- Location: West Portal, Eisenhower Johnson Memorial Tunnels
- State: Colorado
- Date: 2020/03/25
- Summary Description: Roadway buried and infrastructure damaged
- Primary Activity: Backcountry Tourer
- Primary Travel Mode: Snowboard
- Location Setting: Backcountry
- Caught: 0
- Partially Buried, Non-Critical: 0
- Partially Buried, Critical: 0
- Fully Buried: 0
- Injured: 0
- Killed: 0
- Type: HS
- Trigger: AR - Snowboarder
- Trigger (subcode): --
- Size - Relative to Path: R3
- Size - Destructive Force: D3
- Sliding Surface: O - Within Old Snow
- Slope Aspect: W
- Site Elevation: 12200 ft
- Slope Angle: 34 °
- Slope Characteristic: --
This was a hard slab avalanche triggered by a snowboarder. The avalanche was medium-sized relative to the path, and large enough to bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees. The avalanche was triggered by a snowboarder riding through wind-drifted snow on a steep cross-loaded terrain feature. The initial avalanche was relatively small. As it ran down the slope, it stepped down to deeper weak layers and eventually gouged down to the ground. (HS-AR-R3D3-O).The avalanche damaged a remote avalanche control unit, used to manage the hazard to the Loop Road, and covered over 400 feet of the roadway with debris up to 20 feet deep. The Loop Road is a service road that is open to the public, but mostly used by highway maintenance workers. It connects the eastbound and westbound portions of Interstate 70 and runs over the top of the west portal of the Eisenhower Johnson Memorial Tunnels.
Backcountry Avalanche Forecast
The Backcountry Avalanche Forecast for the Vail-Summit County zone on March 25 rated the avalanche danger as Moderate (Level 2) at above and near treeline elevations. The forecast listed Wind Slab avalanches as the only problem on north to east to southeast-facing slopes. The likelihood of triggering was possible and potential size small to large. The Vail-Summit County Summary for March 25 read:
You can trigger avalanches that break at the interface between recently drifted slabs and the older harder snow below. You are most likely to find dangerous conditions on higher elevation, wind-prone, easterly-facing slopes were the prevailing winds thickened these slabs. If you trigger an avalanche at the interface between the recent and old snow it can break 2 feet deep and propagate wider than you might think. Look for recent cornice formation or smooth rounded pillows of snow to identify possibly dangerous areas. You can find safer riding options in wind-sheltered areas and below treeline.
Daytime temperatures recorded near the avalanche were in the upper 20s to low 30s Fahrenheit. Southwest winds increased throughout the day, averaging 15 to 25 mph with gusts up to 35 mph (EJT West Portal CDOT weather station at 11,072 feet). The weather station at the top of Loveland Ski Area’s Chair 9 (0.5 miles to the south of the accident site at 12,700 feet) recorded southwest winds from 40 to 70 mph. In the week previous to the incident, the Loveland Basin SNOTEL site, about 1.6 miles to the east southeast at 11,400 feet in elevation, recorded an increase of 1.8 inches of snow water equivalent and close to 2 feet of new snow.
CAIC forecasters did not visit the upper crown face due to safety concerns. They noted that the snowpack in the upper start zone looked shallow and heavily wind-affected. They observed a snow profile near where the avalanche stepped down to the ground. The snowpack was about 6 feet deep (180 cm). Soft snow from recent storms rested above a thin crust/facet layer. Below the crust layer, the middle layers of the snowpack were relatively dense and consolidated. There was a thin layer of faceted snow below the dense layers. Snowpit tests showed cracking without propagation on the crust/facet layer (ECTN24 @ 155 cm↑), and propagation on the thin layer of faceted crystals deeper in the snowpack (PST 34/100 End @ 100cm↑). The lower 100 cm of the snowpack was a mix of large facets and depth hoar.
Events Leading to the Avalanche
A party of two snowboarders went for a backcountry tour on the afternoon of March 25. They started from a parking area on the west side of Eisenhower Johnson Memorial Tunnels. They intended to ride a steep line above the tunnels. Rider 1 had been watching this particular line for several weeks prior. Recent southerly winds drifted snow into the chute, and it was significantly filled in. The riders ascended an existing skin track in the drainage north of the chute and gained the ridge.They walked south along the ridge and to the slope above the chute. Riders 1 and 2 put their boards into downhill mode and started down, planning to descend the slope to an area where they could see further down their route.
Rider 1 made slow turns down the slope. Rider 2 followed the same general path, but stayed higher on the slope. Rider 1 was traversing when he triggered a small avalanche. The slide was 4 to 6 inches deep and broke in wind-drifted snow. It was a small avalanche initially. As it ran down the slope the avalanche stepped down to a layer about 18 inches deep. Cracks shot widely across the slope and the avalanche gained more volume. As the debris continued to run down and through the chute it stepped down to a deeper layer, a foot or more above the ground. The slide eventually eroded into the very weak basal facets (depth hoar layer), removing the entire snowpack. This created a very large avalanche that rumbled down onto the Loop Road.
Neither Riders 1 or 2 were caught in the avalanche. They watched from above as the avalanche descended to the road below.
The Riders called 911 to report the avalanche. They descended along and adjacent to the avalanche and met law enforcement on the Loop Road. They helped law enforcement determine that no people were involved and no rescue was needed.
Both Riders 1 and 2 are experienced riders and familiar with this area. They carried avalanche rescue gear and established a plan prior to their descent. The pair saw loose avalanches in recent snow. They saw obvious wind transport of snow and cross loading of the slope they were on as they descended.
The CAIC documented very few avalanches breaking on deep layers in the four weeks before this accident. Forecasters and observers were reporting a relatively strong snowpack in areas of deep snow. This slope was not one of those areas. The snowpack on the upper portion of the slope was shallow. The slope was windward, resulting in snow erosion throughout the winter and a shallow snowpack with weaker basal snow layers than leeward slopes or areas with deeper snowpacks.
The riders assumed that the avalanche mitigation to protect the tunnel infrastructure decreased the avalanche hazard on the slope. Mitigation to protect infrastructure like highways is focused on reducing the potential of large natural avalanches. This differs from mitigation within ski areas, where the focus is reducing the potential for even small human-triggered avalanches. Backcountry travelers who are unaware of the differences often overestimate the hazard reduction from an infrastructure mitigation program.
Figure 11: A snow profile observed near the location where the avalanche stepped down to deeper weak layers, March 26, 2020.