Alternate gravel bars usually appear in embanked gravel-bed river channels regulated by dams, causing safety risks to hydraulic structures and modifying fluvial ecology. Managing bars remains challenging and crucial for a river restoration project. Gravel replenishment is one measure to mitigate sediment deficits caused by dam regulation, while its efficiency in embanked rivers with bar morphology remains still unclear. We conducted a laboratory experiment to investigate the morphological effects of single high-flow stockpile replenishment after a flood event in a straight embanked channel characterized by steady hybrid bars. Results showed that erosion of the stockpile and sediment wave propagation mainly occurred during the high-flow period. The stockpile was laterally more eroded when the upstream sediment supply was maintained, while the sediment wave propagated further when the sediment supply was eliminated. Topographic and flow velocity data showed bar migration and suppression triggered by the forcing of the implemented stockpile during high flows regardless of upstream sediment supply during the flood. After the flood, bar morphology reappeared with maintained upstream sediment supply, while a single-thread incised channel was observed without the upstream supply. These observations suggest that sediment replenishment, combined with different upstream dam release manipulations, can strongly modify the initial bar morphology of an embanked river the efficiency of which also depends on upstream dam operations during high flows. This experiment also benefited from a detailed data acquisition plan and analysis, providing a dataset suitable for constructing and calibrating a 2D numerical morphodynamic model for future sediment replenishment design.

Clogging of gravel-bed rivers is a major issue for fish and macro-invertebrate habitats as well as for groundwater-river exchanges. River clogging consists mainly of the deposition and infiltration of fine sediments within the bed matrix, which is a natural phenomenon but can be enhanced by human activities. Although there are several methods for estimating the degree of clogging, quantitative assessment of clogging remains challenging, not to mention the lack of comparison between these methods. We therefore implemented three quantitative methods for estimating clogging (bed material sampling, infiltration test, and interstitial water sampling), and assessed their effectiveness and suitability for application as a long-term monitoring solution. These methods were applied to a natural river bed characterised by high spatial heterogeneity of clogging. The results show good correlations between the different methods despite some scatter that can be linked to limitations of some methodologies in term of sampling itself or sampling depth. Indeed, sampling of the bed material using a McNeil sampler is limited to the first 15 cm below the surface, whereas the infiltration test or interstitial water pumping using a standpipe can be achieved for a depth deeper than 15 cm only. The infiltration test appears to be effective for relatively low clogging, while interstitial water sampling is more efficient for high clogging. The results also show that dissolved oxygen estimation alone may not be sufficient to accurately estimate physical clogging, particularly for cases of low clogging. However, it appears to be a good and simple complementary method to the interstitial water-sediment pumping method. The infiltration test and interstitial water-sediment sampling are simple, quick and easy to apply methods. The two methods are complementary, and suitable for a spatial clogging assessment, with the possibility of investigating a wide range of clogging degrees.