Fine sediment dynamics over a gravel bar. Part 2: Impact of hydro-meteorological conditions

In this study, the temporal evolution of fine sediment deposits on a gravel bar is analysed during a two-year period. The time-series of fine deposit surface area is obtained using the methodology proposed in Part 1 of this work. By combining the hydro-meteorological data, the main evolutions of the fine deposits observed on the bar surface are found to be triggered by events such as natural floods, dam flushes, and strong wind valley breezes, and are quantified for each of these events. A simple processed-based flow model is proposed to investigate the sediment dynamics during the flood events, which balances the erosional/depositional processes of fine sediments. Variability in the fine deposit evolution after floods is mainly due to the different flow and sediment concentration conditions. For small events for which the gravel bar is not totally submerged, a slight erosion generally occurs. For larger events for which the gravel bar is totally submerged, a large net deposition is often observed. The fine deposit surface area time-series also shows sharp decreases of the surface deposit coverage after some strong valley breeze events, with an evolution of the same order of magnitude than floods. The application of a wind transport model shows that strong wind valley breezes can transport fine particles from the bar surface to the river channel, thus leading to a loss of fine deposits on the bar surface. Also, the deposit initial moisture is a crucial element having impact on the fine sediment transport on the bar surface. Dry deposits are much more likely to be transported than wet deposits. A discussion is also provided on the potential impact of wind and vegetation on the long-term dynamics of fine sediment deposits.

Automated summary

This study analyzes the temporal evolution of fine sediment deposits on a gravel bar over a two-year period. It finds that the main changes in deposits are triggered by natural floods, dam flushes, and strong wind valley breezes, and quantifies these changes for each event. A simple flow model is proposed to investigate sediment dynamics during flood events, accounting for erosional and depositional processes. The study also discusses the impact of wind and vegetation on the long-term dynamics of fine sediment deposits.