Every winter, snow accumulates in the Cascade Mountains. When the mountains cannot support the weight of the snow avalanches result.

For the Washington State Department of Transportation, it means travelers over mountain pass highways would be at risk without a comprehensive program to control when and how the unstable snow pack above the roadway is brought down.

Each winter, specially trained avalanche control teams are stationed at Hyak near the summit of Snoqualmie Pass on I-90 and at Berne Camp near the summit of Stevens Pass on US 2. The teams' purpose is to decrease the hazard of avalanches for travelers and reduce the duration of winter highway closures.

When it is performed WSDOT active avalanche control is the intentional triggering of avalanches. In a controlled environment, traffic is safely stopped, snow removal equipment is at hand and the roadway can be re-opened to traffic most quickly. The work is usually necessary during times of heavy snowfall but to be most effective, avalanche control is done when the snow is becoming unstable before it would slide naturally. When possible, the timing of avalanche control work is scheduled during non-peak traffic hours to minimize travel disruptions.

Predicting Avalanches

Avalanche hazard is evaluated using a variety of tools including weather observations, forecasts, snow analysis and historical records. Daily observations of wind speed, direction, air temperature, sky conditions, and other factors are analyzed. Remote automated weather stations relay information by phone, radio or satellite providing technicians with data on conditions on top of avalanche starting zones that can't be easily accessed on foot or other means.
 
Avalanche technicians also dig snow pits to determine the snow pack structure. After snow falls, it begins to undergo changes driven by time, temperature, water content and weight. Some of these changes can weaken layers, causing the snow layers above to slide. Detailed records archived for more than 50 years for some avalanche zones are maintained to provide another key tool for avalanche control technicians. They can use that data to predict how the snow might behave.

Performing Avalanche Control

When an avalanche hazard is present, avalanche control is conducted using artillery, explosives to start the slide. The explosives are delivered by a variety of methods depending on the topography and accessibility of the avalanche chute. Explosives are placed by hand, bomb trams or by use of surplus military weapons.

In addition to active avalanche control, WSDOT uses passive controls. These include snow sheds over the highway; building elevated roadways so avalanches pass under the highway or creating catchment basins to stop the avalanche flow; and divert dams or deflect snow-berms to keep the snow off the highway.

Passes Closed All Winter

Three passes are usually closed during the winter because avalanches are so prevalent that control measures would be too hazardous and costly.

• Highway 410's Chinook Pass (5,430') connects Enumclaw and Yakima, skirting Mount Rainier.
• Highway 123's Cayuse Pass (4,675') connects Chinook and White Passes along the east slope of the Cascades.
• North Cascades Highway (SR 20) including Rainy Pass (4,855') and Washington Pass (5,477') connects the Skagit Valley in western Washington and the Okanogan Valley on the east side. This portion of SR 20 and the east side of Chinook Pass hold the distinction of being among the top areas in the United State for most avalanche chutes per mile of highway.

Passes Open During Winter

Avalanche control is a winter-long activity on two primary travel corridors that must remain open year-round. I-90's Snoqualmie Pass (3,022'), the primary east-west corridor serving Seattle and Tacoma and US 2's Stevens Pass (4,061') connecting Everett and Wenatchee. Snoqualmie Pass is the only Interstate Highway link through the Cascades. It averages nearly 450 inches of snowfall each winter and has traffic volume over 32,000 vehicles per day (including 8,000 trucks). It is estimated that a two-hour closure of the pass costs the state's economy over $1 million.

Snoqualmie Pass

The avalanche character of each of the passes is unique and so are the tools and techniques needed for control. The most persistent avalanche zone requiring control measures through Snoqualmie Pass is east of the summit along Lake Keechelus.

While a snow shed provides passive protection for the westbound lanes below two avalanche chutes, the entire zone can fill quickly during a storm. WSDOT typically barricades the traffic lanes at either end of the zone and run a "sweep" vehicle to ensure that no cars or trucks are still inside the zone. In areas without permanent bomb trams, avalanche technicians drive along the crest of the ridge above the highway in a specially built track vehicle, placing hand charges at the top of the unstable avalanche chutes. The explosives are detonated, releasing the snow. Any excess snow that reaches the roadway is quickly removed by maintenance crewmembers using snowplows, loaders and snow blowers. The entire operation can be completed in a short period of time. Traffic is usually held up for less than an hour. During severe conditions, the amount of snow in the chutes and the time to remove it from the roadway can extend to several hours.

Stevens Pass

The "Old Faithful" avalanche zone just west of the summit of Stevens Pass requires the most frequent control in that corridor. Due to the shape of the highway and the explosives delivery system, travel delays seldom exceed 20 minutes except during unusual severe conditions. Traffic on US 2 is typically held at the summit and at Scenic, eight miles west. Surplus military equipment is positioned to fire shells into the tops of six avalanche chutes. The operator can observe nearly the entire section of closed highway from his location eliminating the need to run a "chaser" vehicle. Traffic is stopped while the shelling is underway and until any required clean up is completed.

Recreational Impacts

Avalanche control activity affects more than travelers. Backcountry recreation has become very popular over the years. From the Stevens Pass Ski Area at the summit, skiers and snowboarders can access backcountry areas and venture into the avalanche zones. Signs alerting recreationists to the danger are sometimes ignored. While dangerous in itself, the hazard on the road increases as well. Skiers and snowboarders can release avalanches that might not otherwise release. They also create a hazard hitchhiking back to the summit because vehicles stop to give hitchhikers a ride, creating a traffic hazard, and sometimes cannot get started again, blocking traffic behind them. The Washington State Patrol petitioned WSDOT to post the zone to prohibit hitchhiking and vigorously enforces that ban. Skiers and snowboarders face similar personal hazards at two Snoqualmie Pass ski areas when they ignore signs and venture outside ski area boundaries.
 
Thousands of avalanches occur in the mountains of Washington State every winter. Hundreds can affect travel over the mountain pass highways. They all present the potential for accidents and delays. WSDOT Avalanche Control technicians are committed to reduce that potential using all the experience and tools at their disposal.