Recent evolution of debris-flow fans in the Central Swiss Alps and associated risk assessment: two examples in Roseg Valley

This paper reports the results of research carried out in the Roseg Valley (Lower Engadine, Switzerland) that examines debris-flow fan typology, debris-flow activity, and the impact of debris flows on forest vegetation. Depositional landforms from debris flows were subdivided according to their degree of geomorphological hazard. An inventory of debris-flow fans and a detailed geomorphological, vegetational, and dendrochronological study of two of them was undertaken. The dendrochronological analysis allowed dating of historical events and reconstruction of their recent evolution. By dating scars and growth anomalies on Larix decidua Mill. and Pinus cembra L., 24 debris-flow events were identified on the Misaun fan and 16 on the Tschierva fan, and differences in their distribution due to drainage basin characteristics identified. Fans chosen for this investigation are crossed by a hiking trail that is frequently used by tourists during the summer season. A qualitative hazard and risk evaluation were undertaken. This approach contributes to hazard assessment on fans, and may also be applied in other environmental contexts (e.g., in trail planning).