8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 AbstractThe study presented here is focussed on the question of the hydraulic nature of the threshold that allows a rill to start. A rainfall-simulation experiment was carried out to produce highresolution flow-velocity data. The experiment employed a 10 m × 4 m experimental plot with a 1 % slope, which had been previously eroded and had a small rill formed in the middle. The experiment consisted of a 2 h 15'-long rainfall at a constant intensity of 69 mm h -1 . Surface elevation was measured before rainfall at a horizontal resolution of 2.5 cm across, and 5 cm along the slope direction. During rainfall, flow velocities were measured at 68 locations on the plot with the Salt Velocity Gauge technology, an automated, miniaturized device based on the inverse modelling of the propagation of a salt plume. The experiment led to the collection of flow-velocity measurements which are novel in three ways: (i) the small size of the measured section, which was only 10-cm long and 1-cm wide, (ii) the wide range of measured flow velocities, which ranged from 0.006 m s -1 to 0.27 m s -1 and, (iii) the large number of measured locations.The flow-velocity field was simulated with three models: PSEM_2D solves the Saint-Venant equations in 2D, MAHLERAN uses a 1D kinematic wave in the slope direction coupled with a 2D flow-routing algorithm, and Rillgrow2, which involves an empirical runoff algorithm that is close in principle to the diffusion-wave equation in 2D. The Darcy-Weisbach friction factor (ff) and the infiltration parameters were calibrated in all cases to investigate the capabilities of the different models to reproduce flow hydraulics compatible with the onset of rilling. In a first 1 set of numerical experiments, ff was set uniform, and calibration used only the hydrograph. The comparison of simulated and observed flow-velocity field showed that PSEM_2D was the most satisfying model, at the cost of longer computational time. MAHLERAN gave surprisingly good results with regards to the simplicity of the model and its low computational needs. However, all models largely underestimated the highest velocity values, located in the rill. Furthermore, none of the models was able to simulate the Reynolds (Re) and Froude (Fr) numbers. The next numerical experiment was done with PSEM_2D. Non-uniform ff values were calibrated by fitting the simulated flow-velocity field to the observed one. The latter simulation produced realistic simulations of Re and Fr. The hydraulic conditions at the transition from interrill flow to rill flow are discussed. The results support the theory that supercritical flows are a necessary condition for a rill to emerge from a smooth surface.

The large slow‐moving landslide of Maca is located in the upper Colca valley (southern Peru), a region characterized by a well pronounced rainy period, and intense and recurrent sustained seismicity. The landslide, developed in deep lacustrine deposits, has recently accelerated, threatening the Maca village. This work aims at understanding the rupture mechanism and the causes of the recent landslide reactivation/acceleration. We present a multidisciplinary characterization of the Maca landslide that includes: (i) geological and morphological mapping in the field; (ii) remote sensing analysis using an historical aerial photograph of 1955 and the Pléiades satellite images (2013); (iii) global positioning system (GPS) including time‐series of surveys over 13 years, and continuous measurements over 14 months; (iv) a geophysical campaign with deep electrical resistivity tomography profiles acquired across the landslide mass. Our study shows that this 60 Mm3 landslide, which can be classified as a clay/silt compound landslide, moved by 15 m between 2001 and 2014 with a large inter‐annual velocity variation (up to a factor of 500) depending on the rainfall intensity. We suggest that these dramatic changes in velocity are the result of the combination of a threshold mechanism and the short intense rainy season in Peru. This study reveals three main driving factors acting at different timescales: (i) over several decades, the river course has significantly changed, causing the Maca landslide reactivation in the 1980s due to the erosion of its toe; (ii) at the year scale, a minimum amount of rainfall is required to trigger the motion and this amount controls the landslide velocity; (iii) transient changes in slide velocity may occur anytime due to earthquakes. This study particularly highlights the non‐linear behaviour of the motion with rainfall. Copyright © 2016 John Wiley & Sons, Ltd.

International audienceThe spatial and temporal distributions of landslides in six catalogues are analyzed in order to better understand landslide triggering mechanisms. The six landslide catalogs are New Zealand, Yosemite (California), Grenoble (French Alps), Val d'Arly (French Alps), Australia, and Wollongong (New South Wales, Australia). Landslides are clustered in time for all catalogs. For New Zealand, Yosemite, Australia, and Wollongong, the frequency of landslides varies between 1 and 1000 events per day and is well fitted by a power law: there is no characteristic scale for daily rates. When the large rates of daily landslides are known to be rain or earthquake triggered, our results suggest the same triggering may hold for the small daily rates. Earthquakes are found to trigger landslides for the New Zealand, Yosemite, and Australia areas at distances as large as 20 times their fault lengths. There is no evidence of landslides triggered by earthquakes for the three other catalogs. Small M ≤ 4 earthquakes have little influence on landslide triggering, if any, for all catalogs. For New Zealand, Yosemite, Val d'Arly, Australia, and Wollongong, the number of landslides per month is significantly correlated with monthly rainfall. A correlation with temperature is found only for Grenoble and New Zealand. Landslide triggering (strong clustering in time and space) is more important in New Zealand than in Grenoble, probably because the forcing (seismicity and climate) is stronger in New Zealand than in the French Alps but possibly also because of a high sensitivity to landslides in New Zealand. We suggest that intensity of clustering in space and time can be used to assess the importance of landslide triggering and the processes responsible for triggering