Debris - flow hazard assessment in Andorra

(ENG) Debris flows represent a significant hazard in the Pyrenees, although they do not occur as frequently as in other mountain ranges. In the case of the Principality of Andorra, the rapid urban growth and the limited space in the valley floors have generated a high amount of elements at risk. In 2002, the government of Andorra started with hazard and risk assessments on the most urbanised debris-flow fans. The present contribution is part of this assessment project and was carried out at five different test sites. The hazard assessment included both a detailed geomorphologic-geologic analysis and numerical simulations. The results show that debris flows with a volume of several thousands of cubic meters have occurred at least once during the 20th century in two of the five studied sites. Smaller events up to 1000 m3 have occurred more frequently. The simulations indicate that future debris flows may cause important damages in one of the five sites studied, and that debris flows defined in the "worst-case scenario" could affect buildings and infrastructure in all of those sites.