Reconstruction of debris-flow dynamics and triggers since AD 1570 - looking back to a decade of tree-ring research at Ritigraben (Valais, Switzerland)

Abstract

Records derived from trees growing in temperate regions can provide annually resolved data on past debris-flow activity that span several centuries. They therefore allow identification and accurate dating of events prior to instrumental records or missing in historical archives. As a result, dendrogeomorphic methods have repeatedly been used over the last decades to reconstruct debris-flow frequencies in mountain regions of Europe or North America. While these studies furnished valuable data on the minimum frequency of events that would have occurred in the channels and cones chosen for analysis, they did, however, not explicitly take account of e.g., depositional processes on cones, volumes, periods of activity in currently abandoned channels or on the age of individual lobes and levees. Over the past eleven years, dendrogeomorphic investigations on past debris-flow activity in the Ritigraben catchment (Valais, Swiss Alps) and its intermediate cone went several steps beyond the as- sessment and pure dating of debris-flow “signatures”. Through the analysis of some 2450 tree-ring sequences obtained from more than 1200 trees growing on the cone and along the current flow path, we were able to identify evidence of 123 events since 1570. The large amount of data also allowed analysis of the spread and deposition of material on the cone as well as determination of activity in currently abandoned channels. A combination of tree-ring with ground survey data, channel recharge rates and debris delivery in the de- parture zone (i.e. a rock glacier) allowed estimation of frequency-magnitude relationships. We also related debris-flow events reconstructed for the Ritigraben torrent with ooding records of neighboring rivers.