# Glossary

Note

This is still under construction

Note

This glossary has the main purpose to introduce and consolidate an unambiguous language and definitions with respect to avalanche dynamics, avalanche simulations and modelling for the digital toolbox AvaFrame and particularly the Avalanche Modelling Atlas (AMA). Terms are organized in alphabetical order and the terminology builds on the UNESCO Avalanche atlas and relates to existing guidelines provided and used by Avalanche Associations and Warning Services (Canadian Avalanche Association [CAA16] , American Avalanche Association (AAA) or the European Avalanche Warning Services (EAWS)). The glossary has the goal to follow common conventions used in publications and related projects.

alpha point
alpha angle

The alpha point is the point of the furthest reach of an avalanche (visible deposition or affected area). The alpha angle is the angle between the line connecting the release point and the alpha point and the horizontally projected travel length along the thalweg (see travel length and runout angle). The alpha angle is also termed the angle of reach or the runout angle.

beta point
beta angle

The boundary between the transition and deposition zone (start of deposition) is often called beta point and associated to a certain slope threshold of the thalweg. The corresponding travel angle is referred to as beta angle. For major avalanche path (s) that may produce avalanches of size 4-5, the beta point is associated to the 10° Point (or beta_10), i.e. the point where the slope angle of the thalweg decreases below 10°. For avalanche path (s) with event (s) of avalanche size 1-3 the beta point may accordingly be associated to larger thalweg slope angles (i.e. up to 30°, see start of transition).

coordinate transformation

Coordinate transformations refer to the operation of changing coordinates, e.g. between a fixed, Eularian, global coordinate system with east and north orientation, to an avalanche path dependent coordinate system along the thalweg, or even a Langrangian coordinate system, moving along particle trajectories.

cycle

An avalanche cycle describes a series of event (s) that occur across a region over a relatively short time span.

danger scale

The avalanche danger scale refers to the avalanche hazard and is an inherrent part of avalanche warning.

dense flow

Dense flow is a form of movement in the transition zone of the avalanche. Dense flow avalanches (DFA) are flowing along the ground and are rather associated to warm flow. Mixed types of movement are often observed, combining different flow regimes and their partial or complete transitions, e.g. ‘mixed flow and powder avalanches’ or ‘flow avalanche with powder component’, towards the evolution of a fluidized layer in the avalanche flow.

density

Release, entrainment, flow, or deposition density. Important quantity relating mass and volume, influencing impact pressure and particular friction relations.

deposition
depth

Release, entrainment, flow, or deposition depth refers to the extent of the avalanche measured in the direction of gravity.

entrainment

Entrainment describes the process of mass intake during the avalanche flow.

event

see scenario

flow variables

Flow variables include thickness, velocity, or density and are determined by the form of movement. Flow models that are implemented usually calculate the spatio-temporal evolution of these variables and where the maximum over the whole flow or computational duration, I.e., their peak values are the most used results. The flow variables are used to derive other variables such as impact pressure or kinetic energy of the flow.

form of movement

Is an avalanche criterion in the zone of transition and has powder snow or dense flow as characteristics.

manner of starting

Is an avalanche criterion in the zone of origin and has the possible characteristics loose, slab, or gliding.

origin
path

The avalanche path summarizes the total catchment and is divided into different zones (zone of origin, transition, 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 avalanches (PSA) refer to the form of movement in the zone of transition, referring to the dust or suspension cloud in avalanches. PSA are associated with cold, dry cohesionless snow. Mixed types of movement are often observed, combining different flow regimes and their transitions, e.g., ‘powder avalanche with flow component’.

projection

The projection refers to the projection of coordinates within a coordinate system, I.e. the projection of the runout point (as furthest reach of the avalalanche) to the thalweg. Or projecting a 3D travel length (xyz) to a 2D travel length measured only along xy.

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.

return period

Return periods are related to return levels describing the size or magnitude of design or recorded event (s) on a respective avalanche path. The return level is often determined by the run out length of historically documented avalanche event (s) accompanied with return period estimates, which are associated to the occurrence probability.

runout area
runout angle
runout length
runout point

Runout lengths and angles are intricately linked to the alpha point, utilizing the projection to the thalweg. In the same manner as travel length, run out lengths are measured as horizontally projected lengths along the thalweg, from the uppermost point of the release area to furthest reach of the runout area. 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 and is usually defined via flow thickness, velocity, kinetic energy or impact pressure thresholds.

scenario

One or multiple avalanche events or corresponding simulation scenarios are associated to a certain avalanche path and have distinct criteria and characteristics such as avalanche size, release area (s), or runout area. These properties are morphologically connected to the different zones (origin, transition, deposition) of an avalanche path and allow to define other associated properties, such as alpha angle or runout length that are defined in combination with the avalanche thalweg. Besides observed and documented avalanche events, design events of particular return period (s) are of particular interest for engineering applications.

size

Avalanche size refers to the magnitude or intensity of an event, classified by destructive potential, runout length and dimension according to the EAWS size classification, which is closely related to the CAA destructive 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 deposition zone of infrequent avalanche path (s)), challenging (well defined avalanche path (s), starting zones, or terrain traps), and complex (exposure to multiple overlapping avalanche path (s), large expanses of steep, open terrain, multiple starting zones, and terrain traps below).

thalweg

The thalweg is defined by the main flow direction of an avalanche path of one or multiple, i.e. not regarding a specific event or scenario. avalanche event (s). Technically it is the two-dimensional terrain representation, displaying the terrain altitude along the horizontally projected travel length.

thickness

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

trajectory length

Used in com1DFA particle dictionaries, where the trajectory length is computed as the distance traveled by a particle from one time step to the next and then accumulated over time. Three different trajectory lengths are computed (1) trajectoryLengthXY - computed in the x, y plane, (2) trajectoryLengthXYZ - also taking the slope of the topography into account and (3) trajectoryLengthXYCor - same as trajectoryLengthXY but corrected for the potential angle difference of the slope and the normal.

transition
travel angle
travel length

Travel lengths are measured as horizontally projected travel length ($$s_{XY}$$) along the thalweg and are associated with the corresponding travel angle, measured between the line connecting the current location with the uppermost point of the release and the horizontal plane. Alternatively, the surface parallel travel length ($$s_{XYZ}$$) may be defined as the three-dimensional length travelled by the avalanche.

velocity

Flow velocities are usually measured in a surface parallel direction. Alternatively approach velocities are measured along the line of sight.

wet snow

A wet snow avalanche (WSA) implies the presence of liquid water within the avalanche and is usually associated to dense flow type of movement in the transition zone of the avalanche.

zone of deposition

The zone of deposition is where the runout area of the avalanche is located and where the avalanche stops due to frictional energy dissipation. The boundary with the transition zone (start of deposition) is often called beta point.

zone of origin

The zone of origin delineates the area, in which typical release area (s) are located, and an avalanche’s appearance is characterized by the manner of starting. The uppermost possible point is referred to as start of origin.

zone of transition

The zone of transition is the area between zone of origin and zone of deposition along the thalweg. The form of movement is linked to the flow variables. The start of transition links the zone of origin and transition and is usually associated with a slope inclination of about 28-30°.