Avalanche Probability Maps

[randoskier]

There are a lot of slope angle maps in use now. Norway has had a great GIS system for avalanche (snoskred) zones on-line for many, many years now (NGI Snoskred Kart), they have now integrated them with the topo planning maps at UT,no, They are extra layers you can activate ending the necessity of switching back and forth between maps to check your route. They show slope angle, declenchment zones and run-out zones.

On planning our cabin to cabin trip through the Snaasa mountains we came across a potential problem as illustrated in these maps.

We planned to ski out the last few days of our trip from the abandoned historic mountain farm (fjellgard) of Gressamoen spending the night at the Kleivstugga cabin (Statskog). Cabin indicated by red circle. We wanted to ski up the unplowed dirt road,which will not be free of snow until 1 June normally (later this year), and reach this cabin travelling westward from the farm in the the east, more or less along this road.

As you can see there is a severe terrain-trap just before reaching the cabin with declenment zones on both sides of the road/river where the road from the east swings north towards the cabin (by the bridge). Right now the snow in these mountains is the most they have received since 1976, and it is still snowing a few days a week. The recent snows were accompanied by high wind. RED FLAG.

We looked at another option- a southern approach to the Klievstugga from another cabin, skipping Gressamoen. As you can see from the map that is not so simple and would require skiing in that steep gully on the map- that is a no go with a pulk and I would not even try skiing it with a backpack either as gullies like that can have all sorts of physical traps like exposed boulders, downed trees, drops that fall between the contour lines, and are not visible on topo maps, etc.

All seemed a bit desperate so we are taking a different way out and giving this whole valley a miss- saving it for a future trip coming from the north instead of the south or east. Looks very beautiful.

Flying Venice-Norway > sleeper train to Snaasa > ski! Post a trip report after- we never do social media during or even use any email during backcountry trips, this is about zero cell coverage in the mountains there which is as it should be.

[CwmRaider]

My understanding is that these are "hazard maps" based on topographic functions (slope continuity, outrun) from digital elevation models, not where avalanches have actually been observed. The situation on the ground will be what tells you what is OK to ski. In heavily forested areas for example the hazard may be much lower.
I say this without a lot of knowledge about avalanches, but quite a good knowledge of maps

[randoskier]

My understanding is that these are "hazard maps" based on topographic functions (slope continuity, outrun) from digital elevation models, not where avalanches have actually been observed. The situation on the ground will be what tells you what is OK to ski. In heavily forested areas for example the hazard may be much lower.
I say this without a lot of knowledge about avalanches, but quite a good knowledge of maps

I would never ski that road before that cabin if there was any significant snow on it, that is a classic terrain trap. How are you assessing the situation on the ground?

Snow pits only go so far, they tell you what the snow is like where you dug the pit, and you can create some theories off of that but that is far from conclusive. .

I think there is a lot of luck involved in Norway (except for some AT skiers who are up on ava prevention/avoidance)- I asked the DNT chief in Narvik about skiing from the end of the Tysfjord (the north arm of the fjord- I forget its name) to the Roysvatn hytta. It is a steep up from sea-level at the fjord. They said "no problem very stable, ski it on tours several times a winter". . We wound up getting a ride up (Yes!) it because they were building a misadvised wind-farm on top (ruining a pristine landscape/Sami reindeer area). The construction worker driving the giant dump-truck told us two weeks before a massive avalanche had swept down it at night- he said luckily nobody was on it their trucks and heavy equipment would have been in the fjord. We saw the remnants of it, big cone, it was a huge slab avalanche from the very top of the slope, it broke off about 200 meters above the road and wet all the way to the fjord just missing their container-city by the dock.

These slope maps give you a good idea of what can slide and how far it will go with historic snowfall levels. There is no way to predict an avalanche, but you can predict where they are likely to happen and under what likely conditions. With 2 meters of wind deposited snow in the Snaasa mountains I am not taking any chance.

[CwmRaider]

This map does not take into account tree cover or historic snowfall, it is just a slope gradient map with emphasis on the 30 degrees and up slopes. The colors are red there because its a ravine and it's steep on the sides. Heck, even the city centres of Bergen, Oslo and Trondheim are mapped with red zones where its steep enough, but they aren't "no go zones" you have to avoid. This is what I mean by "situation on the ground". I have a difficulty picturing your ravine and how the wind accumulated the snow against those cliffs, which is ultimately what would define it as dangerous or not.

Just to be clear, I use this map as well on fjellski tours and avoid anything steeper than 30 degrees above the tree line. I had an avy course but otherwise have no further expertise in analysing snow layers, so I don't take risks either.

So, by all means be extremely careful, but the point is this is a terrain map with a slope function, not an avalanche probability map. It just tells you it's steep there. Nothing more, nothing less.

And yeah, windmill parks suck for the local environment.

[CwmRaider]

PS have a great trip and treat us to some pictures when you're done

[lowangle al]

The construction worker driving the giant dump-truck told us two weeks before a massive avalanche had swept down it at night- he said luckily nobody was on it their trucks and heavy equipment would have been in the fjord.

The driver probably wasn't exaggerating. Down the road from me a slide pushed a Cat D9 dozer a couple hundred yards into Turnagain Arm, and that was after the slope had been blasted and slid once.

[randoskier]

The construction worker driving the giant dump-truck told us two weeks before a massive avalanche had swept down it at night- he said luckily nobody was on it their trucks and heavy equipment would have been in the fjord.

The driver probably wasn't exaggerating. Down the road from me a slide pushed a Cat D9 dozer a couple hundred yards into Turnagain Arm, and that was after the slope had been blasted and slid once.

Wow! Was that in Alaska? You must get a lot of rippers up there.

@roelant Forests can offer a false sense of security- in the Alps this year they already killed two people in a forest, You can also frequently see their paths through trees they can snap 'em like match sticks. There was an entire forested village in CZ wiped out many years ago.

If you want to see how unpredictable they are have a look at the link I put on the other avalanche thread of the Nat Geo program Alpine Tsunami from the seconds to disaster series- that hit a several hundred year-old village right in the center, right in the middle of the day, in the middle of high ski season- nothing had even been close to the village- it killed the priest in the church right in the center of the village, knocked the old woman ironing his robes under the sink of the laundry room where she had an air-bubble and lived- it killed 31 people. It took weeks to dig out and a long time to reach the village. Very interesting program that studies the cause. There is a good museum dedicated to this tragic event in Galtur, well worth a visit.

[randoskier]

PS have a great trip and treat us to some pictures when you're done

Tusend Takk!

[randoskier]

https://www.avalanche.ca/glossary/terms/terrain-trap

[sastrugi]

My understanding is that these are "hazard maps" based on topographic functions (slope continuity, outrun) from digital elevation models, not where avalanches have actually been observed. The situation on the ground will be what tells you what is OK to ski. In heavily forested areas for example the hazard may be much lower.
I say this without a lot of knowledge about avalanches, but quite a good knowledge of maps

You are correct, this map is solely based on slope angle, and the blue runout zones are calculated by computer based on the potential release areas determined by the topography.

The different shades of blue are based on statistics. The darkest blue shade is for short runouts, where apparently 50% of avalanches stop (32 degree alpha angle). The middle shade of blue is set where 25% of avalanches would slide further (27 degree alpha angle), and the lightest shade of blue is where only 5% of avalanches would slide further (23 degree alpha angle). This is based on a paper from 1980, as well as a study of 18000 avalanche observations in Norway, and 19000 avalanche observations in the Alps. This paper describes the methodology in more detail.