Poster: Experimental Meandering: From braided towards meandering by addition of cohesive floodplain material (original) (raw)
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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.
Braided rivers: perspectives and problems
Geological Society, London, Special Publications, 1993
Progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems. Although many advances have been made within recent years in interpreting the mechanics of flow, transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid bar initiation; confluence-diffluence dynamics, the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration.
From nature to lab: scaling selfformed meandering and braided rivers
Proc. River Flow, 2010
Traditionally, rivers were downscaled to the laboratory through similarity of the Froude, Shields and Reynolds numbers. This has worked well for rivers with fixed banks and for braided gravelbed rivers. For self-formed dynamic meandering rivers in experiments, Froude scaling is incomplete without a constrained width-depth ratio. This aspect ratio should be small enough to obtain alternate bars and bank erosion should somehow be limited. Our objective was to develop a scaling and design strategy for experimental meandering that includes bank strength so that width-depth ratio, bar pattern and channel dynamics can be designed. Scale effects of water surface tension and of smooth boundaries were inferred from first principles. Bank strength cannot be predicted well and was experimentally evaluated with two fast, repeatable experimental setups for a range of sediments with additives and vegetation. With selected sediments, we produced moderately dynamic meandering rivers on a 1.25x7.5 m flume. A sediment mixture ranging from silt to fine gravel produces richer morphodynamics and less scale effects with adjustable slightly cohesive banks and self-formed floodplains. Width-depth ratio reduced and floodplain sedimentation prevented that chute cut-offs led to braiding. Different vegetation species and controlled conditions allow adjustable growth rate, bank strength and hydraulic resistance. With the appropriate theory and our preparatory experiments, a range of dynamic river patterns can now be designed for the laboratory with the essential characteristics of their natural counterparts.
Braiding and meandering parameters
Geological Society, London, Special Publications, 1993
Modifications to standard definttrons of brarding and meandertng are proposed to indicate the morphology of every rrver channel reach quantrtatrvely, whether tt_has srngle channel or multiple chinnels. Srnuosrty (P) rs defined as, P = L, ^'lLR, where L".", ts the length of the midline of the channel (rn single-channel rivers), or the wldest channel (inlnulti-channel rivers). and Lp is the overall length of the reach' Braiding rs a measure of channel multiplicity and a new term 'brard-channel ratlo' (B) has been defined as, B= L.rorlL" o,, where L.,o, is the total of the mrd-channel lengths of all the channels tn a reactt.'Anoiiiir e"p."ssion for the brard-channel ratto which gives the same numerical rCsult iS, B= P"rorl'P, where the total sinuosity, P.,o,= L",o1/Lp, and P has been defined above. Scatter pio$ on braid-channel ratlo/slnuoslty axes show a negatlve correlatton between these parameters, as would be expected from the above relatlonshrps. Stnglechannel rivers (-B = l) have relatrvely hrgher srnuosrtres and the upper hmrt of these rs the point at which cutoff becomes hrghly probable. For multi-channel rivers (A > I), sinuoslty iemains low, reflecting the limrtrng effect of braid bars on the development of fully dcveloped spiral seconciary flow. Data analyses showing that increasesin channel slope and bankfull diiharge are associated with changes from meandering to braided morpiology are diverting attention from the importance of the rncreased avarlability of bed-load-grade sediment as a control.
Braid-scale channel geometry changes in a sand-bedded river: Significance of low stages
Geomorphology, 2007
This study focuses on the importance of different stages of bed formation and transportation processes along the partly managed lowland section of Maros River (South Hungary). The role of different stages was determined, applying a series of cross-sectional data. Besides, major morphological zones of over-widened, braided sections were defined, along with their role and function in sediment storage and transport processes.
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.
Water Resources Research, 2009
1] The number of bars that form in an alluvial channel cross section can be determined from a physics-based linear model for alluvial bed topography. The classical approach defines separators between ranges in which river planform styles with certain numbers of bars are linearly stable and linearly unstable. We propose an alternative method that is easier to apply. Instead of defining separators between stable and unstable conditions for certain river planform styles, the method directly estimates the most likely number of bars. It is based on a demonstration that conditions of zero spatial damping in a linear model for steady bars are representative for the bar mode that develops inside a river channel. We argue that a method based on steady bars is more appropriate for real rivers than a method based on free migrating bars. We verified the method by applying it to several existing rivers at bankfull conditions. The results are good for width-to-depth ratios up to 100 but deteriorate for higher width-to-depth ratios. We explain the deficiencies for large width-to-depth ratios from the linearity of the model. The results show that our method can be used as a reliable predictor for whether reducing or enlarging the width of a river will lead to a meandering, transition, or braided planform.