Processes, spatio‐temporal factors and measurements of current erosion in the French Southern Alps: a review

Abstract

Present erosion in mountainous areas of Western Europe causes land management problems, particularly for areas located downstream of erosion zones. Except for transalpine roads and ski resorts, economic activities no longer require as much space as they did in the past. Therefore, natural reforestation has provided significant protection for alpine hillslopes during the 20th century. However, extreme floods continue to cause severe damage in intra-alpine valleys, as well as in piedmont and surrounding plains, making the study of present water erosion phenomena very important.Many studies have investigated the processes and factors of water erosion on slopes at both the catchment and plot scales. They have focused on rock fragmentation and transportation in different fields, the spatial and temporal explanatory variables, the consequences downstream (flooding, sedimentation, river bed evolution) and the impact of floods.In the French Alps, present erosion has been studied in a variety of outcrops, with several recent studies conducted in fields such as marls, clayey deposits, molasses and moraines. These kinds of outcrops are found throughout the alpine massif, including an area of special interest on the great Jurassic black marl outcrop where badlands are frequently observable. Geomorphologists and hydrologists have been particularly interested in the strong erosion processes in marls, seeking to determine the main patterns and the impact of spatial and temporal factors on soil loss quantities.The main climatic factors of rock disaggregation were found to be the freeze-thaw and wet-dry cycles, which destroy rock cohesion, and the splash effect of rain. The principal site variables are vegetation cover, exposure and dip-slope angle. Erosion rates are two or three orders of magnitude higher on bare soils than on pastures; northern aspect slopes suffer two to four times as much soil loss as southern aspect slopes. Finally, the angle formed by the slope and the dip also determines different behaviours: erosion rates are higher when slope and dip are perpendicular than when they are parallel. The transportation agents are mostly debris flows and runoff caused by intense precipitation. Annual erosion depth in the marls is generally assumed to be substantial, up to 10 mm. The high value can be explained by the severity of the climatic conditions and the brittleness of the lithology, which results in numerous fractures.

An attempt to estimate tolerable soil erosion rates by matching soil formation with denudation in Alpine grasslands

“…Forests, shrublands and wetlands usually have considerably lower erosion rates than the more intensively managed grasslands. As it has been shown by Frankenberg et al (1995), Descroix et al (2003), Konz et al (2012) and Alewell et al (2013), intensively used areas in the Alps might have considerably higher erosion rates than the average values of the whole catchment of the Urseren Valley. At strongly affected sites, erosion rates are estimated to be between 1.4 to 3 mm year −1 which would result in a total erosion depth of 1.4 to 3 m at such slopes during the last 1,000 years when human impact was considerable (settlement and deforestation).…”

“…Since the late 19th century the abandonment of agricultural sites, especially in high alpine and remote areas is a well-known phenomenon (Swiss Federal Statistical Office 2001;Descroix and Mathys 2003;Piégay et al 2004;Lasanta et al 2006;Tasser et al 2007). Simultaneously, the remaining farmland in lower areas having good accessibility has been utilised with increased intensity .…”

Consequences of Land Use Changes on Hydrological Functioning

“…As in other mountainous areas of Southern Europe, a hard policy of land reclamation was initiated in the second half of the 19 th century (French 2 nd Empire and 3 rd Republic). Reforestation and torrent correction were strongly developed (Table 1); then, socioeconomic evolution and three wars (against Prussia in 1870, and the 1 st and 2 nd World Wars) depleted these areas of their inhabitants (Descroix & Gautier, 2002;Descroix & Mathys, 2003 The most important sediment providing material in the French Southern Alps is the black marls (Table 2), which outcrops exceed 1000 km 2 ; but some other fields are also very sediment productive, as indicated in Table 2. For centuries, these very degraded marly terrains were providing stream flow and suspended sediment load to the Alpine rivers.…”

Bilan d'erosion d'un cours d'eau alpin: l'Isere a Grenoble (France)

“…Ils peuvent même dépasser localement 60 000 t km -2 an -1 (Lhénaff et al, 1993). Cette forte variabilité spatiale de l'érosion spécifique se retrouve également dans les Alpes du Nord (Vivian, 1981 ;Descroix et Gautier, 2002 ;Descroix et Mathys, 2003) , plus en aval, sur un bassin de 1303 km², et de 181 t km -2 an -1 sur un bassin de 3264 km² (Vezzoli, 2004).…”