Alternate bars manifest as wave-like patterns in which the crest and trough cyclically alternate between the opposing riverbanks. They often appear in the rivers characterized by a significant sediment input and largely constrained by embankments. The wavelengths of these bars typically correspond to several times the river's width, while their heights are proportional to the water depth, although variations occur depending on the mobility of the bars. Forced alternate bars represent a unique category of alternate bars characterized by their stationary nature, originating from distortions in the bank line geometry, such as river bends or bridge piers within the river's cross-section. Subsequent to their formation within rivers, these bars effectively capture fine sediments, including clay, silts, and sands. The accumulation of these fine sediments contributes to gravel bar aggradation and fosters the development of riparian vegetation. This evolution can potentially increase flood risk in the medium to long term. In this context, the current study is focused on elucidating the impact of fine sediment infiltration, particularly silt, within a forced alternate gravel system. This understanding is pursued through a series of laboratory experiments conducted in an 18 m long and 70 cm wide flume. As a first step, we established a reference case where forced alternate gravel bars were created under steady flow conditions. These bars were formed through successive cycles of transport, erosion, and sedimentation. Subsequently, in the second step, forced alternate bars were developed while introducing suspended fine sediments under steady flow conditions. The patterns of fine sediment infiltration observed within the forced alternate bars indicated that the tail end of the bars displayed a relatively higher level of clogging when compared to the bar heads and the central region of the bars. These findings bear similarity to the patterns of fine sediment infiltration observed in alternate bars formed in the Arc and Isère rivers. Moreover, the forced alternate gravel bars developed in the presence of fine sediments display significant distinctions from the reference case in the majority of the experiments conducted. Furthermore, we extended our investigation to examine the impact of the initial bed slope on the development of forced alternate gravel bars, both with and without the presence of suspended fine sediments, in comparison to our reference case. Additionally, we compared the geometric characteristics of the developed forced alternate bars, with and without fine sediments, to two existing theoretical models of forced bars found in the literature. This comparison highlighted the limitations of these models. In the third step, we conducted unsteady flow experiments, both with and without fine sediments. These experiments were designed to investigate various aspects, including the impact of unsteady flow on the dynamics and formation of forced alternate gravel bars; the potential influence of the initial bed topography on the formation of forced alternate bars; the effects of two successive flood events on the forced alternate gravel bar system, among other factors. Specifically, in the unsteady flow experiment conducted over the reference case, where a constant gravel supply rate was maintained throughout the experiment, our findings indicated that the first forced gravel bar became flatter and shorter, and the second forced bar vanished as the water discharge approached its peak. Subsequently, during the falling limb of the hydrograph, the two forced alternate gravel bars redeveloped, each with different shapes.

Cite the thesis

Shashank Gupta. Laboratory experiments on fine sediment erosion and deposition in an alternate bar system and its impact on bar morphodynamics. Fluid mechanics [physics.class-ph]. Université Claude Bernard - Lyon I, 2023. English. ⟨NNT : 2023LYO10353⟩. ⟨tel-04987457⟩

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