Postglacial rockfalls controls on environmental partitioning of the Haute-Romanche valley (Massif des Écrins, french Alps)
Résumé
The Haute-Romanche valley, located in the Massif des Écrins, was widely glaciated during the LGM. Nowadays, the Romanche river is an effluent of two separated glaciers: the Clot des Cavales and the Plate des Agneaux. From the current glaciers fronts to the downstream neoglacial moraines, five successive rockfalls were identified. Their locations are probably linked to preferential areas of postglacial debutressing expression. These large rockfalls produce a semi-permanent environmental partitioning of the valley by controlling sediment flux and fluvial patterns. The objectives of the study are to quantitatively assess the partitioning functions of rockfalls of various volume on the streams long profiles, the downstream fluvial pattern succession and the spatial distribution of archaeological evidences. The topographic surveys of the Haute-Romanche valley floor and rockfalls were done using terrestrial multi-image photogrammetry coupled to a set of DGPS ground control
points. The long profiles of the rivers were achieved with DGPS survey. Volumes of rockfalls were estimated by topographic modeling and volumetric computation using GIS software resources. Fluvial pattern analysis and archaeological mapping were completed based on photo-interpretation and field observations. Results show that river channel morphology is highly dependent of rockfall location and volume. Consequently, the downstream river patterns succession is deeply disturbed inducing a long-term segmentation of the fluvial continuum. This longitudinal partitioning of hydro-sedimentary dynamics has potentially major ecological, geoarchaeological and management implications.