Modelling mid-channel bars in meandering channels

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 field 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 flow-bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to field cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of mid-channel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the flow against the two banks, a process which is proposed as a possible cause of cross-sectional over-widening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this first modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between mid-channel bars and width variations in river meanders.