Changes and trends in debris-flow frequency since AD 1850: Results from the Swiss Alps

Abstract

Although studies have repeatedly focused on feedbacks and impacts of climate change on mass movements in the past, the inter-relations between climatic variables and debris-flow occurrence remain widely unclear and ambiguous to date. Most studies on past debris-flow occurrence remained rather isolated reconstructions for single torrents or they were restricted to short time periods. It is therefore the aim of this study to provide a regional chronology of past debris-flow events for the Zermatt Valley (Swiss Alps) and to go beyond the simple dating of events. Based on tree-ring reconstructed debris-flow histories of eight torrents, we shed light on changes and trends in debris-flow occurrence, climatic conditions prevailing during events and on potential evolutions in a future climate. Based on the analysis of tree-ring records of 2467 conifers (mainly Larix decidua and Picea abies), 417 events between ad 1600 and 2009 were assessed. Decadal frequencies suggest peaks in debris-flow activity after the end of the 'Little Ice Age' and for the period 1920-1929. In contrast, activity was rather low during the most recent part of the record (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), which is in concert with the observed decrease in the number of triggering rainfall events. Long-term trends in debris-flow occurrence were analysed for three time intervals of the period 1850-2009 with Student's t-tests. For the debrisflow frequency of the entire valley, no significant trends can be observed over the last 150 years. We conclude that the occurrence of debris flows would depend on short-term changes in triggering rainfall rather than on long-term climatic changes.

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

“…): a detail of the intermediate debris-flow cone (32 ha) and its mixed conifer stand. b View of the debris-flow system (catchment area: 1.36 km 2 , channel length: 3.5 km) reconstructed debris-flow occurrences with archival records on flooding in neighboring rivers (Stoffel et al 2005a;Bollschweiler and Stoffel 2010a) show a clear peak in activity in July and August (32% each) for the entire period of the reconstruction . In contrast, if only the last 50 years are taken into account, activity is largest in August and September, with almost a complete absence of mass movements in June and July (Stoffel and Beniston 2006).…”

“…For the Valais Alps, highly resolved data exist on the radial growth of L. decidua and P. abies, rendering dating of past debris-flow events at Ritigraben possible with monthly precision and references therein). Results obtained on the intra-seasonal timing of debris flows were also compared with meteorological and hydrological data as well as archival records on flooding in rivers of the wider study region (Lütschg-Lötscher 1926;Röthlisberger 1991;Bollschweiler and Stoffel 2010a) to further improve dating accuracy.…”

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

“…George and Nielsen, 2003;Zielonka et al, 2008), and have, with only a few exceptions, looked at the regional scale (i.e. Ballesteros-Cánovas et al, 2015c, in press;Bollschweiler and Stoffel, 2010;Procter et al, 2011;Schraml et al, 2015;Šilhan et al, 2015;Stoffel et al, 2014b). However, according to Procter et al (2011), the regional assessment of past hydro-climate related processes could be of substantial help in terms of reducing lack of data and maximizing the information available in a given area.…”

Climate change impacts on the Alpine ecosystem: an overview with focus on the soil

“…Beniston 2003), reductions in average summer precipitation and consequent episodes of summer drought may be simultaneously accompanied by a sharp increase in short but potentially-devastating heavy precipitation events (Casty et al 2005;Beniston et al 2011), such as flash floods, more frequent overflow of storm drainage facilities, greater soil erosion and higher risk of landslides in unstable areas (Bollschweiler, Stoffel 2010). Where the climate change is expressed by extreme events, the erosion will be stronger, and the extent of Leptosols might be larger.…”

“…Geotechnical changes of frozen slopes and shortterm changes in triggering rainfall have a notable impact on slope stability, and are thus of interest for dealing with applied problems such as construction stability, debris-flow hazards, or rockfall hazards (Noetzli et al 2007;Kääb et al 2007;Bollschweiler, Stoffel 2010).…”