Passive defense of Avalanches is defined as "The anchoring or or modification of the snow in Avalanche starting zones to eliminate the release of a large Avalanche." The passive method toward Avalanche defense has found favor and sucess primarily in European countries. This is in part due to the long-term commitment of European communities to the mitigation of the Avalanche hazard.
Passive defense techniques can be divided into three main categories, these being "retention structures", "redistribution structures", and "retarding/ catchment structures." Following is a little bit about how each of these work.
1.0 Retention structures
Retention structures consist primarily of snow rakes, snow bridges, and nets. As the class description implies, the driving purpose of these structures is to basically keep the snow where it sits. These structures are effective, but certainly do not beautify the landscape. This is probably their biggest drawback. One other limitation is that of snow depth. For example, these structures are useless in the Pacific Northwest, where the extraordinary amounts of snowfall bury these structures and render them useless.
- A fine example of retention structures in the Swiss Alps.
2.0 Redistribution structures
This is just a fancy name for the good old snow fence. Used all around the world, with places ranging from the biggest ski resorts to my grandparents backyard, the snowfence is probably the simplest and one of the most effective structures out there. The snowfence reduces the effects of snow drifting, and is
effective in reducing the buildup of large, Avalanche causing cornices. Thus, the good old snowfence finds its place as an Avalanche mitigation structure.
3.0 Retarding/ Catchment structures
These structures differ from the other in that they they don't actually prevent the Avalanche, they simply prevent it from destroying anything. In fancy words, these structures are... "used to stop, divert, confine, or slow moving Avalanches." Such structures are ditches, terraces, dams, and mounds. Most of these structures are permanent, with them being either dug from or built of dirt, rock, or both. However, it should be noted that mounds, etc. can be built as temporary structures out of snow, and that they have had success with such structures in California.
4.0 Compaction
Don't read too much into this one folks, the method of Compaction is as simple as it gets. Compaction is simply the stomping down of the snow, which makes it (the snow) more stable. Often done by people, this method is labor intensive, but effective. In certain areas this can be done by mechanical equiptment, such as groomers. This technique is most effective early in the season.
