Sediment dynamics in the subaquatic channel of the Rhone delta (Lake Geneva, France/Switzerland)

With its smaller size, well-known boundary conditions, and the availability of detailed bathymetric data, Lake Geneva's subaquatic canyon in the Rhone Delta is an excellent analogue to understand sedimentary processes in deep-water submarine channels. A multidisciplinary research effort was undertaken to unravel the sediment dynamics in the active canyon. This approach included innovative coring using the Russian MIR submersibles, in situ geotechnical tests, and geophysical, sedimentological, geochemical and radiometric analysis techniques. The canyon floor/levee complex is characterized by a classic turbiditic system with frequent spillover events. Sedimentary evolution in the active canyon is controlled by a complex interplay between erosion and sedimentation processes. In situ profiling of sediment strength in the upper layer was tested using a dynamic penetrometer and suggests that erosion is the governing mechanism in the proximal canyon floor while sedimentation dominates in the levee structure. Sedimentation rates progressively decrease down-channel along the levee structure, with accumulation exceeding 2.6 cm/year in the proximal levee. A decrease in the frequency of turbidites upwards along the canyon wall suggests a progressive confinement of the flow through time. The multi-proxy methodology has also enabled a qualitative slope-stability assessment in the levee structure. The rapid sediment loading, slope undercutting and over-steepening, and increased pore pressure due to high methane concentrations hint at a potential instability of the proximal levees. Furthermore, discrete sandy intervals show very high methane concentrations and low shear strength and thus could correspond to potentially weak layers prone to scarp failures.

Sedimentological processes in the Rhone Delta subaquatic canyons (France-Switzerland)

Nicolas Le Dantec

Deltas are very sensitive environments and highly vulnerable to variations in water discharge and the amount of suspended sediment load provided by the delta-forming currents. Human activities in the watershed, such as building of dams and irrigation ditches, or river bed deviations, may affect the discharge regime and sediment input, thus affecting delta growth. Underwater currents create deeply incised canyons cutting into the delta lobes. Understanding the sedimentary processes in these subaquatic canyons is crucial to reconstruct the fluvial evolution and human impact on deltaic environments, and to carry out a geological risk assessment related to mass movements, which may affect underwater structures and civil infractructure. Recently acquired high-resolution multibeam bathymetry on the Rhone Delta in Lake Geneva (Sastre et al. 2010) revealed the complexity of the underwater morphology formed by active and inactive canyons first described by Forel (1892). In order to unravel the sedimentary processes and sedimentary evolution in these canyons, 27 sediment cores were retrieved in the distal part of each canyon and in the canyon floor/levee complex of the active canyon. Geophysical, sedimentological, geochemical and radiometric dating techniques were applied to analyse these cores. Preliminary data show that only the canyon originating at the current river mouth is active nowadays, while the others remain inactive since engineering works in the watershed occurred. However, alternating hemipelagic and turbiditic deposits in the easternmost canyons, evidence underflow processes during the last decades. Two canyons, which are located close to the Rhone river mouth, correspond to particularly interesting deeply incised crevasse channels formed when the underwater current broke through the outer bend of a meander in the proximal northern levee. In these canyons, turbidites were observed in the sediment record indicating ongoing sediment dynamics during whether extreme flood events or mass-movements due to deltaic scarp failures. The active canyon shows a classic turbiditic system with frequent spillover processes in the canyon floor/levee complex. Geotechnical measurements, a decrease in the frequency of turbidites and a fining upward sequence along the levee suggest that erosion dominates sedimentation in the canyon floor, while sedimentation dominates in the rapid levee building-up process, with sedimentation rates that exceed 2.5 cm/yr in the proximal areas. Therefore, mechanisms controlling the sedimentary evolution on the active canyon represent a complex interplay between erosion and sedimentation.

2012

Triggering mechanisms and geomorphological implications of debris flows in subaquatic canyons: The Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

Subaquatic canyons in deltas are major pathways for the transport of particles from rivers / the upper delta slops to the deep basins. They represent active environments with frequent deltaic failures and massmovements potentially bearing hazard-related (tsunami waves) and economic (infrastructure damages) implications. Understanding sedimentary processes and mass-movement triggering mechanisms is crucial to assess related consequences and to carry out geological risk assessments. The Rhone River delta in Lake Geneva (Switzerland, France) is a complex underwater structure with several active and inactive subaquatic canyons, similar to marine deltas but at a smaller scale. The difference between two bathymetric surveys in 1986 and 2002 revealed an inversion in the topography of the distal active canyon, as a former distal canyon was transformed into a mound-like structure. A 12 m-thick layer was deposited in the canyon and modified the sediment transfer conduit. Sediment cores from this deposit were retrieved in-situ in 2002 and 2011 via the “F.-A. Forel” and Russian MIR submersibles, respectively. These cores contained a homogeneous, sandy material. Its sediment texture, grain-size, high density and shear strength, and low water content suggests that it corresponds to a debris-flow deposit that possibly took place after the initiation of a mass movement due to a scarp failure in proximal areas of the canyon. In addition, in situ geotechnical tests on the modern canyon floor have shown a soft top layer above a stiffer substratum. This soft layer, which increases in thickness towards distal areas, may act as a basal surface for hydroplaning, and might have allowed the debrite to be transported 9 km away from the source of the scarp failure. Similarities in textures and grain-size of the debris flow and levee deposits hint at the proximal northern levee as the source of this material. Rapid sediment loading in this area, at the rate of >2.5cm/yr, steep slopes in the canyon walls and increased pore pressure due to high methane concentrations may have reduced the stability of the canyon wall in this area. Discrete sandy intervals show very high methane concentrations and thus could correspond to potentially weak layers prone to scarp failures. Nevertheless, the probable cause for the 2000 AD Rhone delta event was an exceptional flood in October 2000 which undercut the slope, and subsequently decreased the stability by increasing the shear stress and triggered the mass failure in the already unstable canyon walls. Besides economic and hazard-related implications, such mass failures represent significant and underestimated causes in morphological evolution of underwater canyons by damming the channel and, eventually, forming short-term meanders susceptible to further erosion, as seen in a recent multibeam bathymetric map obtained in 2012.

2012

Sedimentary processes and triggering mechanisms of debris flows in subaquatic canyons in Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

Subaquatic canyons in deltas are major pathways for the transport of particles from land to the deep basins. They represent active environments with frequent deltaic failures and mass-movement deposits potentially leading to hazardous (tsunami waves) and economic (infrastructure damages) implications. Understanding sedimentary processes and mass-movement triggering mechanisms is crucial to assess related consequences and to carry out geological risk assessments. The Rhone River delta in Lake Geneva (Switzerland, France) is a complex underwater structure with several active and inactive subaquatic canyons, similar to marine deltas but at a smaller scale. The difference between two bathymetric surveys in 1986 and 2002 revealed an inversion in the topography of the distal active canyon, as a former distal canyon was transformed into a mound-like structure. A 12 m-thick layer was deposited in the canyon and modified the sediment transfer conduit. Sediment cores from this deposit were retrieved in-situ in 2002 and 2011 via the "F.-A. Forel" and Russian MIR submersibles, respectively. These cores contained a homogeneous, sandy material. Its sediment texture, grain-size, high density and shear strength, and low water content suggests that it corresponds to a debris-flow deposit that possibly took place after the initiation of a mass movement due to a scarp failure in proximal areas of the canyon. In addition, in-situ geotechnical tests on the modern canyon floor have shown a soft top layer above a stiffer substratum. This soft layer, which increases in thickness towards distal areas, may act as a basal surface for hydroplaning, and might have allowed the debrite to be transported ~9 km away from the source of the scarp failure. Similarities in textures and grain-size of the debris flow and levee deposits hints at proximal northern levee as the source of this material. Rapid sediment loading in this area, at the rate of >3cm/yr, steep slopes in the canyon walls and increased pore pressure due to high methane concentrations may have reduced the stability of the canyon wall in this area. Discrete sandy intervals show very high methane concentrations and thus could correspond to potentially weak layers prone to scarp failures. Nevertheless, the probable cause for the 2000 AD Rhone delta event was an exceptional flood in October 2000 which undercut the slope, and subsequently decreased the stability by increasing the shear stress and triggered the mass failure in the already unstable canyon walls. Besides economic and hazardous implications, such mass failures represent significant and underestimated causes in morphological evolution of underwater canyons by damming the channel and, eventually, forming short-term meanders susceptible to further erosion.

2012

Sedimentary processes and triggering mechanisms of debris flows in subaquatic canyons in Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

2012

Triggering mechanisms and geomorphological implications of debris flows in subaquatic canyons: The Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

2012

Sedimentological processes in the Rhone Delta subaquatic canyons (France-Switzerland)

Nicolas Le Dantec

2012