How do you measure avalanche risk?

Avalanche risk measurement criteria

Avalanche Risk Analysis Bulletin (BERA)

The BERA serves as a basis for avalanche risk assessment. Archives of these bulletins for France are available on the Météo France website, and similar bulletins also exist for Switzerland and certain Italian regions, such as Valle d'Aosta, Piedmont and Liguria. Each bulletin is specific to a mountain range.

This avalanche risk assessment quantifies the avalanche danger on a scale from 1 to 4, bearing in mind that level 5 mainly concerns threats to infrastructure and buildings, and is therefore not relevant for skiers. Level 0 means no snow at all. This scale is standardized for Europe and Switzerland.

BERA provides information on both spontaneous and skier-triggered avalanches. It also identifies the most dangerous slopes and provides a general analysis of the situation. In addition, it incorporates recent and forecast meteorological and snow conditions information, offering a perspective on the evolution of risk over the next two days.

Slope inclination

Assessing the steepness ofslopes is essential for backcountry skiers. Although some sources suggest that there is "little risk" on slopes of less than 30°, it is imperative to exercise caution:

• Don't assume that slopes of less than 30° are completely safe;
• it's crucial to take into account the slopes upstream, particularly in view of avalanches triggered at a distance, the presence of other skiers, or possible natural avalanches.

The angle ofinclination can be determined using tools such as the IGN geoportal or the Iphigénie application, where it is also possible to integrate the GPS track of the planned route. Directly on site, the inclination can be measured using a stick (an equilateral triangle represents a 30° inclination) or an inclinometer, a feature available on some phones.

Slope orientation

Slopeorientation plays a crucial role in mountain safety and avalanche risk assessment. In fact, 60% of avalanche incidents occur on north-facing slopes. In these areas, the snowpack tends to have more fragile layers of angular grains, the result of significant temperature variations within the mantle.

However, this data cannot be generalized to all scenarios. It is essential to consider the orientation of upstream slopes, and to consult the ARO to identify the riskiest orientations. Slope direction is often influenced by wind direction, especially after snowfalls, which can accumulate more snow on protected slopes. Avalanche bulletin information does not always cover all slopes and may change during the day.

Slope conditions

Thecondition of the slopes used is essential to consider. It's possible to distinguish between slopes where snow has accumulated naturally and those that have been retraced after each fall. Some slopes may also have been affected by recent flows or have been "cleaned", as at the entrance to a couloir.

However, assessing this condition can be complicated. Simple tracks on the snow are no guarantee of slope stability. A thorough knowledge of a route's history is required to establish a reliable assessment. This criterion is particularly relevant for routes that are frequently skied after every snowfall throughout the season, as is the case for the Vallée Blanche in Chamonix.

5 levels of avalanche risk

The level of avalanche risk is estimated according to 5 categories:

1. Low risk : overall, snow stability is good, offering optimal conditions for mountain activities. Instability is minimal, and is generally found in specific areas with steep slopes. Avalanches are rarely triggered, except under excessive loads.
2. Limited risk: stability is generally satisfactory, with localized areas of instability mentioned by their orientation and altitude. Avalanches are unlikely except under heavy loads, such as a group of skiers.
3. High risk: instability becomes more pronounced, affecting many areas. Avalanches can be triggered by slight overloads, such as the passage of a single skier. Spontaneous avalanches, if they occur at all, are generally of medium size.
4. High risk : most slopes are unstable. Many areas are capable of triggering an avalanche with a slight overload. Level 4 encompasses a variety of situations, some of which present a high risk of spontaneous avalanches, while others do not.
5. Very high risk: instability is extreme and omnipresent. Multiple avalanches, sometimes massive, are likely to be triggered, affecting even less steeply sloping areas. In exceptional situations, avalanches can cause severe damage to people and infrastructure. This category covers particularly dangerous situations, with the potential for considerable damage to property and people.