Towards a frequency-magnitude relationship for torrent events in Austria

Abstract

Hazard assessment and the design of mitigation measures against mountain hazards are usually based on statistically derived magnitude-frequency relationships of process parameters such as discharge,flow velocity, or the volume of debris deposits. However, with respect to debris flows there is a particular lack of such data, as these processes are rare phenomena, the systematic measurements of relevant parameters have only been carried out in selected watersheds within the last decades. In some areas, geomorphic and stratigraphic assessments and dendrochronology studies have been used for estimating magnitudes and frequencies of debris flow events. However, there is still an information gap for quantitative debris flow hazard assessment based on recurrence intervals and associated magnitudes. Our study aims to close this gap by an analysis of an Austrian database of historic events. Information on a local and regional scale has been gathered from records of the Austrian Torrent and Avalanche Control Service and the transcription of the so-called “Brixner Chronicle” (stRele, 1893). The earliest events of our database date back to the 6th century, while in-depth information on the events is available since the 18th century. In total, more than 20,100 torrent events were recorded, and around 5,700 have been identified as debris flow-like events. We report how to best identify different process types and to derive quantitative information from historic texts. Our results may improve the evaluation of frequency-magnitude
relationships to be used in hazard and risk assessment and might also be used for estimates of possible consequences of climate change in the Eastern Alps.