Joshua Theule

Headwater sediment dynamics in a debris flow catchment constrained by high-resolution topographic surveys

Abstract. Debris flows have been recognized to be linked to the amounts of material temporarily stored in torrent channels. Hence, sediment supply and storage changes from low-order channels of the Manival catchment, a small tributary valley with an active torrent system located exclusively in sedimentary rocks of the Chartreuse Massif (French Alps), were surveyed periodically for 16 months using terrestrial laser scanning (TLS) to study the coupling between sediment dynamics and torrent responses in terms of debris flow events, which occurred twice during the monitoring period. Sediment transfer in the main torrent was monitored with cross-section surveys. Sediment budgets were generated seasonally using sequential TLS data differencing and morphological extrapolations. Debris production depends strongly on rockfall occurring during the winter–early spring season, following a power law distribution for volumes of rockfall events above 0.1 m3, while hillslope sediment reworking dominates debris recharge in spring and autumn, which shows effective hillslope–channel coupling. The occurrence of both debris flow events that occurred during the monitoring was linked to recharge from previous debris pulses coming from the hillside and from bedload transfer. Headwater debris sources display an ambiguous behaviour in sediment transfer: low geomorphic activity occurred in the production zone, despite rainstorms inducing debris flows in the torrent; still, a general reactivation of sediment transport in headwater channels was observed in autumn without new debris supply, suggesting that the stored debris was not exhausted. The seasonal cycle of sediment yield seems to depend not only on debris supply and runoff (flow capacity) but also on geomorphic conditions that destabilize remnant debris stocks. This study shows that monitoring the changes within a torrents in-channel storage and its debris supply can improve knowledge on recharge thresholds leading to debris flow.

Comparison of Methods and Procedures for Debris-Flow Volume Estimation

The estimation of debris-flow volume is a fundamental task for the design of any hazard mitigation intervention in a mountain channel. The estimation may be carried out before the occurrence of a debris flow through methodologies aimed at assessing the debris potential in the basin. It can be carried out through instrumentation deployed along the torrent (stage sensors, geophones, etc.). Finally, volume estimation may be performed after the occurrence of the phenomenon, surveying the deposits left in the channel, in a possible retention basin or on the fan. The surveys can be carried out through a conventional topographic survey, through a terrestrial laser scanner (TLS), by photogrammetric techniques, etc. Each method delivers approximated results, due to different reasons. A systematic comparison of the results of these different estimation procedures, carried out simultaneously in the same catchment, might help to better understand their advantages and limitations, to improve their output and to validate and standardize the respective methodologies. An European Territorial Cooperation project, named SedAlp, has been recently funded by the Alpine Space programme. An important focus of the project is on methods and data collection on sediment transport. In this context, one of the activities that is taking place in a pilot area consists in the comparison of the different methods and procedures for debris-flow volume estimation.