
Caltrans' goal is make sure an avalanche never happens, because it can be such a serious threat to public safety, Nogueira said. Avalanche danger for backcountry travelers like skiers, snowmobilers and snowshoers was "HIGH" Wednesday, the fourth degree on a five-point scale.
Danger decreases as the snow compacts and layers of snow from several storms have a chance to bind more firmly, said Bob Moore, a forecaster from Sierra Avalanche Center.
Avalanche forecasters use many techniques to determine danger, most of which involve studying snow layers. Each storm is unique in temperature, wind patterns and snow depth. Conditions after a storm can also be a factor. "If it stays really cold, the snow won't change much," Moore said.
A classic danger sign is melting spring snowpack followed by deep freezing temperatures and light cold powder snow. The powder on top often will not bind well with the bottom ice layer, and will sluff off as easily as powdered sugar on a plate.
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