Water driven processes and landforms evolution rates in mountain geomorphosites: examples from Swiss Alps
Introduction
Water driven processes acting along mountain slopes typically shape spectacular deeply dissected landscapes (i.e., badlands-like landforms; Bl-LFs), with distinctive features due to different substrates (i.e., rocks and deposits), that are various for lithology and texture, and different morphoclimatic contexts which they are inserted in. Mountain chains, like Apennines and Alps, are characterized by peculiar landscapes mainly modelled by running waters.
Widespread and famous are the Italian badlands, modelled in arid, semiarid and humid environments, known as “calanchi”. They are shaped mainly in Pliocene clays outcropping diffusely along the Apennines (Buccolini and Coco, 2013), within regions affected by strong seasonal climatic differences (Della Seta et al., 2009). Such landscapes are characterized by a pattern of dense close-up small valleys and gullies, with steep slopes and sharpened edges, and they are often associated with mostly rounded-edged landforms called “biancane” (Alexander, 1980).
In the Alpine contexts water runoff acts on widely diffuse glacial deposits, constituted by elements of different grain sizes, from boulders to silt and clay, shaping other peculiar badlands-like landforms, the earth pyramids. These are columnar landforms resulting from a big boulder protecting the underlying deposits from water runoff (Erikstad, 2006, Crosta et al., 2014).
Where rocks and deposits consist of soluble components (e.g., evaporites or limestone) also the chemical action of water contributes to the mountain landscape modelling. Where dissolution rates are high, as on gypsum (Nicod, 1976), or where landforms are undergoing to such process for a long time, residual pillars characterize mountain landscapes. These features are in some cases called “pyramids” as well. Hence, rocky pinnacles and earth pyramids may be considered convergent landforms from a geomorphological point of view, due to their similar shape.
The high mountain regions are among the most sensitive to climate change and abundant is the inherent literature (e.g., Evans and Clague, 1994, Chiarle and Mortara, 2001, Ballantyne, 2002, Fischer et al., 2006, Chiarle and Mortara, 2009, Mercier, 2009, Pelfini et al., 2014, Reynard et al., 2012a, Stoffel et al., 2014). Nowadays the action of water under different states (glacier ice, ground ice, melting waters from glaciers, permafrost, ice cores of moraines, rainfall) is regulated by the current climate warming trend, whose effects are inducing landform changes. Glacier ablation is responsible of the huge glacier retreats and of the transition from a glacial to a paraglacial system, subject to the morphological work of water driven and gravity processes (i.e., weathering, splash, rill and gully erosion, through-flow and piping) (e.g., Mercier, 2009, Stoffel et al., 2014). The response of paraglacial systems to climate change may span from immediate reaction until million years as indicated by Mercier (2009) who underlined also that paraglacial systems lifespan may depend on different factors, mainly from sediments at disposal to be reworked, climate conditions and geological constraints.
Therefore, in such evolving landscapes, landforms may be more or less preserved mainly depending on the substrate, which they are shaped in, on the age of landforms and on the rates of geomorphic processes they underwent.
A meaningful example of changing landforms in high mountain environment, and in paraglacial systems in particular, is represented by lateral and ground moraines, well known as key sites for the reconstruction of glacial advancing phases and for the observation of the following modifications under different geomorphic processes. Under the current climatic conditions, moraine ridges are affected by geomorphological instability (e.g., Curry, 1999, Hewitt, 1999, Chiarle and Mortara, 2001, Mortara and Chiarle, 2005, Curry et al., 2006, Mercier, 2009, Smiraglia et al., 2009). Over-incision of frontal moraines, erosion at the foot of the moraine slopes, concentrated linear erosion on the inner flank of lateral moraines, that generate new supra-imposed gullies (Bl-LFs), and burial, due to debris falls/flows, are the main processes affecting moraines on the whole Alpine range (Chiarle and Mortara, 2001). The re-modelling of glacigenic sediments has been recognized as one of the most important paraglacial slope adjustments consisting in increasing gravity processes and the sudden development of gully systems. Sediment transportation and formation of debris cones at the base of moraine inner slopes progressively lead to the reduction of the overall slope gradients and concavity in moraine profiles allowing them to reach a new equilibrium (Curry, 1999, Curry et al., 2006). Huge quantities of water released during intense rainfall events and/or from moraine ice core melting favour the process efficacy.
Also at lower altitudes slopes are mantled by glacial deposits, related to the older glaciations and now covered by soils; nevertheless also deeply dissected moraine ridges, continuously reworked by running and channelized waters and by human impact are present.
In this changing morphoclimatic context, rocky outcrops, characterized by erosion glacial landforms (e.g., roches moutonnées, glacial striae, subglacial potholes), when consisting in soluble lithotypes, can be reworked by karst processes (e.g. Chardon, 1996), whose action is added to the ones from other geomorphological active processes. In this context also pillars can develop, as mentioned before.
Water driven denudation processes are active with different rates in the mountain environment, from paraglacial areas as far as lower altitudes, depending on substrate, relief and climate factors (e.g., Delannoy and Rovera, 1996, Taminskas and Marcinkevicius, 2002). Denudation rates in mountain regions are significant at the drainage basin scale and crucial as they are strictly linked with the downstream physical and chemical water load (Descroix and Mathys, 2003). As reported by Chiarle and Mortara (2001), where huge coverage of loose debris are present extreme rainfall events can trigger mass wasting phenomena (as far as 5 × 106 m3 in a single event; Avisio catchment, Eastern Italian Alps). In general, the climate influence on the water runoff processes has been detected in terms of rainfall regime and typologies of rainfall events. In mountain environments the increase in runoff intensities during wet years, following dry ones, was detected (Bollati et al., 2012b, Bollati et al., 2016a, Bollati et al., 2016c and reference therein) and intense rainfall events demonstrated to trigger slope erosion (Bookhagen and Strecker, 2012) and mass wasting events (Guida et al., 2008).
In this framework, denudation rates (i.e., erosion and dissolution rates in the case of the Bl-LFs examined in the framework of the present research) vary in space and time and different values (i.e., local or averaged) may be obtained from different methods of measurement (i.e., direct and indirect) considering among them the natural data archives like tree rings. They are diversely efficient depending also on substratum and active processes.
In a geoheritage perspective, “pyramids” modelled by water driven processes on different substrates may be sites of great geological-geomorphological interest not only for their scientific importance but also for aesthetic reasons and for their links with different components of culture as literature and art, as well as socio-economic and tourist issues (e.g., Giusti, 2012, Bollati et al., 2016a). Deep is the current attention of the scientific community towards mountain geoheritage for both geoconservation and geotourism purposes due to i) its scientific meaning, ii) the presence of various geomorphological features, also in term of landforms activity degree, iii) the particular sensitivity of this environment to climate change and related hazards and iv) its highly aesthetic value (e.g., IAG – Network on Mountain Geomorphosites; Reynard et al., 2011, Reynard et al., 2016, Giusti et al., 2013, Ravanel et al., 2014, Bollati et al., 2016b, Reynard and Coratza, 2016). Hence, improving knowledge about mountain geoheritage evolution rates is crucial since the processes, which have shaped geomorphosites, can be the same that could degrade or destroy them (Hooke, 1994, Pelfini and Bollati, 2014, Bollati et al., 2016a). Analyses for estimation of changes in denudation rates are hence significant when considering changes in geoheritage for what concerns both conservation and impact and hazard assessment (Bollati et al., 2013, Bollati et al., 2016a).
In this perspective, after a short review on badlands-like landforms in mountain environment and their meaning in the geoheritage framework, two will be the aims to be pursued: i) to present the results of a multidisciplinary analysis on denudation rates characterizing selected geomorphosites shaped by water driven processes (i.e., physical and chemical) acting on different substrates (i.e., glacial deposits and soluble rocks); ii) to integrate site specific results in a discussion on spatio-temporal evolution of geomorphosites, and the related classification, according to geomorphic processes activity they are affected by.
Section snippets
A short review of badlands-like landforms in the framework of climate change
Terminology used for classifying badlands-like landforms is quite diversified (i.e., pyramids, pillars, towers; Perna, 1963) and usually local names are applied, often linked with tradition and legends. A short summary is reported in Table 1 and some pictures are illustrated in Fig. 1. Badlands-like landforms mainly derive from water action (physical erosion and chemical dissolution) on different kinds of substrates characterized by different textures (more or less heterometric in grain-size),
Study areas
The two study sites are included in the Swiss National Inventory of Geosites (SNIG) (Reynard et al., 2012b) (C and E, Fig. 4) and are located in the Western Swiss Alps. Both are named “Pyramids” even if the modelling process is different: i) Pyramides d'Euseigne - PE (Canton Valais) are earth pyramids shaped by water runoff on ancient glacial deposits; ii) Pyramides de gypse du Col de la Croix - PCC (Canton Vaud) are rock pillars deriving from chemical dissolution on gypsum outcrops. Both areas
Materials and methods
The sensitivity of an area to erosion may be determined considering mainly the morphometric factors (e.g., drainage density and setting, length of the slope or relief ratio, exposure and slope angle), the geological features (e.g., lithology, structures affecting drainage patterns) and the vegetation coverage (Latulippe and Peiry, 1996, Descroix and Mathys, 2003, Gyssels et al., 2005). In the Alpine environment, measurements of denudation rates, in relation with different substrates, have been
Morphometric measurements on historical photographs
The analysis of the iconographic material allows evidencing the progressive morphological changes of the site since the end of the 19th century (Fig. 9, Fig. 10).
One of the most evident changes is related to Group 1, characterized by the fall of a big boulder (A1 and A2, Fig. 9) that verified in the time interval 1906–1925. Therefore, after the fall of the block, the earth pyramid underlying the boulder has been evidently thinning more rapidly than the surrounding pyramids. In Group 1, the
Denudation rates at the study sites in comparison with literature data
In Table 7 a summary of the quantitative values of erosion on glacial deposits available in literature is reported, in comparison with those obtained in the present research.
According to Latulippe and Peiry (1996) the erosion on glacial deposits (as are Pyramides d'Euseigne) should be considered qualitatively “very strong” due to their unconsolidated structure. Nevertheless this is not true for very compact lodgement till, as observed in some cases for earth pyramids (Crosta et al., 2014),
Conclusions
From the short review on badlands-like landforms in mountain environment, it emerges that water driven processes related to climate are significant modelling agents in mountain environments. They can change in intensity and frequency, during different time periods and in relation with the involved substratum, producing landforms transforming through times. Results obtained from multidisciplinary and multitemporal analysis on mountain landforms, changing under water action, allowed us to
Acknowledgements
This study has been developed within the framework of the PRIN 2010–2011 project (grant number 2010AYKTAB_006) “Response of morphoclimatic system dynamics to global changes and related geomorphological hazards” (local leader C. Smiraglia and national leader C. Baroni). The Authors are grateful to the Canton of Valais administration (Landscape and Forest Service, director: M. Olivier Guex) for authorizing us to investigate the protected area of Pyramides d'Euseigne and to the Ollon Municipality
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