Avalanche Hazard Reduction Using Wind Drift Disrupters (Snow Sails) Phase I & II: FHWA-WY-05/05F
As a consequence of the rapid urbanization of the greater Jackson, Wyo. area, the hazard to motorists and WYDOT highway maintenance personnel from an active avalanche path at milepost 151 on US 89/191 has also increased. US 89/191 is a principal arterial in and out of Jackson. The average daily traffic (ADT) during the winter period November through March is 7,950 vehicles per day.
Snow sails are a form of passive avalanche starting zone defense. A deployment of snow sails will disrupt the snowpack in an avalanche starting zone and inhibit the formation of coherent, continuous avalanche wind-slabs. A pilot phase deployment of 4 snow sails was initially installed in the 151 avalanche starting zone. The performance of these snow sails was evaluated in terms of their ability to disrupt the snowpack. These sails disrupted the snowpack depositional pattern sufficiently that a full deployment of 50 sails was subsequently installed.
A snow sail performance assessment and evaluation was conducted. This assessment and evaluation lead to the following results:
- The snow sails have reduced the 151 avalanche hazard, but have not eliminated it.
- The 151 site continues to avalanche to the road, at a rate of 1.0 per year, versus 1.5 per year prior to snow sail installation.
Those occasions when the 151 avalanches are when regional winter storm activity is severe, and the attendant avalanche hazard and activity is high. During these periods, the transportation system is under stress and WYDOT winter maintenance resources are at a premium. The 151 avalanche snow sail deployment is an attempt to develop a cost-effective avalanche hazard management solution for the 151 site. Cost estimates for snow supporting structures in the 151 avalanche starting zone or a snow shed at the roadway are $1.4M and $14.0M, respectively.
The net cost to fabricate, transport and install the operational deployment of 50 snow sails on the 151 avalanche and leave 10 sails in reserve was $89.9K. There were one-time development costs for design and technology trial of $82,701. The direct costs for annual snow sail removal, re-installation and maintenance are $8K to $10K.
At present annual rates of snow sail damage, including those that are damage beyond repair and the availability of 10 un-assembled sails in reserve – the snow sail “fleet” service life is estimated at 4 to 6 years before sail replacement costs should start to impact facility cost planning.The hazard to personnel during periods of helicopter sling transport of the snow sails and on-site activity is high and safety concerns remain paramount.
The following recommendations are made:
- When severe winter storms occur, the 151 site should be addressed with artificial (explosive) avalanche control measures.
- The snow sail project – an attempt to address the 151 avalanche hazard with a cost-effective solution has partially achieved its goals. However, with continue growth, investigations into novel and traditional avalanche hazard reduction technologies for implementation at the 151 site is warranted.
- The snow sails should continue to be installed and removed seasonally, pursuant to experience and their operating plan for the next 4 to 6 years of service life anticipated in the snow sail facility.