ABSTRACT
To mitigate torrential hazard associated to sediment transport during floods in steep mountain catchments, improving the understanding of bedload transport or debris flow processes is necessary. The characterization of these extreme phenomena (sediment supply, triggering, and propagation) requires quantitative field observations. For these purposes, three monitoring stations were deployed in late 2010 in a very active site in the French Alps-the Real Torrent-prone to debris flows and bedload transport every year. The site has been instrumented with rain gauges, photo cameras, flow stage sensors and set of geophones. It gives information about rainfall duration-intensity-volume, event nature (debris flow or flood), flow depth and debris flow front velocity. This data collection is completed by field surveys. A previous investigation has presented the flow events of the 2011 season (Navratil et al. 2013 ). During this season, debris flows were triggered by intense rainfall bursts. This paper introduces the 2013 debris flows which were initiated by long-lasting and low-intensity rainfalls, usually rather associated to water-runoff events with bedload transport. It brings out the rainfalls inter-annual variability and the necessity of having long-lasting time series to established discriminating criteria such as the rainfall duration-intensity threshold for debris flow occurrence. Moreover, it highlights the importance of the site chronicle in the occurrence of debris flows which determines if there are available sediment to mobilize or not on hillslope or in the torrent channel.
