Glossary

Note

This is still under construction

Note

The main purpose of this glossary is to introduce and consolidate an unambiguous language and definitions related to avalanche dynamics, avalanche simulations, and modeling for the digital toolbox AvaFrame and in particular the Avalanche Modelling Atlas (AMA). Terms are organized in alphabetical order, and the terminology builds on the UNESCO Avalanche Atlas [DeQuervain+81]. It also relates to existing guidelines provided by avalanche associations and warning services (e.g. the European Avalanche Warning Services (EAWS) [EAW24], the Canadian Avalanche Association (CAA) [CAA16], and the American Avalanche Association (AAA) [AmericanAAssociation16]). The glossary follows common conventions used in related publications and projects (see our References).

alpha angle

The alpha angle is the angle between the origin point and the runout point. It describes the inclination of the line connecting the top of the avalanche release and the furthest point of the deposition. The alpha angle is based on the runout length and the altitude difference between the origin point and the runout point along the thalweg. It is an important element in avalanche simulation and modeling (e.g. alpha-beta model [LB80], FlowPy simulations). The alpha angle is also referred to as the ‘angle of reach’ or the runout angle.

see also zone of origin

path

The avalanche path summarizes the total catchment and can be divided into different zones (zone of origin, zone of transition, zone of deposition) with different criteria and characteristics. An inherent property of the avalanche path is the thalweg and the associated avalanche event.

powder snow

Powder snow is a form of movement, referring to the dust or suspension cloud of a powder snow avalanche (PSA). PSAs are associated with cold, dry cohesionless snow. In reality, mixed types of movement are often observed, combining different flow regimes and their transitions, e.g. ‘powder avalanche with dense flow component’.

projection

Projection refers to the transition of a defined set of coordinates from one coordinate system to another, i.e. the projection of the runout point (as furthest reach of the avalanche) to the thalweg. Or projecting a 3D travel length (xyz) to a 2D travel length measured only along xy.

relative size

The relative size refers to the size of an avalanche in relation to the thalweg. The classification scheme was developed by the AAA ([AmericanAAssociation16]) and relies on the horizontal extent, vertical depth of the fracture, volume and mass of the deposited snow as well as the runout length of the avalanche event. The largest possible event along a thalweg is classified with R5.

release area

Potential release areas are located in the zone of origin. Each documented event or simulation scenario is associated to one or more primary and/or secondary release areas, that can further be described by the manner of starting.

release scenario

One or more release areas, which are associated with a certain avalanche path and release a the same time.

return level

The return level is the magnitude or size of an event that is expected to be exceeded with a certain probability over a specified time period. It is a magnitude-based measure (the size of the event). Return levels are used for designing infrastructure (like dams) to withstand specific events.

return period

The return period is the average time interval between events of a certain intensity or size. The return period is often determined by the runout length of historically documented avalanche event (s). It is a time-based measure (how often an event occurs). Return periods are often used for planning and risk management.

runout angle

The runout angle, also referred to as alpha angle. It describes the inclination of the the avalanche event from the point of release or origin point to the runout point.

runout area

The runout area is associated to a specific avalanche event and usually located in the zone of deposition.

runout length

Runout length is intrinsically linked to the alpha point, utilizing the projection to the thalweg. In the same manner as travel length, runout lengths are measured as horizontally projected lengths along the thalweg, from the uppermost point of the release area (origin point) to furthest reach of the runout area (runout point).

runout point

The runout point is also referred to as the alpha point. It describes the furthest point of the runout, and marks the outer most end of the avalanche deposit. The runout may refer to visible deposition (associated to dense flow), damages or the impacted and affected area (associated to air blast or powder snow) in the zone of deposition. It is usually defined via flow thickness, velocity, kinetic energy or impact pressure thresholds.

size

Two prevalent avalanche size classifications exist.

see destructive size and relative size

terrain classification

Terrain may be classified according to the Avalanche Terrain Exposure Scale (ATES) into ‘simple’ (low angle or primarily forested terrain with some openings that may involve the zone of deposition of infrequent avalanche path (s)), ‘challenging’ (well defined avalanche path (s), starting zones, or terrain traps), ‘complex’ (exposure to multiple overlapping avalanche path (s), large expanses of steep, open terrain, multiple starting zones, and terrain traps below) and ‘extreme’ (exposure to very steep faces with cliffs, spines, couloirs, crevasses or sustained overhead hazard).

thalweg

The thalweg is defined as the line representing the main flow direction of all potential avalanche events within a specific avalanche path. The thalweg is delineated according to the terrain characteristics and is independent of a specific event. Technically it is the two- dimensional terrain representation along the horizontally projected travel length and the altitude difference.

thickness

Release, entrainment, flow, or deposition thickness refers to the extent of the avalanche, measured perpendicular to the slope.