The seasonal cycle of deposition and scouring processes in the channel network as a key process for erosion response in badlands catchments, (Draix, Alpes-de-haute-Provence, France)

In the Southern French Alps, the Black Marls formation, or "Terres Noires" in French, covers a large area. This formation is very susceptible to weathering and erosion. Under the Mediterranean and mountainous climate with frost in winter and high intensity rainfall in summer, it results in "badlands" topography and high solid transport during floods. The experimental site of Draix was created in the period 1983-1984 and aim principally at studying erosion processes and flood generation in this area. Four small watersheds, from 1 000 m2 to 1 km² are equipped to study and quantify runoff and erosion processes, according to the vegetation cover and basin size. The analyses of the data at the event scale pointed out the non-linearity of the response to a rainfall input and revealed the role of the deposition and scouring processes in the channel network. In order to study how this phenomenon control the erosion response at the outlet, field observations were carried out throughout the year in the 2 basins devoid of vegetation : Laval (0.86 km²) and the Moulin (0.09 km²). The paper analyses the field observations and collates it to the rainfall inputs and to the runoff and sediment supplies at the outlet for the same rainfall events or group of events. In addition, the water and sediment response of a monitored gully (The Roubine, 1330 m²) are used to evaluate the contribution of the slopes for the same events. The analysis of the sequence shows how the spring and summer rainfall events increase the temporary stocks in the gully bottoms and channel reaches even when the sediment output is high. At the beginning of autumn, the upstream part of the network begins to empty and the high floods of November scoured most of the remaining material in the larger channels. These results allow to build a seasonal model of the erosion and transport processes in the marly badlands basins.