Downstream and upstream influence in river meandering. Part 2. Planimetric development

An analytical modelling framework is proposed to reproduce the frequently observed but poorly studied occurrence of mid-channel bars in meandering channels. Mid-channel bars occur in meanders and may characterize transitional morphologies between pure meandering and braided rivers. Based on existing fi eld and experimental observations, we propose that two different mechanisms can generate central topographical patterns in meanders. A former mechanism ('width-forced') is related to spatial width oscillations which determine a laterally symmetrical bed shear stress pattern that promotes mid-channel bars. A second mechanism ('curvature-forced') can take place also in curvilinear equiwidth streams since also longitudinal variations of channel curvature can produce laterally symmetrical alterations of the sediment transport capacity. A perturbation approach is employed to model both mechanisms within a common framework, allowing reproduction, at least qualitatively, of several observed features. While width-forced mid-channel bars are a symmetric linear altimetric response, to reproduce curvature-forced mid-channel bars requires modelling nonlinear fl ow-bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to fi eld cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of midchannel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the fl ow against the two banks, a process which is proposed as a possible cause of cross-sectional overwidening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this fi rst modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between midchannel bars and width variations in river meanders.

Section: Perturbation Approachmentioning

confidence: 99%

Modelling mid‐channel bars in meandering channels

Luchi

Zolezzi

Tubino

2010

“…Howard and Knutson (1984) simulated the phenomenon using a kinematic approach in which the reach-averaged channel migration rate had been previously scaled to the river sinuosity to match observations and the local erosion rates were assumed to be a function of the local channel curvature ratio R/B. The phenomenon was simulated using a dynamic model by Seminara et al (2001), who, however, did not provide physical explanations.…”

Section: Problem Descriptionmentioning

confidence: 99%

Physical explanations of variations in river meander migration rates from model comparison

Crosato

2009

In meandering rivers, the local channel migration rate increases with increasing bend sharpness until it reaches a maximum at a certain critical value of the bend sharpness. Beyond this critical value, the migration rate decreases if bend sharpness increases. Similarly, reach-averaged migration rates attain a maximum at a certain river sinuosity. This work investigates the physics of these phenomena by comparing the results of two physics-based models of different complexity, in which the migration rates are proportional to the near-bank fl ow velocity excess. In the computational tests the river was allowed to meander progressively, starting from an almost straight planimetry.Both models reproduced the observed peak in the curve describing the local migration rate as a function of the ratio radius of curvature-channel width (R/B), with a rising limb at lower R/B values and a falling limb at higher R/B values. The rising limb can be explained by the decrease in relative lag distance between near-bank fl ow velocity and forcing curvature as R/B increases. The falling limb results from the decrease in local channel curvature and near-bank fl ow velocity excess. Since the models do not include fl ow separation, the results indicate that this phenomenon is not needed to explain the decrease of channel migration rates in sharp bends. The models reproduced also the peak in the curve describing the reach-averaged migration rates as a function of river sinuosity The increase and then decrease of reach-averaged migration rates as sinuosity increases appears to be mainly caused by the variation of the reach-averaged value of the ratio R/B.

“…This migration rate is usually related to the stream hydrodynamics through a simplifi ed linear relation of the form (Ikeda et al, 1981;Seminara et al, 2001):…”

Section: Mathematical Frameworkmentioning

confidence: 99%

Long‐term river meandering as a part of chaotic dynamics? A contribution from mathematical modelling

Frascati

Lanzoni

2010

In the present contribution we focus our attention on the possible signatures of a chaotic behaviour or a selforganized criticality state triggered in river meandering dynamics by repeated occurrence of cutoff processes. The analysis is carried out examining, through some robust nonlinear methodologies inferred from time series analysis, both the spatial series of local curvatures and the time series of long-term channel sinuosity. Temporal distribution of cutoff inter-arrivals is also investigated. The analyzed data have been obtained by using a suitable physics-based simulation model for river meandering able to reproduce reasonably the features of real rivers. The results are consistent and show that, at least from a modelling point of view, no evidence of chaotic determinism or self-organized criticality is detectable in the investigated meandering dynamics.