Clogging of riverbed substrates by fine sediment

Riverbeds represent the habitat of numerous aquatic species. Exchanges between the groundwater, the hyporheic zone and the surface flow are also essential for river ecosystems. Fine sediment transported by rivers deposits inside or on top of the bed and modifies the characteristics of the substrate resulting in a large variety of conditions. However, the increase of fine sediment yield associated with anthropic activities and the modification of river geometry and flow regime have led to an increase of the areas clogged by fine sediment. These areas are characterized by a low permeability, which reduces exchanges with the hyporheic zone, and is detrimental to the ecosystem. In the last decades, an increasing effort has been made to restore the ecosystem of rivers, by giving more space to rivers, restoring bedload and providing suitable habitat for aquatic species. The clogging of riverbeds has been subject of numerous studies trying to understand the complex interactions between the riverbed substrate, fine sediment, surface and hyporheic flow and biological components. The present research first focuses on the state of the art, with a broad review of the physical clogging process. It revealed in particular that more research was needed to understand the role of the infiltration flow and the link between permeability and the depth of clogging. Also, the clogging process under various flow conditions, including riverbed mobilization, was not sufficiently documented. Laboratory experiments were conducted to analyze in detail the clogging process of silt-size particles in a substrate composed of sand and gravel. The permeability and vertical distribution of fine sediment in the substrate were both analyzed, partly through the use of a new general clogging model, to understand the influence of (1) the substrate and fine sediment grain-size distributions, (2) the percolation gradient, and (3) the surface flow conditions. In addition, the effects of riverbed mobilization and variable flow conditions on the clogging process have been analyzed by reproducing scenarios of reservoir sediment flushing. The results show that the clogging process inside the substrate can be reproduced by combining filtration equations, and using a retention factor for the clogging depth. The limit between surface clogging and inner clogging was also analyzed. Finally, it was observed that clogging can still take place in the presence of riverbed mobilization, but its characteristics differ significantly from the clogging in the absence of bedload. These outcomes allow for a better understanding and estimation of the clogging process evolution in time and its extent in the substrate under various conditions. This research also shows that parameters like the substrate composition can have a major influence on the process. Future challenges will consist in the development of strategies that allow a reduction of the impact of clogging, combined with further investigations that link clogging with the wider dynamic of riverbed morphology.