An attempt to link suspended load hysteresis patterns and sediment sources configuration in alpine catchments

Abstract

A large part of total solid flux is transported as suspension in mountainous rivers. It is crucial for water resource management and for environmental issues to be able to model and to understand these fluxes. However, suspended load is known to be highly variable in time and space, as fine sediments can originate from various erosion processes and from various sources. Among the different methodologies available for analyzing the suspended sediment flux dynamics, hysteretic loops in discharge and suspended load signals are commonly used to assess sediment sources and production processes. However, the shape of these loops is often analyzed qualitatively for a single or a small number of catchments. Hence it is still unclear how the geomorphological catchment properties influence the variability of the flow rate - suspended sediment concentration relationship through the hysteresis effects. This is particularly true in mountainous catchments where important sources of fine sediments may originate from the river bed in addition to hillslopes.In this study we analyzed quantitatively ten long-term series of high-frequency observations of suspended sediment load measured in contrasted alpine catchments. Hysteresis effects were analyzed in a high number of automated sampled events and the dominant response for each catchment was sought. This was done by using a normalized hysteresis index developed by Lloyd et al. (2016), which we weighted by the mass transported during each event. The various catchments were characterized with a normalized geomorphological index expressing the relative importance of sediment sources originating from the river bed or from eroded areas as a function of the distance to the outlet of the catchment.The dominant hysteresis response of the ten alpine catchments studied was found to be greatly linked to their geomorphological index. These results suggest that the sediment source configuration upstream of a measuring station drives hysteresis effects and thus the variability of the flow rate-suspended sediment concentration relationship.