Ravinement en montagne : processus, mesures, modélisation, régionalisation

Cemagref (Agricultural and Environmental Engineering Research Centre) in Grenoble and the Scientific Group "Draix, mountain erosion studies" held a conference in Digne, in the French Alps, from October 15 to 17, 2003, devoted to the theme of gully erosion in mountain areas. The aims of this international seminar were to evaluate the state of the art in research on erosion by water in uplands, to allow scientists to meet and exchange ideas on related topics, and to initiate further collaborations, with particular emphasis on field work in research basins. This meeting, which encompassed 32 oral presentations and 24 posters, was attended by 80 participants from 12 countries. A one-day scientific excursion to the Draix experimental site was dedicated to multidisciplinary researches on mountain erosion and upland floods.Gully erosion represents an important sediment source in river systems and accounts for as much as 70 to 90% of the overall sediment production of a catchment. In mountain areas, the steep slopes enhance gully processes, accelerate sediment transfer from uplands to valley bottoms and generate natural hazards : mudflows, overflowing of heavily loaded floods, silting up of reservoirs, for example. Therefore, there is a need for monitoring and for experimental and modelling studies in order to forecast the effects of climatic or land use changes on sediment dynamics in mountain catchments. In mountain environments, the development of appropriate measurement and observation techniques is quite difficult. Original devices were presented, operating from the small plot scale to the large gully and catchment scales.Several presentations focused on the processes and factors controlling erosion and gullying in mountain areas : these include mechanical breakdown and chemical weathering of rocks, bed load and suspension sediment transport, in which threshold effects have a determining influence. Mass movements are also a substantial source of sediments in steep upland basins. These may occur as superficial earth flows affecting the weathered layer, and as deep-seated landslides or rock-block slides. Several papers examined the susceptibility of the terrain to the mass movements, the conditions for landslide initiation, the relations between mass movements in black marls and rockfalls in the overlying limestone levels, and the formation and propagation of debris flows. The major role of a few paroxystic events in sediment production was pointed out by several authors. Particle detachment, mass movement initiation, and sediment transport are processes subjected to threshold effects. The connectivity in the basin network may vary from one event to the other. The studies on sediment load rates during floods showed that the availability and location of sediment stocks in the channel and gully bottoms play a greater role than the transport capacity of the flow. The role of the vegetation cover in diminishing sediment yields is widely accepted. Therefore, it is still necessary to understand how the aerial parts and roots of vegetation interact with runoff and erosion in order to model the evolution of gully systems subject to climatic or land use changes. These studies show that erosion modelling becomes more complex as one goes from the plot or slope scale to the catchment scale, which necessitates a transport-distance approach. At this scale, the question of connectivity between flow paths becomes crucial and depends on the time scale of the study.The present issue of the journal publishes five papers related to erosion measurement and remote sensing, to erosion dynamics at the catchment or at a regional scale, and interactions between erosion, and vegetation and restoration works.The paper by D. Raclot et al. presents a first attempt of at using low-altitude aerial photographs taken from a drone for erosion surveys in the Draix area. The relatively poor vertical accuracy (70 cm) does not allow yet for the measurement of short-term ablation, but the good quality and resolution of the pictures, and the possibility for of repeated acquisition offer good opportunities for the survey of landform evolution, especially in remote areas of the studied catchment. Two others additional papers (E. Gauché, J. Collinet & P. Zante) present fruitful methodologies for mapping and topographic survey of gully development. Recent progress in GIS and DEM techniques complete the quantitative observation of erosion processes. Addressing scale issues, the paper by J. Collinet and P. Zante presents different techniques in Tunisia covering rainfall simulation on plots in the flat upstream areas, monitoring of gully evolution on the intermediate slopes, sedimentation in reservoirs at the outlets. The authors show the importance of the processes linking these three slope interfaces. In Morocco, E. Gauché found contrasting rhythms of landscape evolution between mountain slopes and alluvial deposits downstream. Lithology and relief play a role, which is reinforced by land use change to a point that an erosion crisis may be identified, mainly due to the decrease of the extent of tilled areas. The paper by F. Rey presents an experimental bioengineering device for trapping sediments in gullies in marls of the French Alps and shows how brush layers 1.2 m thick across a gully may trap 0.1 to 0.3 m3 per year. The paper by M. Cohen and F. Rey discusses the paradox between positive vegetation dynamics at the regional scale and active erosion observed at the local scale and shows that processes vary depending on spatial scales : for instance, gully erosion may co-exist with vegetation spread on interfluves.Temporal and spatial scales as well as connectivity in soil erosion systems appear as key questions for further research on gully erosion, including appropriate measurement techniques, process analysis, modelling, and upscaling