Maintenance - Avalanche Control

Avalanche Contor Video link

Watch our winter videos on our WSDOT YouTube Channel . Crews performed avalanche control work at the snowshed just east of Snoqualmie Pass. See photos at the WSDOT Flickr site

WSDOT is committed to maintaining safe, efficient and reliable transportation systems. This commitment reaches beyond our highways to steep and often avalanche prone mountain slopes that threaten the highways and the traveling public.

WSDOT’s Avalanche and Forecasting Control team is a dedicated crew of experienced professionals who monitor the weather and snow to determine when avalanches may occur. The crew is split into two regional teams with full-time employees and seasonal employees. The Avalanche Control Supervisor for each team leads the crew throughout the year.

When conditions indicate an avalanche is imminent, teams employ various methods and tools to bring down unstable snow in a controlled manner. The team works throughout the winter and spring along Washington’s mountain highways.

Forecasting avalanches

Avalanche forecasting determines the potential risk along a particular mountain slope. Avalanche forecasts are based on past, current and forecasted conditions. Some important factor used to determine the avalanche hazard include:

  • New snow or rain
  • Temperature
  • Wind speed and direction
  • Existing snow conditions

This information is combined with a mountain weather forecast to predict the chance of an avalanche will occur on a particular mountain slope.

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

I-90 Snoqualmie Pass Avalanche Paths   
View the west side  and the east side  avalanche paths .

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

Stevens Pass Avalanche Areas Map  
View the west side and the east side avalanche paths.

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 passes 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.

Recreational impacts

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.