Rockfall travel distance analysis by using empirical models (Solà d'Andorra la Vella, Central Pyrenees)

Abstract

Abstract. The prediction of rockfall travel distance below a rock cliff is an indispensable activity in rockfall susceptibility, hazard and risk assessment. Although the size of the detached rock mass may differ considerably at each specific rock cliff, small rockfall (<100 m 3 ) is the most frequent process. Empirical models may provide us with suitable information for predicting the travel distance of small rockfalls over an extensive area at a medium scale (1:100 000-1:25 000). "Solà d'Andorra la Vella" is a rocky slope located close to the town of Andorra la Vella, where the government has been documenting rockfalls since 1999. This documentation consists in mapping the release point and the individual fallen blocks immediately after the event. The documentation of historical rockfalls by morphological analysis, eye-witness accounts and historical images serve to increase available information. In total, data from twenty small rockfalls have been gathered which reveal an amount of a hundred individual fallen rock blocks. The data acquired has been used to check the reliability of the main empirical models widely adopted (reach and shadow angle models) and to analyse the influence of parameters which affecting the travel distance (rockfall size, height of fall along the rock cliff and volume of the individual fallen rock block). For predicting travel distances in maps with medium scales, a method has been proposed based on the "reach probability" concept. The accuracy of results has been tested from the line entailing the farthest fallen boulders which represents the maximum travel distance of past rockfalls. The paper concludes with a discussion of the application of both empirical models to other study areas.

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“…3). Just like in the case of the reach angle approach, this model demonstrates that the falling block will stop within the intersection of the energy line with the local topography (Copons et al 2009). In the last few decades several papers have been based on this approach (e.g.…”

“…In the last few decades several papers have been based on this approach (e.g. Copons et al 2009;Jaboyedoff, Labiouse 2011;Blahůt et al 2013) and the typical value of the shadow angle is around 27° (Copons et al 2009). …”

“…The need to define the maximum run-out zones and hence to delimit potentially endangered localities led researchers to study the maximum extent to which the boulders occur at footslopes. Copons et al (2009) noticed four different approaches to the problem, based on (i) geomorphic mapping to study the most distant boulders, and on forecasting maximum runout zones using (ii) heuristic, (iii) numerical and (iv) empirical models. These latter approaches have gained particular popularity and are applied in many studies.…”

“…Heim 1932;Corominas 1996) and the shadow angle model (e.g. Evans, Hungr 1993;Copons et al 2009) (Fig. 3).…”

“…The reach angle model, also known as Fahrböschung, travel angle or travel distance angle is the arctangent of the line which connects the rockfall source area with the most distant boulder (Copons et al 2009). The model assumes that the falling block will stop at the point of intersection of the above mentioned line with the slope topography (Fig.…”