High-frequency monitoring of debris-flow propagation along the Real Torrent, Southern French Prealps

The Real Torrent (2.3 km(2) drainage area) is a very active debris-flow torrent in the French Prealps. It was equipped with three high-frequency monitoring stations in late 2010 to investigate the triggering conditions and propagation of debris flows along the 2-km long torrent channel. The monitoring system combines different techniques including rain gauges and measurements of ground vibrations and flow elevations. In spring and summer of 2011, five rainfall-triggered debris-flow events occurred along the torrent. Most debris fronts deposited in the upstream part of the channel but one large front (1.9-2.0 m high) propagated to the catchment outlet with a mean velocity of about 3 m s(-1). The volume of this main debris surge was 3900 m(3) near the hillslope sources and increased downstream: 5500 m(3) in the middle part and 8600 m(3) near the catchment outlet. The debris flow developed a more mature flow pattern as it progressed downstream. Multiple debris and hyperconcentrated surges were observed upstream, while a single debris surge occurred near the catchment outlet. The front-height remained relatively constant during its propagation, whereas its viscosity and velocity tended to decrease downstream as the channel slope decreases. The growth of this main debris surge could be explained by the entrainment of in-channel sediments and the coalescence of the multiple preliminary surges. For the 2011 events, rainfall burst duration and intensity were the main factors controlling the occurrence, volume and runout distance of debris flows. Antecedent channel conditions (sediment recharge, bed wetness and antecedent rainfall) are also important factors, but further investigations are necessary to better understand the role of these factors regarding the triggering and propagation of debris flows in the Real Torrent. (C) 2013 Elsevier B.V. All rights reserved.