Spatial variability of the erodibility of fine sediments deposited in two alpine gravel-bed rivers: The Isere and Galabre

In mountainous environments, high suspended sediment load during runoff or dam flushing events can lead to important amounts of fine deposits in gravel bed rivers. Fine sediment deposits may contribute to bar elevation, riparian vegetation growth and consequently to bar stabilization. Despite their contribution to the morphodynamic of mountain rivers, the erosion properties of fine sediments in this context is not fully understood. In order to investigate the dynamics of re-suspension of these deposits, field monitoring campaigns were performed to explore both the spatial variability and the controlling factors of the erodibility of fine deposits. A cohesive strength-meter (CSM), along with moisture, grain sizes, geographical position and elevation were used to evaluate both the critical bed shear stress for erosion and erosion rate of fine sediment deposits in two rivers of the French Alps: the Ise`re and Galabre. The results highlight a large variety of fine sediment deposition areas, which are discontinuous compared to those in estuaries and lowland rivers. A high spatial variability of erodibility was observed on the reach, the bar and the metric scale. While no upstream-downstream trend was observed at the scale of both studied reaches, the locations of the deposits, elevation from the river surface and their moisture were inter-related variables and with the highest correlations to erodibility. Measurements showed that both dry and humid deposits located at the highest and lowest elevation from the river surface respectively, were more easily eroded than intermediate deposits with medium moisture.

Automated summary

High suspended sediment load in mountainous environments can lead to the deposition of fine sediments in gravel bed rivers, which affects the morphodynamics and stability of the rivers. The erosion properties of these fine sediment deposits are not fully understood, so field monitoring campaigns were conducted to investigate their dynamics and controlling factors. The results show spatial variability and inter-related variables, such as deposit location, elevation from the river surface, and moisture, that affect the erodibility of the fine sediment deposits.