Meandering rivers - feedbacks between channel dynamics, floodplain and vegetation

Abstract

Rivers have distinctive channel patterns such as multi-channel braiding and single-channel meandering. Why these different river patterns emerge is only qualitatively understood. Yet, we have not been able to retain dynamic meandering in laboratory experiments. The main objective of this thesis was to develop experimental settings in the laboratory that lead to dynamic meandering, in order to determine how rivers self-organize their morphology through interactions between channels, floodplain and vegetation. I unraveled these interactions in a series of controlled flume experiments. I found that classical similarity scaling as used in engineering scale models does not work well because the channel width-depth ratio that determines bar formation is a dependent parameter in self-formed channels. Therefore, I relaxed the scaling rules. Systematic small-scale tests showed that there is a narrow range of suitable conditions for continuous meandering regarding sediment mobility and bank strength. Initially, in a straight channel with a low width-depth ratio alternate bars develop and evolve into meander bends. An upstream perturbation is required to initiate bar formation, but the experiments showed that a static perturbation led to low amplitude bends. In contrast, a transversely moving inlet point caused high-amplitude dynamic meandering with scrolls bars and infrequent chute cutoffs. Floodplain development further defined the resulting river pattern. In experiments with only channel sediment, overbank flow led to chute cutoffs that terminated meander bend development and that were the onset of a braided river with multiple parallel channels. Bank erosion by bend migration is balanced by deposition of sediment forming new floodplains and cutoffs, as also confirmed by numerical modeling. I showed that vegetation and fine sediment on the floodplain decreased the number of chute cutoffs, allowing meander bends to grow further. The fine sediment adds cohesion to the floodplain and increased bank strength, while plants increased bank strength and roughness. The combination of flume experiments and numerical model results shows that channel dynamics are controlled by bankfull discharge conditions, but overbank flow is highly important in constructing and destroying the floodplain. The composition of the floodplain, e.g. cohesion of the banks and the presence of vegetation, determines whether a braided or a meandering river pattern develops.