A rockfall is defined as the “detachment, fall, rolling, and bouncing of rock fragments. It may occur singly or in clusters, but there is little dynamic interaction between the most mobile moving fragments, which interact mainly with the substrate (path)".
The most widely used classification system of landslides is the one derived from the Varnes classification [1], which was slightly modified by [2] and more recently by [3]. Excluding slope deformation, five types of rock movements are described by [3]: rock fall; rock topple; rock slide; rock spread; and rock avalanche. A rock fall is defined as the “detachment, fall, rolling, and bouncing of rock fragments. It may occur singly or in clusters, but there is little dynamic interaction between the most mobile moving fragments, which interact mainly with the substrate (path). Fragment deformation is unimportant, although fragments can break during impacts”. A rock avalanche is defined as an “extremely rapid, massive, flow-like motion of fragmented rock from a large rock slide or rock fall”. As there is a continuous transition between rock falls and rock avalanches by a progressive increase of volume, both have been included in the scope of this article. However, for the sake of simplicity, we will use the term “rock fall” to name both. FigureFigure 1 1 shows an example of rockfall.
Ref. [4] divided flow-like movements (where particles interact with each other and travel as deforming mass) into dry granular flows and granular flows with special mobilization phenomena, which have usually a bigger size, but it is difficult to establish a fixed boundary. Their excessive mobility could be the result, for instance, of undrained loading.
Note that the two nouns of “rock fall” are separated according to the general principle of the Varnes classification (the first noun describes the material and the second describes the type of movement), but the term “rockfall” is often used by engineers [5]. Thus, we will use “rockfall” within the rest of this article. The term “fragmental rockfall” may be used to name a rockfall strico sensu.
An element at risk exposed to a diffuse hazard may be impacted periodically by a rockfall. That is why a diffuse hazard is usually assessed by a passage temporal frequency that is obtained from a rockfall inventory (or data base) covering a known period. This inventory may identify rockfall released in a given area or rock fragments deposited in an area of interest. As the frequency is strongly dependent on the volume, an inventory should include the volume of each event or fragment. When using an inventory of rockfalls released in a given area, a propagation analysis is needed to infer the passage frequency from the release frequency. Note that the concept of temporal frequency is not suitable to describe a localized hazard, because the release of a given rock compartment occurs once only.
The definition of risk given by [1222] is: “A measure of the probability and severity of an adverse effect to health, property or the environment. Risk is often estimated by the product of probability of a phenomenon of a given magnitude times the consequences. However, a more general interpretation of risk involves a comparison of the probability and consequences in a non-product form. For Quantitative Risk Assessment the use of the landslide intensity is recommended.” In addition to the product of probability times the consequences, the risk can be described by the annual probability of different levels of loss [1222][2223].
In the case of rockfalls, the risk is often analyzed considering different volumes and energies, which have different probabilities (or frequencies). A method for rockfall quantitative risk assessment is described by [2324].
Different types of risk can be considered [1222].
24]
Table 1. Example of calculation of human risk for a 195 m long trail section exposed to a diffuse rockfall hazard (modified from [2510]).
Volume Class (m3) |
Rockfall Release Frequency (Events/Year) |
Reach Probability |
Temporal Spatial Probability |
Vulnerability |
Annual Risk (Human Life) |
---|---|---|---|---|---|
V < 0.05 |
16.32 |
0.119 |
0.010 |
0.5 |
9.9 × 10−3 |
0.05 < V < 0.5 |
0.25 |
0.328 |
0.019 |
0.9 |
1.4 × 10−3 |
0.5 < V < 5 |
3.3 × 10−2 |
0.590 |
0.022 |
1.0 |
4.3 × 10−4 |
5 < V < 50 |
4.3 × 10−3 |
0.765 |
0.066 |
1.0 |
2.2 × 10−4 |
50 < V < 500 |
5.7 × 10−4 |
0.832 |
0.124 |
1.0 |
5.9 × 10−5 |
V > 500 |
8 × 10−5 |
0.874 |
0.153 |
1.0 |
1.0 × 10−5 |
Total risk |
0.012 |
Individual risk to life (or individual human risk): “The annual probability that a particular life will be lost”.
Societal risk to life (or societal human risk): “The risk of multiple fatalities or injuries in society as a whole”, which can be expressed as the annual number of deaths.
Non-human societal risk concerns “financial, environmental, and other losses”. The elements at risk can be “buildings and engineering works, economic activities, public services utilities, infrastructure and environmental features in the area potentially affected by landslides”.
An example of calculation of human risk for a 195 m long trail section exposed to a diffuse rockfall hazard is given in Table 1.
To learn more about quantitative rockfall hazard and risk analysis, see [