Climate change and integrated analysis of mountain geomorphological systems
Emmanuel Reynard, Christophe Lambiel,
Stuart N. Lane, Lausanne
Geographica Helvetica Jg. 67 2012/Heft 1-2
1 Introduction: sediment transfers and mountain complex systems
Quantification of the effects of climate changes upon the Earth system, both historically and over the next century, is particularly relevant today. Parry et al. (2007), for the Intergovernmental Panel on Climate Change, make a critical observation: little work has been conducted on the expected impacts of climate change on sediment production and transfer in mountain watersheds and the subsequent sediment loads in rivers (Kundzewicz et al. 2007), especially in mid and low latitude mountain environments (Alcamo et al. 2007). This is despite two major concerns: (1) the dynamics of sediment delivery to and transport in mountain river systems is of crucial significance for flood risk (e.g. Lane et al. 2007); and (2) watershed sediment systems are particularly sensitive to climate change, as observed across a variety of scales from the individual basin (e.g. Rumsby & Macklin 1994) to the regional scale (e.g. Knox 1999; Warner 1987), especially where historical land use changes such as deforestation are important (Macklin & Lewin 2003).
Since the early 2000s, the University of Lausanne has developed a research programme concerned with environmental change in both high-mountain and arid regions (Winistörfer & Reynard 2003). The primary geographical focus of this work has been the Val d’Hérens, a watershed of the River Rhône, Switzerland, with an altitudinal range from 4357 m a.s.l. (Dent Blanche) to 493 m a.s.l. (confluence of the Borgne river with the Rhône river), a suite of geomorphic processes (periglacial, glacial, gravitative, fluvial), and multiple influences linked to both climate changes and human impacts (e.g. hydropower plants, roads, mountain villages and tourist resorts). The catchment contains the «Maison des Alpes», a foundation that promotes the natural and cultural heritage of the Alpine region. Since 2008, the Institute of Geography of the University of Lausanne (IGUL) has worked collaboratively with this Foundation. Our aim in this paper is to conceptualise our understanding of high mountain sediment transfer systems in Alpine settings so as to identify critical research questions over the next decade for this environment. We illustrate IGUL’s contribution to three elements of this work: (i) permafrost distribution and characteristics in high catchments; (ii) modelling of sediment transfers in the Borgne d’Arolla watershed, and (iii) mapping of sediment sources in mountain torrents.
Fig. 1: Schematic representation of a typical steep alpine slope affected by various processes Schéma d’un versant alpin typique concerné par différents processus Schematische Darstellung eines typisch alpinen Berghangs und verschiedener Prozesse Graphics: C. Lambiel
Fig. 2: Front view of the Tsarmine research area on the right hand side of Arolla valley Vue de la région de Tsarmine, sur la rive droite du Val d’Arolla Frontalansicht des Tsarmine Untersuchungsgebiets auf der rechten Seite des Arolla Tals Photo: R. Delaloye, graphics: C. Lambiel
Fig. 3. Sediment transport rate dependant on shear stress (a) and flow duration with and without hydroelectric power offtake (b) for the Borgne d’Arolla Variation de la capacité de transport sédimentaire en fonction des contraintes de cisaillement (a) et de la durée du débit avec et sans prélèvement hydroélectrique (b) pour la Borgne d’Arolla Sedimenttransportraten in Abhängigkeit von der Schubspannung (a) und Fliessdauer mit und ohne hydroelektrischer Entnahme (b) für die Borgne d’Arolla Graphics: S.N. Lane.
Fig. 4: Catchments prone to debris-flow activity in the Hérens valley Carte des bassins versants sujets à des laves torrentielles dans le val d’Hérens Murgang-anfällige Einzugsgebiete im Hérens Tal Cartography: B. Maillard
4 Conclusion
The above examples show that current geomorphic process activity and the modelling of future changes must combine: (i) characterisation of the current geomorphological system (e.g. through geomorphological mapping and intensive field survey); (ii) quantification of process activity (e.g. through measurements and monitoring); (iii) mathematical modelling of current processes and feedbacks in response to future forcing. They also indicate why high mountain sedimen-tary systems may be particularly sensitive to climate change and so need significant research attention. Whilst recent work suggests a speeding up of individual elements of the sediment transfer process, there also remains a set of critical and unresolved questions, which, in turn, shape the necessary focus of Swiss contributions to geomorphic research in high mountain environments. In this onclusion, we identify five principles that we believe need to underpin this focus.
First, understanding future climate effects requires us to consider mountain catchments as holistic cryospheric-hydrologic-geomorphic systems. Geomorphic research in high mountain environments has tended to focus upon singular elements of the sediment transfer system (e.g. landslides, rock glaciers, debris flows). Yet, it is the connections between them that will determine whether or not the total basinscale sediment flux is responding to climate change, and these have been less well-considered.
Second, our examples demonstrate the importance of intensive survey at the scale of small catchments coupled to methods of generalisation to the regional scale (e.g. Switzerland, Alps) through field-informed modelling. Whilst studies have sought to develop regional correlations between temperature records and changing sediment transfer activity, such correlations overlook the mediating effects of endogenous environmental processes, such as those that control the annual and local depth of penetration of the summer thaw of winter-frozen ground, leading to possible non-linear responses to temperature rise. A more sophisticated attempt to measure and to model the ambient environmental conditions that are associated with any reconstructed changes is required that captures more sensitively the critical role played by local forcing and feedbacks.
Third, we emphasise that fluvial research in these systems needs to consider more than just the hydrological system but also the links with other geomorphic subsystems (e.g. hillslopes, rock glaciers, landslides). In particular, very few studies have explored the extent to which changing sediment transfer at the hillslope scale can be detected in changing river response and vice versa (Schwab et al. 2008 is an exception).
Fourth, climate change is not the only factor that will influence the intensity and spatial distribution of mountain processes. Anthropogenic factors will also impact on the sediment transfer system. Efforts must be made to delineate human-natural processes interactions, as well as positive/negative feedback loops.
Finally, almost all of the above studies have focused upon the period between the end of the Little Ice Age (c. 1860 in the European Alps) and present, and most commonly upon the period associated with the aerial photographic record (post 1940s). Whilst the latter period allows for the most detailed reconstructions, there is also the need to attempt to quantify changing activity over much longer timescales, extending through to the last millennium. Given recent work that has been done at a European level, we now have a very good understanding, through reconstruction, of climatic variability over the last millennium (e.g. Holzhauser et al. 2005). A challenge is now to reconstruct levels of sediment dynamics over this longer timescale in high mountain settings.
It is the aim of the project «Val d’Hérens», at the University of Lausanne, to apply these five principles to a watershed of regional importance, in order to model future changes with which Alpine societies will have to deal in the future decades.
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