Dec 212013
 

Wet Slabs are a both a regular occurrence for us and a challenging avalanche problem.  After a very wet day on the mountain I went home looking forward to getting into some cotton.  While pulling into the driveway I glanced over to a pole barn I am building which isn’t quite done yet.  I saw snow had been creeping down breaking off the bottom in what came to be about 30cm sections of slab. (I have been a little anxious about it’s first winter although it’s pretty rugged.)  I got in close and watched carefully and noticed a slow creep of about 1mm a second and thought I’d shoot a little video.  Avalanching roofs are good example of what goes on in the field demonstrating the stress and strength relationship.  You likely saw a lot of crown lines these past few days on metal roofs around your neighborhood.  This was a particularly good set of factors to point out that regularly play out on Mount Washington.  Namely, rain and meltwater percolating and pooling to a buried ice lens that gets lubricated decreasing the strength at the interface between the above slab and the wet lens.  Although I was being cautious and thoughtful, ready with an escape plan, I thought the consequences were low. Due to cat like reflexes and baboon speed I avoided getting roofalanched.  Although I joke, this is a serious issue in many ski towns in snow country as well as the urban/suburban environment.  Primarily because of standing seam roofs offering no anchors become more the standard and shed snow very well.  Numerous individuals have been buried by roofalanches, some fatally.  Kids are particularly vulnerable so keep that in mind for you and your family, and even your car.  See data and raw uncut video below. Chris

 

Roofalanche

DATA:

  1. Roof pitch: 21 degrees
  2. Color of bed surface: ugly blue simulating blue water ice
  3. Bedsurface roughness: 6mm screw heads with an even spatial distribution grid at 60cm by 20cm. (used roofing although paint in good shape grit and grime add to anchoring.  Also reused old holes on ridges likely adding a bit to shear strength)
  4. Creep rate: 1mm per second
  5. Slab thickness 21 cm
  6. Precipitation: +/- 5mm of water in the preceding 24 hours at time of avalanche
  7. Temperature: +3.3C inducing additional freewater
  8. Wind: Nil
 Posted by at 9:06 am