Bombs of the Backcountry: How to be safe cutting cornices
Cornices are beautiful: white, transfixed crescendos of nature, icons of the alpine environment. They can also be great snow stability tools when used properly. To safely send one down a slope is an invaluable tool in any backcountry traveler’s bag of tricks.
The advantage of cutting cornices is that it allows backcountry skiers or mountaineers to test a slope without getting onto the slope and putting themselves at risk. It’s also superior than using results from stability tests performed on adjacent slopes. That all seems straight forward, but make no mistake, cutting cornices can be very dangerous.
“The most obvious danger associated with cutting cornices,” says Ron Johnson of the Gallatin National Forest Avalanche Center, “is being too close to the edge and falling off the ridge when the cornice breaks.”
“A second danger,” warns Doug Abromeit, Former Director for the US Forest Service Avalanche Center, “—dropped cornices are called the bombs of the backcountry, and they can trigger large slides that can bury other skiers, climbers, etc., who happen to be in the path.” An average cornice is 30% density with a weight of 300kg per cubic meter (660 lbs). It’s not uncommon for backcountry skiers to cut cornices larger than two cubic meters, which is well over 1200 lbs bounding down a hill. It’s easy to see why they are called “bombs”.
Cornices are notorious for breaking back farther than you think. Here is one rule of thumb to help avoid getting pitched off. If you are cutting a cornice on a 40-degree slope, imagine a line continuing up through the cornice at forty degrees until it reaches the top. As best you can, picture where that line would be and stay well behind it. It’s also worth mentioning that following a large cornice down its path after it has gone down the slope is a relatively safe route in high avalanche conditions.
There are three basic ways to cut a cornice: kicking them with your skis, cutting them with a Bonesaw (attached to a ski pole), and cutting them with a knotted cord. For the first two methods, it’s more than a good idea to be belayed to an anchor, as you’re probably right on the breaking point. While 7mm dynamic rope is acceptable to some professionals, thicker diameter cord is recommended.
“People should practice cutting small cornices on ‘safe’ slopes,” says Abromeit, “and then work up to actually cutting large cornices in potentially dangerous terrain.
” While it’s far from absolute, it’s a pretty reliable test and can help determine slope stability. “The most important thing to remember when cutting a cornice,” says Ron Johnson, “is that it is only one stability evaluation tool and should be used in conjunction with other tests and snowpack observations.”
Things to consider: How far back can the cornice break? Is anybody on the slope below? Can I safely cut this cornice without falling with it?
Things to practice: Start small and work your way up to larger cornices. Belay your partner from a safe spot, i.e., tree anchor, ski anchor. Eyeing up slope angle with regards to the potential breaking point, then take a big step back. Hell, take two.
G3 knows that no technique is absolute in the backcountry, but lowering your exposure and making calculated decisions are based off of experience, restraint, and good information. Be smart and have fun exploring the mountains.