Biomorphodynamics of alternate bars in a channelized, regulated river: An integrated historical and modelling analysis

Abstract

The development of alternate bars in channelized rivers can be explained theoretically as an instability of the riverbed when the active channel width to depth ratio exceeds a threshold. However, the development of a vegetation cover on the alternate bars of some channelized rivers and its interactions with bar morphology have not been investigated in detail. Our study focused on the co‐evolution of alternate bars and vegetation along a 33 km reach of the Isère River, France. We analysed historical information to investigate the development of alternate bars and their colonization by vegetation within a straightened, embanked river subject to flow regulation, sediment mining, and vegetation management. Over an 80 year period, bar density decreased, bar length increased, and bar mobility slowed. Vegetation encroachment across bar surfaces accompanied these temporal changes and, once established, vegetation cover persisted, shifting the overall system from an unvegetated to a vegetated dynamic equilibrium state. The unvegetated morphodynamics of the impressively regular sequence of alternate bars that developed in the Isère following channelization is consistent with previous theoretical morphodynamic work. However, the apparent triggering dynamics of vegetation colonization needs to be investigated, based on complex biophysical instability processes. If instability related to vegetation colonization is confirmed, further work needs to focus on the relevance of initial conditions for this instability, and on related feedback effects such as how the morphodynamics of bare‐sediment alternate bars may have affected vegetation development and, in turn, how vegetation has created a new dynamic equilibrium state. Copyright © 2018 John Wiley & Sons, Ltd.

Channelization of a large Alpine river: what is left of its original morphodynamics?

“…(); (3) Serlet et al . () unless otherwise specified; (4), Vautier (); (5) Stäuble et al . (); (6) Federal Office for the Environment (FOEN) ‐ Hydrol.…”

“…The application of the free‐bar predictor (Colombini et al ., ) to the Adige, the Drau, the Upper Rhone, the Alpine Rhine and the Isère offers a reliable explanation of such diverging trajectories. In the Alpine Rhine and in the Isère, the difference β − β c , which measures the morphodynamic instability of the riverbed, is always positive within a broad range of bar‐forming discharge values, implying bar formation (Figure (b) and (c)), as observed (Adami et al ., ; Serlet et al ., ). In contrast, the results of the present work (Figure and Figure (b) and (c)) indicate that for most of its length, the study segment of the Adige is characterized by a reach‐averaged width to depth ratio β smaller than the critical value β c for bar formation, as also occurs for the Drau.…”

Morphodynamics of alternate bars in the presence of riparian vegetation

“…However, while bar wavelengths of the Isère River have evolved from 5 to 20 channel widths (Serlet et al. ), bar wavelength ranged from to in the biomorphodynamic simulations. Moreover, in the field, bar elongation has been concomitant with bar accretion, with important fine sediment deposits during floods (Allain, ), a process that is not included in the present model.…”

“…Through a historical analysis of the biomorphological trajectory of the Isère River in southeastern France (Figure A), Serlet et al. () have described the development of vegetation on bars of a 33 km reach over a period of 80 years, which has been concomitant to increasing bar lengths and decreasing bar migration celerities. Currently, vegetated bars slowly evolve during floods, mostly through lateral erosion of bar heads (Jourdain, ).…”

Numerical Modeling of Plant Root Controls on Gravel Bed River Morphodynamics

“…However, the condition analyzed simplifies the topography of a real river gravel bar. The modeling framework we developed can be easily extended to take into account both flow unsteadiness and more complex morphologies, such as alternate bar patterns (Serlet et al, 2018). Moreover, we could investigate erosion processes related to changes in sediment supply rate (Diehl et al, 2017;Gran et al, 2015) and bar migration (Bertoldi et al, 2014), which are not considered here.…”