CHALLENGES IN THE MANAGEMENT OF ROCKFALL RISK: THE ANDORRAN EXPERIENCE

The Principality of Andorra is a small country located in the Pyrenees, affected by a variety of natural hazards. Since the late eighties of last century, the Andorran administration promoted several initiatives such as natural hazard maps (1:25,000 and 1:5,000 scales), the Urban and Land-Use Planning Law, and the Geotechnical and Landslide Hazard Zoning Plan. Based on them, private developments in the threatened areas must set up the necessary protective measures in order to obtain building permits and strategies for living with risk were developed.

One of the most important actions is the Rockfall Risk Management Master Plan (RFMP) of the Solà d’Andorra (Copons et al. 2004). It established for the first time the exclusion for development of the most threatened sector of the slope and is legally binding. At the time of approval, there were several buildings in the exclusion zone and the Plan included the construction of defence works. Their performance is satisfactory so far as several events were intercepted with only minor consequences. However, a residual risk exists as large events will not be fully retained. Because of this, the RFMP is complemented with annual helicopter flights aiming at the inventory of the events, the validation of the rockfall modelling and the detection of potential large slope failures. It is assumed that the latter will show premonitory signs.

The issue still to be solved is the assessment of the maximum credible event. Several researchers suggest that frequency-magnitude relations may be used to estimate the occurrence of large unrecorded events. The volume distribution of the rockfall events >1 m³ in the Solà d’Andorra during the last 50 years, fits well to a power law. Despite of this, we argue that the extrapolation of the relation far beyond the historical data in this case, is not appropriate. Neither geomorphological evidences nor the size of the potentially unstable rock masses identified in the slope, support the occurrence of the large rockfall volumes predicted by the power law and a cut-off value must exist.

Copons R, Vilaplana JM, Corominas J, Altimir J, Amigó J (2004) Rockfall risk management in high-density urban areas. The Andorran experience. In: T.Glade, M. Anderson and M.J. Crozier (eds). Landslide hazard and risk. John Wiley and Sons, Chichester. pp. 675-698.