Chapter 4: Avalanche Observations

Objectives

Recording avalanche occurrences have the following applications:

  • To be used with other observations in evaluating snow stability.
  • To identify areas where avalanches have released earlier in the winter; snow stability may vary between these sites and undisturbed sites.
  • To assess the effectiveness of control measures.
  • To correlate past weather and snow conditions with avalanche occurrences for weather and avalanche hazard forecasting.

All avalanches that are significant to an operation should be recorded. Noting the non-occurrence of avalanches is also important for snow stability evaluation.

Recording

A set of core observations is best recorded on the left-hand page of the field book, or on photographs. The right-hand page may be used for comments and additional observations.

Use a “~” in the notebook when no observation was made. Code as “U” if the observation was attempted but no reliable value could be ascertained. Do not leave blanks. Only write “0” when the reading is zero.

Procedure

Date. yyyy-mm-dd of the avalanche occurrence.

Time. 24-hour time using local time, followed by estimated age (in hours) of the avalanche. Note: +/-0 symbol is used when the time of occurrence is precisely known. E.g. 1700, 2.5 +/-0.

Area and Path. Enter the name of the operational area where the avalanche path is located, followed by the avalanche path identifier (name or number). Some road and railway operations may name their paths by the running kilometre, up to two decimal places.

Aspect. Specify the avalanche’s central aspect in the starting zone by a cardinal point.

Incline. Record the estimated incline of the starting zone. Add “M” if measured.

Size. Estimate the destructive potential of the avalanche.

  • Half sizes may be used for avalanches which are midway between defined classes.
  • “0” may be used to indicate no release of an avalanche following the application of control measures.

 Type of Snow Failure. Record the type of snow failure.

Record the slab harness if observed. Hardness can be measured using the hand hardness test in the starting zone or from the deposit in the runout zone, where the slab is still recognizable.

Liquid Water Content. Record the liquid water content in the starting zone and deposit of the avalanche snow. 

Terminus. Describe the tip of the avalanche deposit.

  • Short paths are where the starting zone, track, and runout zone cannot be easily separated.
  • Operations that have avalanche paths with well-defined features may apply additional codes.

Trigger. Record the cause of avalanche release with a basic letter code, and where possible, a modifier. Operations may devise other trigger subclasses that apply to their specific conditions.

A more detailed trigger classification system:

  • Avalanches that start when an aircraft flies overhead should be considered natural.
  • A remote event occurs at some distance >5m from the probable trigger. Specify the distance to the probable trigger in meters.

Comments. Enter information about damage and accidents caused by the avalanche and any other significant information. Describe weather phenomena suspected as the trigger (i.e. rising or falling temperature trend, solar effect, wind loading, wind gusts, precipitation intensity).

Additional Observations

For operations that control avalanches by explosives note:

  • Number of explosive charges applied and confirmed detonations
  • Size of charges (kg)
  • Target
  • Avalanche starting location
  • Sliding surface

When explosives were used but no avalanche resulted or charges misfired note:

  • Number of explosive charges applied and confirmed detonations
  • Size of charges (kg)
  • Target
  • Location of charges

For highway, railway, mine and forestry road operations note:

  • Length of road buried
  • Average and maximum depth of snow on the road
  • Toe distance mass: distance of the toe of the avalanche mass from the uphill edge (fog line) of the road or railway

For ski areas note:

  • Target
  • Avalanche starting location
  • Failure plane and bed surface (note the date of burial and predominant grain type where possible)
  • Width and thickness of slab avalanches at their crown

Avalanche Starting Location. Describe the location of the avalanche fracture with one of the following code letters, physical features or elevation, and, when applicable, add the key for the starting sub-zone or the target.

  • Avalanche Starting Location Incline. Record the incline of the steepest pitch in the avalanche starting zone (excluding cliffs) averaged over a fall-line distance of 20 m.

Bed Surface. Record the level of the bed surface in the snowpack.

Form and Age of Failure Plane. Record the predominant grain form observed in the failure plane.

Where possible, identify the weak layer by its probable date of burial. Note the occurrence of a shear fracture that steps down to other layers in Comments.

Slab Width. Estimate or measure width (m) of the slab between the flanks near the fracture line. Add “M” when width is actually measured. Observers may wish to use a hip chain to calibrate their estimates.

Slab Thickness. Estimate or measure in a vertical direction to nearest 10cm, the average thickness of the slab at the fracture line. Add “M” when thickness is actually measured.

Deposit on Road. Record in metres the length of road, railway line, ski run, power line or other facility buried in avalanche snow. Record average depth and maximum depth. Add “M” when length and depth are measured.

Toe Distance Mass. Measure or estimate the distance between the uphill edge or fog line of the road, or other development, and the farthest point reached by the mass of avalanche. Negative values are used when the deposited mass failed to reach the road or facility.

Some operations may also wish to document the occurrence of snow dust on the road. Dust results from the fallout of an avalanche’s powder cloud and may impact on driver visibility.

Total Deposit Dimensions. Record the average width and length of the deposited avalanche snow in metres.

Record the average deposit depth in metres and tenths of a metre. Add “M” after each value is measured by tape and probing.

Elevations. With reference to a contour map:

  • Fracture line or point of failure in starting zone
  • Deposit in runout zone

Length of Path. Some operations may wish to record the estimated distance an avalanche ran along a slope.

  • Up to a distance of 300m, estimate the length of path to the nearest 25m.
  • Beyond a distance of 300m, estimate the length of path to the nearest 100m.

Road / Line Status. Transportation operations should record the status (open or closed) of any roads or railway lines at the time when the avalanche occurred.

Multiple Avalanche Events

An operation may wish to group large numbers of similar avalanche events into one record or report, especially if that information is to be sent to a central information exchange. Grouping is achieved by allowing certain fields to hold a range of values (i.e. by specifying lower and upper bounds, separated by a dash). The report should be repeated for different types of activity (i.e. natural versus artificially released avalanches).

Significant avalanches (larger than size 3), and events involving an incident, damage, or injury, should not be described in this method. They must be described individually.

Avalanche Summary

Objectives. Avalanche summaries provide a clear and concise overview of avalanche conditions to assist stability and hazard analysis and forecasting for the operational region. The objective of such a summary is to organize and reduce data.

Avalanche summary parameters are different from avalanche observation parameters in that they are not recorded at a specific location and time but are a general characterization of the range of conditions encountered in a broader geographical area during the day. This not only includes average conditions but also potential anomalies and outliers.

Frequency. Summaries are generally done once a day, after the field day is complete.

Procedure. The following avalanche observations should be recorded:

  • Location and elevation range.
  • Percent of area observed.
  • Date of occurrence.
  • Time period.
  • Number of avalanches for each type, trigger, and occurrence date.
  • Typical size of the avalanches observed and the maximum size observed.
  • Triggers (described above).
  • Type of snow failures (described above).
  • Average incline of the average starting zone for each type, trigger, and occurrence date.
  • Average or range of starting zone elevations for each type, trigger, and occurrence date.
  • Average or range of slab thickness values for slab, loose snow + slab, cornice fall + slab, or ice fall + slab avalanches for each trigger, failure plane, and occurrence date.
  • Average or range of slab width values for slab, loose snow + slab, cornice fall + slab, or ice fall + slab avalanches for each trigger, failure plane, and occurrence date.
  • Average or range of length of path values for each type, trigger, failure plane, and occurrence date (described above).
  • Predominant grain form and level in the snowpack of the failure plane (described above).
  • Additional comments, which may include notes on whumpfs, fracture propagation, slab properties, and weak layer attributes.