Author(s): Shashank Gupta; Celine Berni; Benoit Camenen

Linked Author(s): Shashank Gupta

Keywords: Forced alternate bars; Unsteady flow; Gravel bar morphodynamics

Abstract: Alternate bars are wave patterns, of which the crest and trough alternate between the banks of the river. They often appear in the rivers characterized by a significant sediment input and largely constrained by embankments. Their wavelengths are generally of the order of several river widths and heights scale with the order of water depth, but both differ depending on bar mobility. Forced alternate bars are non-migrating and formed due to distortion in the bank line geometry, such as a river bend or a bridge pier along the river cross-section. After their formation in the river, they alter local water levels and sediment transport and induce side bank erosion. Predicting the geometrical features of forced alternate bars and understanding the change in those features due to discharge variability in the rivers are crucial for river managers to maintain rivers. In particular, it is difficult to know what is the formative discharge of such large scale bed-forms. To understand the dynamics of forced alternate gravel bars under unsteady flow, first, we developed a reference state of two forced alternate gravel bars with the water worked bed methodology in our 18-meter long and 0.7-meter wide inclined flume. Forced alternate bars were formed through sequential cycles of transport, erosion and sedimentation, which is a way to reproduce the surface and subsurface properties of gravel-bed rivers. Secondly, two types of experiments were performed on the reference state by launching a flood event (unsteady flow) with and without upstream sediment supply. In the first experiment, when the sediment was supplied continuously from upstream to maintain the bed slope, we found that the forced alternate bars disappeared at the peak of the flood event. But, they reemerged with different bar lengths during the fall of the flood event. In the case of no upstream sediment supply, the bed slope decreased due to global erosion of the bed, but eventually, similar behavior is observed in the bar lengths at the end.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022231

Year: 2022