The new avalanche mitigation study, which looks at Behrends Avenue and White Subdivision avalanche paths, gave eight recommendations to the city — including home buy-out options and a second crossing to Douglas Island.
The City and Borough of Juneau received a Federal Emergency Management Agency grant to update its hazard’s mitigation plan, including avalanche mitigation. City Emergency Programs Manager Tom Mattice, also the avalanche forecaster for the city, explained the report for the Assembly Committee of the Whole on Monday.
The study was conducted by WSL Institute for Snow and Avalanche Research, a company in Davos, Switzerland. Mattice said the company was chosen because it is using new computer technology that shows impact pressure.
Mattice emphasized that the study was not a hazards map for the city, but instead a mitigation plan that evaluated past studies and included modeling and mapping.
Mattice showed an example of how the company looked at avalanche zones by playing a video clip. The video showed a computer animation of a swath of snow at the top of Mount Juneau represented in green. As the video played, it showed that snow pack become an avalanche and how it fell down the mountain. As it fell, the colors changed to how dense the snow was. This computer program for SLF is called Rapid Mass Movements, RAMMS. This system does not currently evaluate “powder snow” avalanches — very dry avalanches.
The company did those analyses for 10-year, 30-year, and 300-year avalanche scenarios. The consultant visited Juneau in April to evaluate the avalanche paths.
The study was also submitted with a note of caution.
“The city should know that while SLF can and does attempt to uphold the highest professional standards, the state of science and engineering knowledge is incomplete, and does not always permit certainty,” it states. “The complex phenomena involved in avalanches cannot be perfectly evaluated and predicted, and methods used to predict avalanche behavior change as new research becomes available.”
The study looked at the Hart report from 1967 and 1968, the La Chapelle report of 1968, the Geophysical Hazards Investigation by the city in 1972, and the Juneau Area Mass-Wasting and Snow Avalanche Analysis of 1992.
The new report describes the Behrends Avenue avalanche path as “very complex” with a mean inclination of 35-40 degrees — which is actually pretty steep, with some points as much as a 50 degree inclination.
“This steep part of the avalanche track favors the formation of powder snow avalanches,” it states.
Further in the Behrends zone, the consultant references an incident in Galtuer.
“...Galtuer has a mean inclination from the crown line to the destruction area of 29 degrees, in February 1999 an extreme avalanche in this path killed 31 people,” it states. “If the Behrends Avenue avalanche path would be situated at a higher elevation, for example the Swiss Alps, large avalanches would probably be frequently observed.”
Mattice said Juneau is fortunate because of its elevation and because of the rain/snow in winter, making the mountainside more stable.
SLF evaluated the avalanche path and found that the 10-year avalanche does not reach Behrends Avenue, the 30-year avalanche would stop at Egan Drive, and the 300-year avalanche extends into Gastineau Channel.
Mattice said SLF evaluated several options for mitigations including artificial release (it did not recommend further detonation because it could trigger a significantly larger and several avalanches), preventative road closures on Glacier Highway and Egan Drive when the risk of a powder avalanche is high (though it notes it would be quite time consuming), snow supporting structures and deflecting dams. The consultant did not recommend snow supporting structures at this time because of the quantity needed. Deflecting dams would not be effective in the case of powder avalanches. It also looked at direct protection of buildings, which essentially would built structures around the mountain side of a home — like a stone wall. Mattice said the structure would cost more than homes on the site. There are 28 residential houses in the severe hazard zone and 12 in the zone designated as “special engineering.”
The White subdivision also was similarly assessed. SLF found the concern of a secondary avalanche release when creating an artificial avalanche release is still present. Damage potential in this zone is five residential houses in the severe hazard zone and eight buildings in the “special engineering” zone.
“The exposed buildings are not protected, e.g. reinforced, and seem to be very vulnerable against avalanche impacts,” the study found.
The study recommends closure of Egan Drive and Glacier Highway and evacuations should the city conduct artificial releases, since this location would be easier to conduct those mitigations. It does not recommend artificial release without evacuation. It did find that the White Subdivision is better suited for an artificial release. SLF recommended further study to determine if some kind of snow supporting structures would be appropriate for this location, starting with snow stakes and a geotechnical evaluation because of the mudflows. It advised that avalanche dams in this area would be very difficult because of the steep slope, but catching dams may be feasible. It could also address the mass-wasting issue during the rest of the year.
SLF recommended implementing mitigations in this area before considering housing buyouts for this subdivision.
For more the full report, see bit.ly/zJhWhp.
• Contact reporter Sarah Day at 523-2279 or at firstname.lastname@example.org.