Author(s): Junjian Deng; Tea Piednoir; Lionel Penard; Benoit Camenen

Linked Author(s): Junjian Deng, Benoît Camenen

Keywords: Lpine river; Fine sediment stocks; Bed mobility; 1D numerical modelling; GIS

Abstract: Fine sediment is a fundamental component in the river system. Pollutants and nutrients can be transported by fine sediments. Fine sediment conditions also support good ecological status in different environments. Moreover, fine sediment transport can lead to security issues for hydroelectric buildings in river channel, i.e. sedimentation in reservoirs leading to siltation problems for hydropower plants; and also resulting in a loss of reservoir storage capacity. In alpine rivers, a large amount of fine sediments travels over the gravel-bed system as suspension and interacts with the gravel matrix. Recent studies highlight that fine sediment stocks in river bed can be a significant source of suspended load at event scale, has and can have a non-negligible effect on sediment budget estimation. However, there exists not proper estimation of fine sediment stocks in gravel-bed rivers, especially for the sand fraction. One can also question the potential spatial and temporal variability of these stocks, which makes the estimation of such source of sediments challenging. In this study, we intend to quantify fine sediment stocks in an alternate bar system (Arc and Isère Rivers) and estimate the potentially re-suspended fine stocks from the gravel bar matrix for different discharges. Fine sediment stocks in the gravel bars are firstly measured using a field protocol optimised from Misset et al. (2021). The measured fine stocks show a significant amount of sand present in the river bed, which is rarely even not measured in most studies. The evaluation of the total stocks of fine sediments is made by combining these local measurements to GIS analyse based on LiDAR data. Then, in order to predict the re-suspended fine stocks, a 1D numerical hydraulic model is used to calculate bed shear stresses on the bar surface and evaluate the thickness of the potential remobilized coarse sediments using the model proposed by Pugh & Wilson, (1999). Having the volume of sediments remobilized, one can evaluate the potentially re-suspended fine stocks for different discharges. The silt-clay part of the calculated re-suspended stocks is found equivalent to around 30% of annual Suspended Sediment Matter flux for a 15-year return period flood event during one day. However, this part of stocks represents only around 20% of the total fine stocks. Therefore, a large amount of sand could be re-suspended from the gravel bar matrix and should not be neglected when estimating the downstream sediment budget.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022681

Year: 2022