Incipient meandering and self-formed floodplains in experiments (original) (raw)
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Meandering rivers - feedbacks between channel dynamics, floodplain and vegetation
2013
Includes only chapter 1 and 7, see other chapters main page. 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.
Earth Surface Processes and Landforms, 2015
Hydraulic interactions between rivers and floodplains produce off-channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel-bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off-channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer-grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation.
Experimental evidence for the conditions necessary to sustain meandering in coarse-bedded rivers
surfaces, yet the conditions necessary to maintain meandering channels are unclear. As a consequence, self-maintaining meandering channels with cutoffs have not been reproduced in the laboratory. Such experimental channels are needed to explore mechanisms controlling migration rate, sinuosity, floodplain formation, and planform morphodynamics and to test theories for wavelength and bend propagation. Here we report an experiment in which meandering with near-constant width was maintained during repeated cutoff and regeneration of meander bends. We found that elevated bank strength (provided by alfalfa sprouts) relative to the cohesionless bed material and the blocking of troughs (chutes) in the lee of point bars via suspended sediment deposition were the necessary ingredients to successful meandering. Varying flood discharge was not necessary. Scaling analysis shows that the experimental meander migration was fast compared to most natural channels. This high migration rate caused nearly all of the bedload sediment to exchange laterally, such that bar growth was primarily dependent on bank sediment supplied from upstream lateral migration. The high migration rate may have contributed to the relatively low sinuosity of 1.19, and this suggests that to obtain much higher sinuosity experiments at this scale may have to be conducted for several years. Although patience is required to evolve them, these experimental channels offer the opportunity to explore several fundamental issues about river morphodynamics. Our results also suggest that sand supply may be an essential control in restoring self-maintaining, actively shifting gravel-bedded meanders.
Experimental rivers and deposits in a self-formed floodplain
"Meandering rivers have proven difficult to reproduce experimentally, while braided rivers are relatively easily formed in the laboratory. Our objective is to create self-formed dynamic braided and meandering rivers in a laboratory, and to understand and quantify the necessary and sufficient conditions for meandering. In addition, we have tested existing theories to reconstruct the original morphology from preserved stratification as stratification is the key to reconstruct formative channel dimensions and palaeoflow conditions from the fluvial record. The experiments were carried out in a flume of 6 m wide and 11 m long, which was split up into two separate fluvial plains (each 3x10 m). We designed experimental conditions that minimize the most important scaling issues. The morphological and stratigraphical evolution was recorded by high-resolution line-laser scanning and Digital Single Lens Reflex (DSLR) camera used for channel-floodplain segmentation, water depth approximation, silica flour distribution and particle size estimation. Key findings include that the experimental river without silica flour evolves from alternate bars to a multi-thread braided river. Addition of silica flour to the feed resulted in a single-thread meandering river with chute cutoffs. Large bends developed with scroll bar complexes and sinuosity reached a maximum of 1.4. Furthermore, we find that the mean set thickness agrees well with the theoretical prediction from channel morphology. The mean preserved set thickness is 30% of the mean channel depth. Finally, there is much systematic spatial variation in set thickness related to repetitive point bar growth and chute cutoff. We find undisturbed and thick sets close to channel belt margins and more irregular stratification with stacked thinner sets in the channel belt center."
Meandering-like river channel evolution due to the formation of fluvial bars
2019
Meandering has been studied as one of the most interesting processes among river evolution phenomena. Many theoretical studies have been done on the development of meandering. If a small sinusoidal disturbance is imposed to the planar shape of river channels, the curvature of the river channel causes a deviation of the flow towards the outer bank, and accumulation on the inner bank occur, and meandering makes further development under some conditions. Ikeda, Parker, Sawai provided a first theoretical explanation on the mechanism of meandering as a shear instability on both banks in terms of linear stability analysis. Afterwards, the influence of interaction and resonance with fluvial bars has been taken into account from the condition satisfying the continuity of sediment and the influence of secondary flow due to meandering has been included into the linear stability analysis. According to these theories, the development of meandering and the formation of sand bars interact with ea...
Chapter 6- Floodplain construction and destruction
Field studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers. However, it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering. This is needed because only then a dynamic equilibrium exists and channels maintain meandering with low width-depth ratios. Our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering. In this study we present two experiments with a self-forming channel in identical conditions, but to one we added cohesive silt at the upstream boundary. The effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2. The experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels. In contrast, in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel. Overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed; first, overbank vertical-accretion of silt, e.g. levee, overbank sedimentation or splays; and second, lateral point bar accretion with silt on the scrolls and in the swales. The first style led to a reduction in bank erosion, while the second style reduced excavation of chutes. We conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering.
Influence of floodplain erosional heterogeneity on planform complexity of meandering rivers
Geophysical Research Letters, 2011
1] Although it is widely acknowledged that spatial heterogeneity in floodplain erosional resistance should affect the planform evolution of meandering rivers, past studies have not systematically explored the importance of this effect. Using a physically-based model of river meandering and stochastically-generated heterogeneous landscapes, we analyze in detail how spatial variability in erosional resistance influences meander evolution and the emergence of planform complexity. The simulated planforms are remarkably similar to natural freely-meandering rivers, both visually and in their spectral signatures. We quantify the dependence of meander morphology on both the scale of erosional heterogeneity and the magnitude of erosional variability in external forcing. We also show how the sensitivity of autogenic meandering processes to stochastic variability in the environment leads to different patterns of planform evolution even in landscapes with the same mean spatial heterogeneity and magnitude of variability. The results demonstrate that heterogeneity in erosional resistance has a major influence on meander evolution and should be incorporated into morphodynamic models to enhance our understanding of meandering dynamics at the landscape scale. Citation: Güneralp, İ., and B. L. Rhoads (2011), Influence of floodplain erosional heterogeneity on planform complexity of meandering rivers, Geophys. Res. Lett., 38, L14401,
Experimental observations on the retreat of non-cohesive river banks
The topic of bank retreat is of particular relevance in the hydro-morphodynamic models aimed to the study of the planform evolution of rivers. In gravel-bed rivers, banks are often composite with an upper cohesive layer, and a basal layer composed by granular, relatively coarse sediments (gravel and cobbles). This research aims at gaining a better comprehension of some of the basic processes responsible for the retreat of coarse riverbanks; in particular, it focuses on the mass instability mechanisms due to the oscillations of water levels and pore water pressures, and on the particle erosion due to seepage flow. To this aim, a series of first laboratory experimental observations on the behavior of a bank mostly composed by gravel are here reported. The evolution of bank geometry from its initial configuration to the final one at the end of hydrograph is here reported and interpreted using both the measurements about the groundwater flow and mathematical modeling.