Runoff and erosion in the Black Marls of the French Alps: Observations and measurements at the plot scale

The experimental basins of Draix in the southern French Alps have been monitored since 1984 in order to quantify and analyse the incidence and patterns of erosion in the badlands developed on Black Marl formations. In order to analyse the rainfall-runoff behaviour of the mudstone slopes, rainfall simulations were conducted on small plots 1 m2 in area. Four different local geomorphologic conditions were selected for the experiments: (a) steep slope parallel or (b) perpendicular to the bedding, (c) moderate slope and (d) gravel-covered surface, both parallel to the bedding. Rainfall simulations were performed at a moderate intensity (12-25 mm h- 1) for 30 min on three plots in each location. Different delays between simulations offered different initial soil wetnesses. A simulation during high-intensity rainfall (90-150 mm h- 1) for 30 min was made on one plot of each type. One plot of each type was selected for monitoring during the following summer and autumn.In dry conditions, runoff was low or negligible. A second simulation carried out 1 day later again produced very low runoff coefficients. Runoff increased slightly when the interval between rainfall simulations was reduced to 30 min. In this case, a few grams of sediment were eroded, whereas no erosion was observed in the first two cases. Under high-intensity conditions, runoff began quickly and the runoff coefficients were high (20-50%). Erosion was notable, but not significantly different from dry conditions.The behaviour was very different during natural rainfall events, in particular summer storms: even if the mean rainfall intensity was lower than 60 mm h- 1 in 30 min, short periods of high intensity (more than 100 mm h- 1 in 5 min) revealed substantial erosion activity: more than 700 g m- 2 on all the plots for a rainfall event of 54 mm in 3 h and more than 600 g m- 2 for an event producing 26 mm. However, the runoff coefficients were on the same order as those of the last experiment under high-intensity conditions (32-55%). The rainfall intensity over the short duration and the kinetic energy of the raindrops were the main factors influencing particle detachment and movement.