The value of historical documents for hazard zone mapping

Abstract. The assessment of historical data of small mountain torrents in Alpine catchment areas has shown a significant difference between the results of a regular hazard zone mapping and a thorough historical analysis. The Gemsbach in the Ostrach valley near Hinterstein (Municipality of Hindelang, southern Bavaria/Germany) serves, among others, as an example. A "traditional" hazard zone for the Gemsbach has been mapped in the 1990s. The oldest event included in the analysis was the flood of 1954. But historical data collected and analysed by the HANG-project shows flood-prone areas around the Gemsbach differing greatly in size and location from the one shown in the hazard zone maps. The inclusion of all data available for the Gemsbach area (15 events between 1671 and 1960 collected from four different archives) leads to a completely new picture of the regional hazardous activity and demands a re-structuring of the traditional way of hazard zone mapping.