Geomorphological factors influencing hysteresis patterns between suspended load and flow rate in Alpine rivers

Suspended sediment load represents a large part of total solid fluxes transported in most rivers. Thus, for hydropower plan management or for environmental issues, it is crucial to understand how these sediments are produced, stored and transported in a given catchment. Hysteresis loops in discharge-suspended load signals are commonly used to assess sediment sources and production processes but most of the time the shape of this relation is analyzed qualitatively on short time series or for few events. In this study we analyzed quantitatively 10 long time series of suspended sediment load of various alpine catchments. This method allows us to compare events and to assess to which extent fine sediments originate from hillslope erosion processes or from river bed remobilization. We found that watersheds with braided bed morphology are dominated by clockwise loops while those with narrower bed as step-pool morphology are dominated by counter-clockwise hysteresis or have no general trend. These results suggest that storage and remobilization of fine sediments within the bed could play a major role in suspended sediment transport in Alpine streams, especially in large braided rivers

Geomorphological factors influencing hysteresis patterns between suspended load and flow rate in Alpine rivers

Suspended sediment load represents a large part of total solid fluxes transported in most rivers. Thus, for hydropower plan management or for environmental issues, it is crucial to understand how these sediments are produced, stored and transported in a given catchment. Hysteresis loops in discharge-suspended load signals are commonly used to assess sediment sources and production processes but most of the time the shape of this relation is analyzed qualitatively on short time series or for few events. In this study we analyzed quantitatively 10 long time series of suspended sediment load of various alpine catchments. This method allows us to compare events and to assess to which extent fine sediments originate from hillslope erosion processes or from river bed remobilization. We found that watersheds with braided bed morphology are dominated by clockwise loops while those with narrower bed as step-pool morphology are dominated by counter-clockwise hysteresis or have no general trend. These results suggest that storage and remobilization of fine sediments within the bed could play a major role in suspended sediment transport in Alpine streams, especially in large braided rivers

Interactions entre flux en suspension et lit d’une riviere alluviale alpine – cas de La Séveraisse

Suspended load often represents the most significant fraction of sediment load in rivers. The transfer of these fine particles produced on hillslopes and transported through the river system is poorly understood in Alpine rivers. However, fine particles transport is associated with social, economic, and ecological issues such as pollutant transfer, riverbed clogging, vegetation growth, or reservoir siltation. In this context, the objective of this study is to characterize the interactions between suspended load and the riverbed of La Severaisse (Ecrins Massif), a typical Alpine river. To this aim, we applied a short-term sediment budget approach based on high frequency suspended load measurements on a 3.5-km reach. In addition, we measured bedload transport through direct sampling. The results indicate that suspended load is significantly buffered in the studied reach during the two-month campaign. Moreover, conditions leading to the mobility of coarse particles were found to evolve concomitantly with conditions leading to fine particle mobilization. These observations bring new insights in understanding the transfer of suspended material in Alpine rivers

Sing und Spiel mit Uns CE1, Méthode de langue

International audienceMéthode d'enseignement de l'allemand en primaire (classe de CE1

Temporal variability of suspended sediment sources in an alpine catchment combining river/rainfall monitoring and sediment fingerprinting

International audienceInfluence of the rainfall regime on erosion and transfer of suspended sediment in a 905‐km$^2$ mountainous catchment of the southern French Alps was investigated by combining sediment monitoring, rainfall data, and sediment fingerprinting (based on geochemistry and radionuclide concentrations). Suspended sediment yields were monitored between October 2007 and December 2009 in four subcatchments (22–713 km$^2$. Automatic sediment sampling was triggered during floods to trace the sediment origin in the catchment.Sediment exports at the river catchment outlet (330 $\pm$ 100 t km$^{‐2}$ yr$^{‐1}$) were mainly driven (80%) by widespread rainfall events (long duration, low intensities). In contrast, heavy, local and short duration storms, generated high peak discharges and suspended sediment concentrations in small upstream torrents. However, these upstream floods had generally not the capacity to transfer the sediment down to the catchment outlet and the bulk of this fine sediment deposited along downstream sections of the river. This study also confirmed the important contribution of black marls (up to 70%) to sediment transported in rivers, although this substrate only occupies c . 10% of the total catchment surface. Sediment exports generated by local convective storms varied significantly at both intra‐ and inter‐flood scales, because of spatial heterogeneity of rainfall. However, black marls/marly limestones contribution remained systematically high. In contrast, widespread flood events that generate the bulk of annual sediment supply at the outlet were characterized by a more stable lithologic composition and by a larger contribution of limestones/marls, Quaternary deposits and conglomerates, which corroborates the results of a previous sediment fingerprinting study conducted on riverbed sediment