Visual observation of erosion processes on the Black Marl badlands in the southern Alps, France

Abstract

Abstract To clarify the erosion processes on a marly bare slope in the southern Alps, the erosion processes on a steep and erodible slope composed of Black Marls formations were observed using a time-lapse video camera. The observations revealed that miniature debris flows (MDFs) occurred at the time of the rainfall–runoff event during which the most severe erosion took place over the 3 month observation period. Analysing the camera images, we show some characteristics of the MDFs and discuss them in the context of real rainfall–runoff phenomena observed at the outlet of the small experimental basin that contained the visually observed slope. The following results were obtained. (1) It is roughly estimated that the total amount of sediment discharged by the MDFs was not quantitatively negligible in comparison with the total sediment discharge from the entire experimental basin. (2) The MDFs occurred only during the rising limb of the hydrograph, which lasted 6 min. The visual observation also provided information on triggering conditions. Compared with the precipitation data, it was found that a rainfall intensity of 1 mm min −1 triggered MDFs and an antecedent cumulative rainfall of 10.4 mm was also enough to trigger an MDF. This is consistent with a previous study in Draix. (3) Although the Darcy–Weisbach friction factor and Manning's n of the MDFs observed differ considerably, they are consistent with results in the literature reporting erosion phenomena on very steep slopes up to 36°. (4) Based on this observation and a review of the literature, on very steep and highly erodible slopes, MDFs or similar phenomena might play an important role in erosion and transport processes. It is important to consider this possibility when examining erosion models for catchments composed of steep and erodible slopes.