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.