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Etude géomorphologique de la dynamique sédimentaire de torrents à lave (Alpes Françaises)

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. The purpose of this research was to investigate the coarse sediment transport through steep catchments and how channel storage can influence debris-flows. This required intensive field-based geomorphic monitoring of flow events in the Manival and Réal torrent catchments which can experience debris-flows and bedload transport every year.In the Manival Torrent, the sediment transfers were characterized at a seasonal time scale by a complete sediment budget of the catchment derived from multi-date topographic measurements between important flow events (cross-section surveying and terrestrial laser scanning). Sediment budget reconstitution of two debris-flows revealed that most of their 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 magnitudes is proposed.In the Réal Torrent, post-event surveying and high-frequency monitoring stations were used to compare and compile measurements of flow events. Three debris-flow events and three periods of bedload transport with small headwater debris-flows were observed. Sediment transport volumes for debris-flows were very similar to the Manival with important volume growth in the channel. The largest observed debris-flow revealed a downstream decrease of maximum flow heights, shear stress, velocity, and flow resistance. We hypothesize that the debris-flow front scours and destabilizes the channel, but it cannot transport the material because of its high sediment concentration. Therefore, the trailing hyperconcentrated surge picks up the remaining material, grows in volume, and coalesces with the decelerating debris-flow. Both the front and following surges play an integral role for net erosion during a debris-flow event.Multi-date cross-sections in the Manival and Réal have shown that debris-flows have significant scouring with large spatial variability. Bedload transport was observed to be at equilibrium with little variability. Field observations of channel deformations show that debris-flow scouring is strongly controlled by upstream slope and storage conditions. A logarithmic relationship is proposed as an empirical fit for the prediction of channel erosion. The most susceptible materials for erosion in the Manival are the unconsolidated gravel wedges developed from bedload transport. This material has a smooth surface within the rugged channel which can be automatically mapped with a 20 cm digital elevation model from either terrestrial or airborne laser scans by calculating roughness with a one meter window. This provides an automatic assessment of erodible areas in a channel at the time of the laser scan survey.This study has contributed to the need of quantitative field observations in the realm of debris-flow research. Complete and thorough databases were obtained by integrating multi-date cross-section surveys, multi-date laser scans, and high-frequency monitoring stations. Quantified evidence revealing sediment transfers, channel interactions/controls, debris-flow dynamics, and storage characterizations in two different catchments provides a strong basis in the development of conceptual and statistical models. These observations also highlighted the significant field parameters that have an influence on debris-flows and steep catchment systems.
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Joshua Theule. Etude géomorphologique de la dynamique sédimentaire de torrents à lave (Alpes Françaises). Sciences de la Terre. Université de Grenoble, 2012. Français. ⟨NNT : 2012GRENU021⟩. ⟨tel-00864986⟩

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