Avalanche Zoning in France –Regulation and Technical Bases

Abstract

AbstractAvalanche zoning in France comprises two main parts:1. A map of the probable location of avalanches was produced between 1970 and 1976, at a scale of 1 : 20000, and covering nearly 600000 ha. This was prepared by using a novel method based on:(i)the photo-interpretation of air photographs: the relief, extent of vegetation, notably forests the structure of deposits and alluvia, the comparison of several areas, thus enabling the zones …. which appear to have been struck by avalanches in the past to be identified;(ii)information acquired from inhabitants and the study of archives;(iii)a detailed inspection of the ground which can reveal physical or anthropogenic signs not disclosed by photo-interpretation.This map is only concerned with the past and gives no indication of frequencies.2. French law places an obligation on the State to define the limits of the zone subject to natural risk betore a building permit can be obtained.Consequently, since 1974 when France ventured into zoning, plans have been made on a large scale (1 : 2000) showing the zones exposed to avalanches. Plans are made in three colours: white presumed to be without danger, red dangerous and therefore prohibited to constructors, and blue zones, doubtful.Blue zoning, the most difficult to establish, relics on the evaluation of specialists, supported by the classical calculation of Voellmy, as well as by some first results received from experiments in a water channel for powder-snow avalanches, or from numerical simulations for other avalanches.The blue zone includes heavy avalanches with a time of return greater than 300 years and slightly more frequent avalanches (return time 30-50 years) but of very little importance.According to circumstances in every blue zone, several types of measure are recommended: police measures (evacuation or regulation in the home), architectural measures of reinforcement protective structure construction or annual checking by the authorities that natural (forest) or artificial (structures) defences are in a good state.