Rainfall characteristics for periglacial debris flows in the Swiss Alps: past incidences–potential future evolutions

Abstract

Based on observational meteorological data since A.D. 1864 and tree-ring records of debris-flow activity, this paper assesses changes in rainfall characteristics and their impact on the triggering of geomorphic events in a high-elevation watershed of the Swiss Alps since the end of the Little Ice Age. No trends are visible in the frequency of heavy rainfall events, but we observe a reduced number of heavy, shortlived rainfalls in summer and a concentration of advective storms is recorded in late summer and early fall since the late 1980s. These changes in triggering meteorological conditions resulted in a cluster of debris flows in the early decades of the twentieth century and a lowering of debris-flow activity since the mid 1990s, and may be mirroring the observed changes in persistent high-pressure systems over the Alps. We also observe intra-seasonal differences in debris-flow system response reflecting the state of the permafrost body in the source area of debris flows, allowing for very small debris flows to be released by limited rainfall inputs (<20 mm) in June and July. The same quantities of rain will not trigger debris flows in August or September, when a thick active layer of the permafrost body is capable of absorbing water. With the projected amplitude of climatic change, seasonality, return intervals and volumes of debris flows are likely to be altered. RCM projections based on the IPCC A2 scenario suggest a decrease in heavy summer rainfalls which will most likely result in a (further) reduction of the overall frequency of debris flows, leaving more time for sediment to accumulate in the channel. Such an increase of channel accumulation rates along with the projected destabilization of the steep rock-glacier body is likely, M. Stoffel (B) · M. Bollschweiler · M. Beniston

Landslides and synoptic weather trends in the European Alps

“…The top classifications are all automated, while the best of the subjective classifications are highlighted in the shaded rows. Rank is based on the BSS result, name is from the COST733 catalogue often associated with convective storms and heavy downpours in the region (Rudolph and Friedrich 2011;Stoffel et al 2011) potentially causing the high landslide numbers observed. In contrast, the anticyclone over central Europe (type #18; Fig.…”

Regional reconstruction of flash flood history in the Guadarrama range (Central System, Spain)

“…Since Pinus spp. has a different genetic setup and does not generate tangential rows of resin ducts (or TRD; Stoffel, 2008) after mechanical damage, these species could not be used as a proxy to infer the seasonality of flash flood events (Stoffel and Beniston, 2006;Stoffel et al, , 2011. Flash flood seasonality has instead been inferred from those samples showing wound borders on the increment rings and from a comparison between damage and meteorological record.…”

Methods for detecting channel bed surface changes in a mountain torrent – experiences from the Dorfbach torrent

“…Inhabitants and visitors in the Matter Valley (Switzerland, Valais) are threatened by various natural hazard processes; moreover, an observed change in the frequency of debrisflow events enhances this peril (Graf and McArdell, 2005;Stoffel et al, 2011;. The Dorfbach torrent case study is based on the assessment of one of these hazardous torrents in the Matter Valley (Fig.…”