Multivariate characterization of meandering

2020

The development and evolution of meandering channels has long held a fascination for scientists of many disciplines. Researchers have used field, archival and modelling sources to investigate the dynamics and long-term behaviour of meandering channels to help understand and predict future changes. There is still a great deal of uncertainty about why different meander bends are active and why similar bends are stable. Questions also remain about how the impact of change on one part of the river is propagated upstream and downstream. These questions are important as dynamic rivers can be challenging for human settlements, but also provide an opportunity to increase biodiversity and habitat opportunity in the riparian zone. The aims of this thesis are to explore the variability of meander migration in space and time for two actively meandering rivers in the UK. The long-term evolutionary behaviour of individual bends is explored using historic Ordnance Survey maps. Finally, a data mode...

Water Resources Research, 2014

The continuous wavelet transform is applied to the analysis of curvature signals from both synthetic meanders and 52 realizations from 16 natural meanders ranging from class B to class G (Brice classification), thus providing information on the spatial distribution of their arc-wavelength spectrum, and therefore, representing an objective characterization of meanders. Past research has studied the meander dynamics by using the centerline (short-term frame) and the valley centerline (long-term frame). The present study introduces a medium term frame, termed the mean center (MC), which is defined as the medium term coherent wave being present in the meander planimetry for a period that is strongly governed by the occurrence of cutoff events; although in the absence of them, it is present for $10 to $30 years. The MC is obtained by using a methodology that combines the capabilities of the principal component analysis and the discrete wavelet transforms. The application of wavelet cross correlation shows that peaks in the centerline curvature are strongly correlated with those of the MC suggesting that (1) a linear relationship between them may be associated to bank processes and, (2) in all other cases, a higher nonlinear relationship may be induced by autogenic hydrodynamic processes. In freely meandering rivers, compound bends, multiple loops, and cutoff events are associated to peaks in the MC local curvature. We define the planform amplitudes as the orthogonal distance of the centerline from mean center. Planform amplitudes (orthogonal distance of the centerline from mean center) are normally distributed and ranges from 2 to 20 river mean widths.

2008

The complex feedback between process and form that governs planform migration of meandering rivers is still incompletely understood. Current theoretical models aimed at predicting planform migration relate the rate of meander migration at a particular location to the channel curvature at and upstream of that location. However, these models are still incapable of reproducing complex forms of bend development, such as compound loops. Evaluation of such models requires a representation of planform curvature better than that provided by traditional methods where the curvature is calculated from directional change between the successive digitized points-an approach that provides a discrete representation of curvature dependent on the density of the digitized points used to characterize the channel planform. This article presents and evaluates a methodology that provides a continuous functional representation of the planform geometry and curvature of meandering rivers. The method involves the fitting of splines, specifically parametric cubic splines (PCSs), to discrete digitization data of the channel centerline (CL) and the arc-length parameterization of the resulting composite curve. The arc-length parameterized PCS-interpolated curve is then used to compute analytically the channel CL location and curvature for any value of the streamwise axis. Evaluation of the method involves examination of the effects of digitization interval size and digitization error on the characterization of the planform geometry and curvature using PCS fitting. The derivation of curvature values from continuous planform function for any location and any spatial interval overcomes the reliance either on average bend curvature values or on discrete curvature values based on digitized points.

2002

The morphodynamics of the lower River Vecht in the Netherlands and the influence of geomorphological setting and bank composition on meander migration was studied by means of reconstructing the pre-channelisation landform configuration on a scale of 1:25,000, using historical maps from 1720, 1850 and 1890 and other data. A downstream sequence of reaches was observed, each with a typical fluvial style and channel migration rate: (A) a narrow meander belt and a highly sinuous channel with intermediate migration rate, in the middle of an extensive floodbasin, (B) a wide meander belt and high rates of lateral channel migration, especially where large meanders impinged upon valley bluffs, as part of an incised setting, (C) a low sinuosity, embanked channel with low rates of downstream migration confined by dikes, occurring in an inland delta with sandy sediments. Local variation was observed within reach B. Different migration rates were caused by the spatial variability of bank resistan...

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,

Sediment Transport, 2011

have limited widths and lengths. The flow and the hydraulic properties of the meandering river, the amount of sand moving along the bed, and the rate of bank erosion determine these limits. When a bend reaches this width, a chute forms and a new bend develops farther downstream. Distorted bends and natural cutoffs are caused by local changes in the character of bank materials. Several hypotheses have been brought forward to explain meandering. Meandering has been attributed to the earth's rotation; to the excessive slope and energy of a river (

Journal of Fluid Mechanics, 2014

This research implements a recently proposed framework for meander migration, in order to explore the coevolution of planform and channel width in a freely meandering river. In the model described here, width evolution is coupled to channel migration through two submodels, one describing bank erosion and the other describing bank deposition. Bank erosion is modelled as erosion of purely non-cohesive bank material damped by natural armouring due to basal slump blocks, and bank deposition is modelled in terms of a flow-dependent rate of vegetal encroachment. While these two submodels are specified independently, the two banks interact through the medium of the intervening channel; the morphodynamics of which is described by a fully nonlinear depth-averaged morphodynamics model. Since both banks are allowed to migrate independently, channel width is free to vary locally as a result of differential bank migration. Through a series of numerical runs, we demonstrate coevolution of local curvature, width and streamwise slope as the channel migrates over time. The correlation between the local curvature, width and bed elevation is characterized, and the nature of this relationship is explored by varying the governing parameters. The results show that, by varying a parameter representing the ratio between a reference bank erosion rate and a reference bank deposition rate, the model is able to reproduce the broad range of river width-curvature correlations observed in nature. This research represents a step towards providing general metrics for predicting width variation patterns in river systems.

1998

A topographic steering river meandering (TSRM) model based on continuity of a simplified flow field through bends is developed. The equilibrium coupling between helical flow and sediment transport, as in Ikeda [1989] determines downstream variation of transverse bed slope. The model tests the hypothesis that meander development patterns can be captured under the assumptions: (a) bank shear stress arises from forces associated with topographically induced convective accelerations; (b) turbulent boundary layer dissipation of these forces at the banks is sufficiently represented by gaussian smoothing at a parameterized scale; and (c) lateral migration of the channel is proportional to bank shear stress. The resulting TSRM model produces realistic complex meander patterns and scroll bar-like topography. Model compound bend formation is compared to a field case and found to arise from the nonlinear interaction of bank roughness and channel hydraulics scales. When the latter is short relative to the former, maximum bank shear stress occurs early in the bend and leads to compound bend formation. New statistical stream sinuosity and spatial coordinate variation measures are applied to both natural and model streams and reveal secondary sinuosities arising from compound bend formation in both cases. Scroll bar topography and channel bank roughness are studied in the field to compare natural and model mechanisms. A channel-hillslope integrated landscape development (CHILD) model incorporates the TSRM model. The CHILD model represents the landscape as an irregular, Delaunay triangulated mesh of landscape nodes that may be moved, deleted, or added to accommodate meandering channels that are in general discretized at different spatial resolution than the surrounding landscape. The interactions among meandering, bank erodibility's bank height dependence, and uplift rate in a detachment-limited river valley are examined. An equilibrium landscape adjusts to the onset of meandering and approaches a new dynamic equilibrium. For the detachment-limited case, the hypothesis that meandering is more active when uplift is quiescent is rejected. When bank erodibility's bank height dependence is greater, bend scale sinuosity is smaller, but the tendency toward multi-bend loop formation is reinforced.

Geomorphology, 2010

The extent and pattern of width variations along a meandering channel and its association with variation in bed topography and development of mid-channel bars have been examined through field survey evidence for a reach of the River Bollin in NW England. The wet width has been quantified along the reach by applying a hydraulic model to surveyed cross sections under a range of discharges between low flow and defined bankfull conditions. This approach allows an objective, modelling-based method to compute channel width. The high spatial resolution of the topographical survey allows capture of significant variations in the crosssectional morphology at the meander wavelength scale. Results disclose some features of longitudinal and stage-dependent width variability in meanders. Width variation is shown to be highly correlated with curvature: at bankfull conditions width peaks in bend apex sections exceed those at inflection sections and can be up to twice greater. The width-curvature behavior is correlated with the pattern of bed and banks morphology, which is different in bend apices and in meander inflections. The survey shows that the bedform morphology can be characterized by a mid-channel bar pattern that is initiated at the inflection section and that the bedform dynamics can be closely associated with channel width variations.

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.