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

- We monitored debris-flow propagation along a steep torrent.
- Rainfall bursts control debris-flow occurrence, volume and runout distance.
- The largest debris surge led to a more mature flow in the downstream area.
- Front velocity and viscosity decrease downstream while its height remains stable.
- Debris volume increases with surge coalescence and erosion of in-channel sediment.

The Réal Torrent (2.3 km2 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 m3 near the hillslope sources and increased downstream: 5500 m3 in the middle part and 8600 m3 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 Réal Torrent.