Snow slides are a fact of life in the Cascade Mountains. An avalanche happens when weaker snowpack layers cannot support the layers of snow above.
WSDOT avalanche control technicians work to reduce the potential hazard using all available experience and tools. This means operating a comprehensive program to control when and how to bring down unstable snow.
Each winter, WSDOT stations specially trained avalanche control teams at Hyak, near the I-90 Snoqualmie Pass summit and at Berne Camp, near the US 2 Stevens Pass summit. The teams work to reduce the avalanche hazard as well as the number and duration of highway closures.
Active avalanche control is when crews intentionally trigger an avalanche. To do this, WSDOT stops traffic and triggers the avalanche. Avalanche control must be done during heavy snowfall. However, to be most effective, active control work is done just as the snow is becoming unstable; but before it slides. Whenever possible, the control work is scheduled outside of peak traffic hours.
Avalanche technicians dig snow pits and use sophisticated instruments to determine the snow pack structure. After snow falls, it begins to undergo changes driven by temperature, water content and weight. These changes can weaken the structure of some layers, causing adjacent layers above them to slide.
WSDOT evaluates avalanche hazards using a variety of tools, including weather observations, forecasts, snow analysis and historical records. Crews also analyze daily observations of new snow, wind speed and direction, air temperature, sky conditions, and other factors. Remote automated weather stations relay information by phone, radio or satellite. This provides technicians with conditions at the tops of often-inaccessible avalanche starting zones.
Our technicians also use more than 50 years of detailed records. This provides more data to help forecast how the snow might behave.
Performing avalanche control
When an avalanche hazard develops, WSDOT uses artillery, or explosives to trigger the avalanche. These are various methods of delivery depending on the topography and accessibility to the avalanche path. Explosives are placed by hand, cable-pulley bomb trams, or with surplus military weapons.
In addition to active avalanche control, WSDOT also uses passive control methods to control snow slides. These include snow sheds over the highway; elevated roadways so avalanches pass under them, or with catchment basins to stop the avalanche before snow reaches the highway. WSDOT also uses diversion dams and snow berms to keep the snow off the highway.
Passes open during winter
Avalanche control is a winter-long activity on two primary travel corridors; I-90 Snoqualmie Pass (3,022') and US 2 Stevens Pass (4,061'). These mountain passes average more than 450 inches of snowfall each winter. Typical traffic volumes over Snoqualmie Pass is about 28,000 vehicles per day and about 5,600 of those vehicles are freight. The typical traffic volumes over Stevens Pass is about 4,500 vehicles and about 450 of those vehicles are freight.
I-90 Snoqualmie Pass
The avalanche character of each pass is unique and so are the control tools and techniques. The most persistent avalanche zone through Snoqualmie Pass is east of the summit along Lake Keechelus. This area is known as the East Shed.
While a snow shed provides passive protection for the westbound lanes below two avalanche paths, the eastbound lanes are not covered. When active control is required there, we stop traffic a safe distance away from the foot of the avalanche zone. A "sweeper" vehicle drives through to ensure that no vehicles are still inside the zone. Once the area is clear, cable trams deliver the explosives to the top of the avalanche paths where they’re detonated, allowing our technicians to remain at a safe distance.
In areas without trams, team members use a variety of methods to place explosives at the top of unstable avalanche zones. Options range from snowshoes and hand charges to snowmobiles and snow cats. Technicians will often use surplus military artillery to deliver explosives to the top of otherwise inaccessible avalanche paths. The crew on Snoqualmie Pass used a 105mm Recoilless Rifle for many years. As surplus munitions became scarce, the giant rifle has now been replaced with a surplus M60 tank. That tank is identical to a pair of tanks are used successfully on Stevens Pass for several winters.
The desired result is the same; detonated explosive, release the avalanche. Then maintenance crew members remove any snow that may reach the roadway, so it can be reopen to traffic. Traffic delay is typically less than an hour. However, during severe conditions, the volume of snow released and the time to clear it from the roadway can extend to several hours, so be prepared for such delays.
US 2 Stevens Pass
The "Old Faithful" avalanche zone just west of the Stevens Pass summit requires the most frequent control. Traffic on US 2 is typically held at the summit (MP 64) and at Scenic (MP 56), eight miles to the west. The Stevens Pass crew uses trams and those two surplus military tanks to fire shells into the top of twelve avalanche paths. Traffic is stopped while avalanche control and any required cleanup is completed. Most of the control is done between midnight and 6 a.m.
Mountain passed closed all winter
WSDOT closes some passes during the winter because avalanche control work becomes too hazardous.
- SR 410 Chinook Pass (5,430') Enumclaw to Yakima, skirting Mount Rainier.
- SR 123 Cayuse Pass (4,675') Chinook Pass to White Pass along the east slope of the Cascades.
- SR 20 North Cascades Highway-Rainy Pass (4,855') and Washington Pass (5,477') the Skagit Valley to the Okanogan Valley. SR 20 east of Washington Pass holds the distinction of being among the top areas in the United State for most avalanche paths per mile of highway.
WSDOT avalanche control activity affects more than travelers. Backcountry recreation has become very popular. From the US 2/Stevens Pass Ski Area, skiers and snowboarders can access backcountry areas and potentially venture into the highway avalanche zones. WSDOT posts warning signs at the top of the ski area and in key locations, but are sometimes ignored. Besides risking injury, skiers and snowboarders sometimes trigger avalanches. They also create a hazard for themselves and others by hitchhiking back to the summit. When vehicles stop to give hitchhikers a ride, it creates a traffic hazard. The Washington State Patrol petitioned WSDOT to post the avalanche zones from milepost 58 to 66 to prohibit hitchhiking and WSP troopers vigorously enforce this ban. Skiers and snowboarders face similar personal hazards at two Snoqualmie Pass ski areas when they ignore signs and venture outside ski area boundaries.
New technology now and for the future
WSDOT Avalanche Teams test new technologies that could be incorporated into our control program. As an example, WSDOT is evaluating whether remote-controlled drone aircraft could successfully deliver explosive charges under severe mountain weather conditions.