Technical Note: Preliminary estimation of rockfall runout zones

M. Jaboyedoff,V. Labiouse

doi:10.5194/nhess-11-819-2011

Abstract

Abstract. Rockfall propagation areas can be determined using a simple geometric rule known as shadow angle or energy line method based on a simple Coulomb frictional model implemented in the CONEFALL computer program. Runout zones are estimated from a digital terrain model (DTM) and a grid file containing the cells representing rockfall potential source areas. The cells of the DTM that are lowest in altitude and located within a cone centered on a rockfall source cell belong to the potential propagation area associated with that grid cell. In addition, the CONEFALL method allows estimation of mean and maximum velocities and energies of blocks in the rockfall propagation areas. Previous studies indicate that the slope angle cone ranges from 27° to 37° depending on the assumptions made, i.e. slope morphology, probability of reaching a point, maximum run-out, field observations. Different solutions based on previous work and an example of an actual rockfall event are presented here.

Highlights

Rockfall hazard is a delicate task to assess because it is very difficult to predict the exact trajectory of any block of rock
The CONEFALL method described in this paper is based on a simple frictional model assuming that the rockfall propagation areas can be modelled by analogy with a block sliding along a slope (Heim, 1932)
The friction can be linked to an apparent friction angle equivalent to γ. This principle was modified and applied to rockfalls without volume dependency using a predefined angle of the line joining the source to the stop point of blocs. φp ranges from 22◦ to 37◦ depending on assumptions (Fig. 1a) and based on field evidence (Wieczoreck et al, 1999; Evans and Hungr, 1993; Toppe, 1987; Onofri and Candian, 1979). Such a model can be quickly applied to large areas, where a preliminary investigation of the potential rockfall propagation areas is needed starting from known source areas, or for very large rockfalls based on the Heim theory, which is based on the estimation of friction angle depending on volumes

Summary

Introduction

Predicting the rockfall runout distance and propagation areas, i.e. the areas potentially under the threat of rockfall, is still a challenge. The friction can be linked to an apparent friction angle equivalent to γ This principle was modified and applied to rockfalls without volume dependency using a predefined angle of the line joining the source to the stop point of blocs (φp). Φp ranges from 22◦ to 37◦ depending on assumptions (Fig. 1a) and based on field evidence (Wieczoreck et al, 1999; Evans and Hungr, 1993; Toppe, 1987; Onofri and Candian, 1979) Such a model can be quickly applied to large areas, where a preliminary investigation of the potential rockfall propagation areas is needed starting from known source areas, or for very large rockfalls based on the Heim theory, which is based on the estimation of friction angle depending on volumes. This method can be applied to prioritize detailed investigation based on a global risk analysis by crossing objects at risk and potential rockfall areas

Previous work on rockfall trajectory modelling

Theoretical background of CONEFALL

Cone method implementation

Applications

Findings

Discussion and conclusions