Scroll bar formation in experimental meandering rivers
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Even though scroll bars are often observed in meandering rivers, it is still not evident what exactly causes their formation. According to several studies on natural rivers, scroll bars only form when there are discharge variations and therefore pulses of outer bank erosion. Otherwise a flat point bar would develop as a continuous process. However, numerous flume experiments have reported on scroll bar formation while discharge is constant. Also, there are observations in natural rivers that sedimentation on the inner bank causes outer bank erosion. The objective of this research is to determine if erosion of the outer bank or variation in sediment supply is the leading process for the development of scroll bars. In order to study the development of scroll bars while controlling the boundary conditions, an experimental meandering river was created in a 3x11m flume. Eight cases were performed in developed bends, which included the addition of sediment pulses and removal of part of the outer bend. Sediment pulses were added in order to determine if variations in sediment supply from upstream causes individual scroll bars to form and whether inner bend accretion is the dominant process in bend migration. This was done in an already migrating bend and in a bend that did not migrate before the perturbation. Part of the outer bend was removed several times in order to determine if outer bend erosion is the dominant process leading to scroll bar formation and bend migration. This corresponds to discontinuous outer bank erosion in nature. Also, in one bend the outer bank was fixated, in order to see if scroll bars would still develop if there is no erosion of the outer bend . In nature this corresponds to a meander bend with a high bank strength. In all cases the accretion of the inner bend and the erosion of the outer bend were monitored and flow velocities were measured. Scroll bars were observed in all bends that migrated and were formed by multiple bedforms that were deposited by secondary currents. Channel width increased downstream of the apex of these bends, causing flow velocities to decrease. Therefore, scroll bars were formed at the downstream end of a bend. Forced sediment pulses did not cause individual scroll bars to develop and had no significant effect on flow velocities and outer bend erosion. In the bend that was fixated, eventually no sediment was deposited on the point bar anymore and therefore also no scroll bars formed. However, each removal of part of the outer bend caused flow velocities to decrease at the inner bend due to the increase in width, which caused a scroll bar to develop. Erosion of the outer bend is thus the leading process in scroll bar formation and channel width variations are essential to explain sediment deposition and meander bend migration. Further research can be done on scroll bars in relatively sharp bends where flow separation zones are present. Also, in several natural rivers relatively wide scroll bars are observed, which differ with the scroll bars in the experiment.