Erosion processes at the gully scale: observations, quantification and interpretation of field data from the Draix laboratory

In the Southern French Alps, the black marls formation covers a large area and is highly susceptible to weathering and erosion. It has a badlands topography and is subject to high solid transport, bringing high sediment yield downstream and silting up reservoirs. The scale is decisive in the study of erosion processes and quantification of sediment production, de Vente and Poesen (2005). This paper focuses on erosion at the slope and gully spatial scale and at the event temporal scale and is based on the data collected since 1983 by cemagref on a small gully, called Roubine (1330 m²). For the 19852003 period, 1016 rainfall events were registered, 472 produced runoff at the outlet and 373 yielded measurable erosion. A total of 288 sediment trap measurements are available and 205 five floods were sampled for suspended sediment. For the 19 years of the study period, the 19 highest values represent 33% of the total production of the period. For the data of all the events, there was no relation between suspended sediment concentration and flow discharge, but for one event hysteresis curves were observed. The total amount of sediment trap deposit represents a mean 9 m3 per year, i.e. 10 kg y1 m2. The ratio of suspended sediment in the total yield was 15% on average, 20% in cumulated amount, but reached 5050% for a few events. The analysis conducted on 19 years of rainfall-runoff-erosion data and field observations allows us to propose an erosion production model at the gully scale in marly badlands catchments. A seasonal pattern was observed with the renewal of the weathered mantel in winter, substantial displacement of material with spring events, high production of numerous and intense summer storms, and a decrease in sediment availability in autumn.