Frequency–Magnitude Relationships, Seasonality and Spread of Debris Flows on a Forested Cone

Abstract

Debris flows repeatedly cause damage to infrastructure or even loss of life on cones or at the mouth of gullies (Jakob and Hungr 2005). With the projected greenhouse warming (Christensen and Christensen 2007), there is much debate about changes in the frequency, magnitude and seasonality of precipitation events and related flooding or mass-wasting processes (Milly et al. 2002). Before establishing any cause-and-effect relationship between global warming and the incidence of geomorphic processes, the natural variability of extreme weather events must be examined as well as detailed information obtained on past process dynamics on debris-flow cones, alluvial fans or floodplains. Sletten et al. (2003) pertinently emphasize that records of past debris-flow activity may be particularly useful in the recognition of past process dynamics and precipitation events. Individual debris flows are usually released during intense rainfall (Caine 1980; Blikra and Nemec 1998) and repetitive debris flows usually occur in sediment-rich catchments, as long as the triggering precipitation events are recurring (Zimmermann et al. 1997). In addition, debris-flow deposits have a high preservation potential on cones and can therefore be used for the analysis of spatial patterns of past events (Bollschweiler et al. 2007).