Winter driving - Avalanche control
WSDOT is committed to maintaining safe, efficient and reliable transportation systems. This commitment reaches to our steep and often avalanche prone mountain slopes that threaten the highways and the traveling public.
WSDOT's Avalanche Forecasting and Control team, check out a recent video highlighting their avalanche control efforts, 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 fall, winter, and spring along Washington's mountain highways.
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
- Wind speed and direction
- Existing snow conditions
This information is combined with a mountain weather forecast to predict the chance 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 elevated roadways so avalanches pass under them and 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.
Avalanche control is a winter-long activity on two primary travel corridors; I-90 Snoqualmie Pass (3,022 feet) and US 2 Stevens Pass (4,061 feet). These mountain passes average more than 450 inches of snowfall each winter. Typical traffic volume over Snoqualmie Pass is about 28,000 vehicles per day and about 5,600 of those vehicles are freight. The typical traffic volume 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 on the west side of the pass area. When active control is required there, crews close the roadway to ensure safety. A "sweeper" vehicle drives through to ensure that no vehicles are still inside the zone. Once the area is clear, cable trams and or artillery are used to deliver the explosives to the top of the avalanche paths where they're detonated, allowing our technicians to remain at a safe distance.
The crew on Snoqualmie Pass has used a 105mm Recoilless Rifle, and an M60 Tank for many years. A 105mm howitzer is now used when artillery is need to trigger avalanches. The desired result is the same; detonated explosives to release the avalanche. Then maintenance crew members remove any snow that may reach the roadway, and then reopen the road to traffic. Traffic delays are typically 30 minutes to two hours. However, during severe conditions, the volume of snow released and the time to clear it from the roadway can extend to several hours. WSDOT crews strive to perform avalanche control work at night or during periods of lower traffic volume, but they can't control the weather so winter travelers should always be prepared for possible delays
US 2 Stevens Pass
The "Old Faithful" avalanche zone just west of the Stevens Pass summit requires the most frequent control. During an avalanche control road closure, traffic on US 2 is typically held at the summit (MP 64) and at Scenic (MP 56), eight miles to the west. Among other methods, the Stevens Pass crew use trams and a 105 mm Howitzer to fire shells into the top of twelve avalanche paths. The road is reopened after the avalanche control work and clearing snow from the roadway is completed. When possible, most of the control is done between midnight and 6 a.m.
Seasonal pass closures
WSDOT seasonally closes some passes each winter because avalanche control work in those areas is too hazardous.
- SR 410 Chinook Pass (5,430 feet) Enumclaw to Yakima, skirting Mount Rainier.
- SR 123 Cayuse Pass (4,675 feet) Chinook Pass to White Pass along the east slope of the Cascades.
- SR 20 North Cascades Highway - Rainy Pass (4,855 feet) and Washington Pass (5,477 feet) 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 having the 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.
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 can provide information about snow conditions in the avalanche paths. Crews are also evaluating Remote Avalanche Control Systems (RACS) to eventually replace the aging surplus military artillery.