1

Work of an avalanche control service /

local hazard assessment

(example from Switzerland)

Lukas Stoffel, SLF Davos

WSL Institute for Snow and Avalanche Research SLF, Davos

Content

A) GENERAL REMARKS

B) GUIDELINES (GUIDA PRATICA)

C) LOCAL HAZARD ASSESSMENT

Switzerland: 17 fatalities in houses or on roads

Event analysis: deficiencies in organization, decision

making process, documentation of local avalanche

safety services (villages and roads).

Alps: Disastrous winter 1998-1999

A) GENERAL REMARKS After 1999: Effort to temporary

measures

Education: special courses SLF for avalanche

services (dt, fr, it)

Several reports and guidelines:

-Specifications («cahier des charges») 2001 *

-Guideline for avalanche services 2007 * (IFKIS-

EVAL 2009)

-Artificial release: insurance questions 2004

-Artificial release: secondary avalanches 2012

-Artificial release: comparison methods 2013

Iniatitive: SLF, Governement (BAFU), SILS

Avalanche history: analysis

Return period to hit a certain area

• Area often hit by avalanches (T<1-2J.)

• Area seldom hit by avalanches (T=5-20J.)

• Area very rarely hit by avalanches (T>25J.)

T=1J.

T=7J.

Certain amount

of new snow

necessary

IS IT POSSIBLE

TO FIND OUT

A CRITICAL

VALUE?

New snow

Snow drift

Wet snow

Available data (a.o. new snow)

Altitude Data source Parameters

Often approx.

2500 m

Automatic

weather station

(AWS):

Snow station

Wind station

New snow (sum): calculated value

Snow height

Temperature

Wind speed + direc.

e.g. 1500 m

(valley

bottem)

Observation field

(e.g. Snow pole)

New snow

Snow height

Temperature

2

Salezerlawine: Analysis Schweizer, Mitterer,

L. Stoffel (2008)

1950/51 – 2007/08: 12 Avalanches reaching the area

of the shed. Typical conditions:

• New snow sum 1-3 d Davos Dorf (1550 m): > 55 cm

= critical new snow sum + other factors:

• Snow height Davos Dorf: > 50 cm (before snow fall)

• Air temperature Δ24h (at 2500 m): +3°C

• Dry snow at valley bottem («entrainement possible»)

Other study: Analysis A. Stoffel,

Meister, Schweizer 1998

Analysis of avalanches from Zuoz (1982-1996):

large avalanches more likely for 3-day new

snow sum in Zuoz > approx. 50 cm

Salezerlawine: Analysis Schweizer, Mitterer,

L. Stoffel (2008)

BUT, many situations with approx. (1950/51–2007/08):

• New snow sum 1-3 d Davos Dorf (1550 m): > 55 cm

• Snow height Davos Dorf: > 50 cm (before snow fall)

And no avalanches or no avalanches on the shed

Forecast problem:

Many similar

situations exist

where avalanches

can not be excluded

→ Critical new snow

sum gives an idea,

additional consi-

deration necessary

Local hazard assessment for

persons/objects: single slope, road section

During a current new snow situation: Is the road endangered?

• „Not possible to predict exactly the time of a spontaneous release

and the avalanche size“

• BUT: the scope is to determine times with an elevated probability for

avalanches hitting the road

Combination of factors Remarks

Avalanche bulletin a.o. avalanche danger level

Data autom. weather station,

observation field (snow/wind/temp)

Critical new snow sum (ADAPTATION ON

CURRENT SITUATION) + snow height

Observations Occured avalanches during the snow fall period;

Snow fall limit, snow line

Inspection helicopter flight or snow

cover investigations

Weather forecast incl. new snow, wind, temperature

Avalanche history ev. NXD / Nivolog

In critical avalanche paths,

considerations

- Situation in release zone? (Release height,

release area?)

- Situation in track and run out? (Roughness? /

avalanches already occured? / Entrainement?)

- Avalanche type? (Powder snow, wet snow etc)

- Perm. Protection measures effective? (Dam)

- Possible avalanche size? (Hazard map)

→ Road endangered: Yes or no

Local hazard assessment: aval. size

(data and additional considerations)

Hazard assessment: Data from automatic

weather station

3

Local hazard assessment: high

release probability B) GUIDELINES (dt, fra, ita)

The guidelines shall help local

avalanche services to structure

their work.

Content:

1) Principles

2) Organization

3) Working basis

4) Scheme for hazard assessment

5) Documentation

1. Principles

A local avalanche service has to:

• Suggest measures corresponding to the local hazard (estimated danger level can be different from danger level of public bulletin)

• Document its evaluation and justify preventive measures

3. Working base = Safety concept

3.1 Avalanche paths (endangered houses /

roads / ski tracks etc.)

3.2 Evaluation of the avalanche situation

Snow data, observations (incl. Avalanche

activity), weather forecast, avalanche

bulletin

3.3 Closurse concept (e.g. Road closures)

3.4 Artificial avalanche release

3.1 Avalanche paths

Avalanche map:

• Starting zones

• Flow direction

• Protection measures

(e.g. a dam)

• E.g. Ski track

3.1 Avalanche paths

Inclination map

or

table with terrain

characteristics

(inclination,

aspect,

elevation)

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3.1 Avalanche paths

Avalanche history, e.g.

Extent of (big)

avalanches, date

Existing permanent

protection measures

Avalanche hazard map

3.1 Avalanche paths: scope

Good knowledge of area to evalutate.

Areas with different hazards:

a) Most critical avalanche paths (aspect, inclination, altitude)

b) Areas seldom hit by avalanches

c) Areas very rarely hit by avalanches

• Endangered objects

• Existing permanent protection measures

3.2 Evaluation avalanche situation

Data, observations etc Parameters

Automatic snow and

wind station (often

approx. 2500 m)

Snow height, new snow (modelled),

temperatures

Wind (direction, strength) at ridges

Observation snow

field

(often 1000-1600 m)

Snow height, new snow, temperatures

Observations Snow drift, avalanche activity (e.g. Results

of artificial avalanche release), snow poles

Information from

other avalanche

services

e.g. Avalanche activity

Weather forecast Weather, snow, wind, air temperature

Avalanche bulletin Avalanche danger level, ...

Recommentation to use a form, e.g. IFKIS-EVAL

Use of which parameters?

IFKIS – EVAL (pagina 1+2)

3 day sum

All data automatically

Davos

3.3 Closure plans A-D (for residental areas)

Scenarios:

A: major snow fall

B + C: extraordinary snow fall

D: extreme situation

3.4 Artificial avalanche release

In case of artificial release:

Avalanche path, Detonation points, method,

closures, flight route, etc.

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C) SCHEME FOR HAZARD ASSESSMENT

Step 1: Data analysis and danger evaluation

How is the avalanche situation today?

Persons (Objects) endangered: yes / no

Step 2: Assessing the hazard

to people and infrastructure

Where are the problems?

Avalanche with damage: possible or unlikely

Measures: yes / no

Step 3: Preventive measures

What needs to be done? When?

Foto

Step 1: Is there an avalanche problem today?

e.g. medium snowfall: problem only in ski area

(pink arrows)

scale: Region of Davos

IFKIS – EVAL (pagina 1-3)

People or infrastructure

endangered?

a) If no time of next

evaluation

b) If yes Step 2 (or eva-

luation for explosive control)

Estimation of local avalanche

danger and its trend

p. 1+2:

Snow data

Observations

Weather forecast

p. 3

Davos

Davos:

Dorfbach, Dorfberg, Saleza

Endangered area

for extreme run-out

Step 2: e.g. large

snowfall: local focus,

where are the problems

(possible avalanche

size)?

? ? ?

Step 2: Hazard to people, infrastructure

General remarks:

Road section (ski track), several avalanche

paths:

- Situation in most critical avalanche path is

decisive

Residental areas:

- Most critical avalanche path(s), measures

yes or no

- Avalanche situation is getting worse:

measures in additional avalanche paths yes

or no

Combination of factors Remarks

Avalanche bulletin a.o. avalanche danger level

Data autom. weather station,

observation field (snow/wind/temp)

Critical new snow sum (ADAPTATION ON

CURRENT SITUATION) + snow height

Observations Occured avalanches during the snow fall period;

Snow fall line, snow line

Inspection helicopter flight or snow

cover investigations

Weather forecast incl. new snow, wind, temperature

Avalanche history ev. NXD / Nivolog

In critical avalanche paths,

considerations

- Situation release zone? (Release height, area?)

- Situation in track and run out? (Roughness? /

avalanches already occured? / Entrainement?)

- Avalanche type? (Powder snow, wet snow etc)

- Perm. Protection measures effective? (Dam)

- Possible avalanche size? (Hazard map)

- Release probability?

→ Road endangered: Yes or no («Scenarios»)

Local hazard assessment: aval. size

(data and additional considerations)

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Step 3: Preventive measure: location + time

e.g. Choice of closure plan B Dorfbach

Step 3: Measures

Documentation:

Hazard situation +

measures (e.g.

IFKIS-EVAL)

Record of use of

explosives

Avalanche activity

Summary

Work of avalanche services:

- To follow a snow fall period (data analysis,

observations, forecast etc): evaluation of

actual situation and forecast (a.o. new snow

amount in which time)

- Guidelines for avalanche control services

exists download www.slf.ch. For practical

use a Web-based documentation tool exists

(IFKIS-EVAL)

- Temporary measures (e.g. closures) still

contain considerable uncertainty