Fluvial Meanders And Their Sedimentary Products...

This book includes some of the most recent advances in the study of the morphodynamics and sedimentology of meandering rivers, and is an important resource for those who want to investigate fluvial systems and their deposits.

Fluvial Meanders and Their Sedimentary Products...

Abstract:Over the past few millennia, meandering fluvial channels drained coastal landscapes accumulating sedimentary successions that today are permeable pathways. Propagation of pollutants, agricultural exploitation and sand liquefaction are the main processes of environmental interest affecting these sedimentary bodies. The characterization of these bodies is thus of utmost general interest. In this study, we particularly highlight the contribution of noninvasive (remote and ground-based) investigation techniques, and the case study focuses on a late Holocene meander bend of the southern Venetian Plain (Northeast Italy). Electromagnetic induction (EMI) investigations, conducted with great care in terms of sonde stability and positioning, allowed the reconstruction of the electrical conductivity 3D structure of the shallow subsurface, revealing that the paleochannel ranges in depth between 0.8 and 5.4 m, and defines an almost 260 m-wide point bar. The electrical conductivity maps derived from EMI at different depths define an arcuate morphology indicating that bar accretion started from an already sinuous channel. Sedimentary cores ensure local ground-truth and help define the evolution of the channel bend. This paper shows that the combination of well-conceived and carefully performed inverted geophysical surveys, remote sensing and direct investigations provides evidence of the evolution of recent shallow sedimentary structures with unprecedented detail.Keywords: electromagnetic induction; depth inversion; sedimentary processes

Meandering channels extensively dissect fluvial and tidal landscapes, critically controlling their morphodynamic evolution and sedimentary architecture. In spite of an apparently striking dissimilarity of the governing processes, planform dimensions of tidal and fluvial meanders consistently scale to local channel width, and previous studies were unable to identify quantitative planimetric differences between these landforms. Here we use satellite imagery, measurements of meandering patterns, and different statistical analyses applied to about 10,000 tidal and fluvial meanders worldwide to objectively disclose fingerprints of the different physical processes they are shaped by. We find that fluvial and tidal meanders can be distinguished on the exclusive basis of their remotely-sensed planforms. Moreover, we show that tidal meanders are less morphologically complex and display more spatially homogeneous characteristics compared to fluvial meanders. Based on existing theoretical, numerical, and field studies, we suggest that our empirical observations can be explained by the more regular processes carving tidal meanders, as well as by the higher lithological homogeneity of the substrates they typically cut through. Allowing one to effectively infer processes from landforms, a fundamental inverse problem in geomorphology, our results have relevant implications for the conservation and restoration of tidal environments, as well as from planetary exploration perspectives.

Highly sinuous meandering channels are among the most fascinating morphological patterns existing in nature1,2. Their extensive presence typically characterizes both tidal and fluvial landscapes (Fig. 1) and is central in controlling the eco-morphodynamic evolution and sedimentary architecture of the landscapes they cut through3,4,5,6,7. Questions concerning analogies and differences among tidal and fluvial meanders (TM and FM hereafter) have long been debated8,9,10, mainly because of the different chief-landforming processes they are formed by. While the basic flow in FM is invariably directed downstream, a periodic flow reversal in response to varying tidal phases controls TM dynamics3,10. In addition, maximum flow discharges in fluvial settings are attained when water level is at the top of river banks (i.e., bankfull stage) and generally remain constant along the river course until a major confluence is found. On the contrary, the highest water stages in purely tidal landscapes correspond to low flow conditions (i.e., high slack-water periods), and the maximum along-channel discharges rapidly decline as channels extend landward, leading to a peculiar progressive reduction of channel widths (i.e., channel funneling)9,11,12 (Fig. 1).

In starking contrast to these different chief-landforming processes, however, TM and FM display several remarkable analogies: they are characterized by similar values of sinuosity, as well as ratios between channel width and other relevant features such as mean flow depth, meander amplitude, wavelength and radius of curvature (Fig. 2). The proportionality of relevant morphometric features of TM and FM to channel width, as well as theoretical treatments of the flow field in fluvial and tidal settings13,14, also suggests that a proper normalization is ensured by the mean meander width, which allows one to compare meanders of different sizes. Thus, when normalized with local channel width, observed migration rates of TM and FM are shown to be quite similar15. In addition, such a scaling substantially mitigates planform morphological differences (e.g., due to channel funneling in tidal environments) and hinders a clear distinction between TM and FM. Looking at the width-normalized planforms of meandering channels shown in Fig. 3, one can hardly disclose their fluvial or tidal nature. Thus far, only sedimentary facies of deposits associated with TM and FM have shown to be different, in spite of apparently similar large scale architecture of TM and FM point bars16,17,18,19. Differences in sedimentary facies are mainly due to the periodic flow reversal in TM, which produces distinctive sedimentary structures16,17. Only in some cases the effect of bidirectional flow significantly impacts the planform shape of individual TM bends. For example, segregated flood and ebb channels, and banks carved in cuspate fashion (Fig. 1c,d), can form as a result of distinct patterns of maximum near-bank velocities attained during ebb and flood3,16,20. We therefore wonder whether or not tidal and fluvial meanders consistently display distinct planform features, and to what extent possible differences can be objectively detected. Previous studies were unable to identify suitable, distinctive metrics to quantitatively discriminate between TM and FM planforms9,10,13 (Fig. 2), and it still remains unclear whether and how signatures of the different landforming processes operating in tidal and fluvial settings are retained in the corresponding planforms. Developing quantitative frameworks capable of unravelling origins of meandering channels from their planforms might provide remarkable insights into subsurface investigations based on 3D seismic data21, and represents a fundamental step to disclose similarities and differences between TM and FM morphodynamics3,4,9,14. In addition, it might result crucial for testing different formation theories of extraterrestrial sinuous channels22,23, such as those observed in Marsian and Venusian landscapes24,25,26,27. Here we compare, by means of statistical and spectral tools, the normalized planform features of about 10,000 meander bends observed worldwide, aiming at characterizing differences in footprints of flow fields and morphodynamic processes shaping TM and FM. The detected morphological differences are then interpreted based on a number of available theoretical, numerical, and field studies on meandering channels in order to relate channel morphology to meander dynamics.

where \(\lambda \mathrm=2\pi /\ell _F\) is the meander wavenumber, while cF and cS represent the fattening and skewing coefficients, respectively. The lack of even harmonics, particularly of the second one, in Eq. (1) was justified by the cubic geometric nonlinearity of the integro-differential equation describing the planimetric-evolution of meandering rivers14. Nevertheless, our results highlight the presence of non negligible second harmonics in both TM and FM spectra, with almost identical power density (Fig. 5a). This proves that, unlike previously suggested9, TM and FM cannot be distinguished based on the possible presence of even harmonics. In general, TM display higher power density within the first harmonic, followed by a more rapid decay of spectral energy within higher modes compared to FM. Our results indicate a lower complexity of TM planforms, whereas FM are likely to exhibit more complex patterns owing to the marked convolutive (nonlinear) interactions among spectral modes. We applied the Singular Spectrum Analysis32 (SSA, see Methods) to full-meander curvature series to further substantiate such hypothesis29. The resulting eigenvalue spectra (Fig. 5b) provide information on the separation between the meaningful part of the signal and the noisy background, the latter being mainly confined in the spectrum tail (i.e., components from the 5th to 20th). The significance of each spectrum component can be quantitatively estimated by comparing the variance contributed by each SSA eigenvalue. Even though an exponential decay is observed for both fluvial and tidal spectra, the former exhibit a more distinct slope break that usually corresponds to signal to noise (S/N) separation32. In addition, the relative importance of each SSA spectrum component, with the sole exception of the first one, is higher in the fluvial than in the tidal case, with the most marked difference observed for the second and third components (Fig. 5b). The slower decay of the SSA spectrum of FM points at a higher morphological complexity relatively to TM, due to the increased weight of high-order spectrum components. The same line of reasoning is supported by the results obtained from the multivariate extension of SSA (M-SSA, see Methods). We employed M-SSA to investigate the planform homogeneity among series of M-adjacent half-meanders, with M equal to 2, 5, and 8. Regardless of M-size, the M-SSA spectra obtained for TM (Fig. 5c and Supplementary Fig. S1) display a faster decay than FM. Increasing the window length M, the eigenvalue spectra of FM show, on average, a progressively clearer break, standing for S/N separation, that is conversely absent in TM. Hence, FM belonging to a series of consecutive bends are more likely to display morphological features different from each other, whereas in tidal realms the characteristics of a given individual meander are more similar to those of the adjoining bends. 041b061a72