Abstract
Abstract. Steep mountain catchments typically experience large sediment pulses from hillslopes which are stored in headwater channels and remobilized by debris-flows or bedload transport. Event-based sediment budget monitoring in the active Manival debris-flow torrent in the French Alps during a two-year period gave insights into the catchment-scale sediment routing during moderate rainfall intensities which occur several times each year. The monitoring was based on intensive topographic resurveys of low- and high-order channels using different techniques (cross-section surveys with total station and high-resolution channel surveys with terrestrial and airborne laser scanning). Data on sediment output volumes from the main channel were obtained by a sediment trap. Two debris-flows were observed, as well as several bedload transport flow events. Sediment budget analysis of the two debris-flows revealed that most of the debris-flow volumes were supplied by channel scouring (more than 92%). Bedload transport during autumn contributed to the sediment recharge of high-order channels by the deposition of large gravel wedges. This process is recognized as being fundamental for debris-flow occurrence during the subsequent spring and summer. A time shift of scour-and-fill sequences was observed between low- and high-order channels, revealing the discontinuous sediment transfer in the catchment during common flow events. A conceptual model of sediment routing for different event magnitude is proposed.
Highlights
Channelized debris-flows are common, natural processes in the French Alps
Eight periods of significant geomorphic activity were observed along the study reach between July 2009 and December 2010, two of these periods being characterised by the occurrence of a debris-flow (August 2009 and June 2010)
A frequency analysis of maximum daily rainfall based on the nearest long-term rainfall time series (Meteo France station of Saint-Hilaire-du-Touvet, 1964–2010, elevation of 970 m a.s.l, located 5 km from the Manival, on the same mountain side) gave a 10-yr daily rainfall of 88 mm (90 % confidence interval: 83–94 mm)
Summary
Channelized debris-flows are common, natural processes in the French Alps. They typically occur in steep, small to average size mountain streams (torrents) and induce each year disturbances and/or damages to infrastructure. The most striking feature of debris-flows is their ability to transport a considerable volume of sediment over long distances (typically several kilometres) and at a relatively high velocity (generally between 2 to 20 m s−1). The prevention of natural hazards related to debris-flows requires a better understanding of sediment transfer in debrisflow catchments. Some authors proposed to discriminate supply- and transport-limited debrisflow catchments as a function of the sediment recharge rate for low-order channels, this being defined as the rate at which colluvium fills the scoured channel after the passage of a debris-flow (Bovis and Jakob, 1999). The higher the recharge rate is, the higher the susceptibility of the catchment to produce a debris-flow during high-intensity rainfall events
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
