Morphodynamics of alternate bars in the presence of riparian vegetation

Abstract

Alpine gravel‐bed rivers are dynamic systems that have been subjected to many anthropic alterations in the past centuries. Riparian vegetation development on previously bare sediment bedforms has been a common adjustment, raising important management issues in terms of flood risks and biodiversity. Many of these rivers are also channelized, and as a result present a pattern of alternate bars. Considering recent advances in numerical biomorphodynamic modeling, this study aims at exploring numerically the morphodynamics of alternate bars in the presence of riparian vegetation. To this end, a dynamic vegetation module has been implemented on top of an existing morphodynamic model, accounting for ecological processes of seed dispersal, seedling recruitment, growth, and mortality. Numerical simulations have been performed on a simplified reach of a gravel‐bed river with free migrating alternate bars at initial state. In this work 96 scenarios have been simulated, each representing 50 years of channel evolution, with different flood regimes characterized by various peak discharges and flood durations. Yearly peak discharge variability is explicitly modeled in 48 scenarios. Model outcomes present two possible equilibrium biomorphodynamic behaviors: stationary vegetated bars, or free migrating bars in the case of frequent vegetation removal during floods. This binary behavior holds true when the stochasticity of annual peak discharges is represented, and for a wide range of parameter values included in vegetation dynamic modeling. Transient mobility of vegetated bars is observed in specific configurations where large sediment deposits deflect the flow field, eroding bar heads. Modeled bar wavelengths are in the range of values predicted for free bars by linear bar theory, and remain far from the theoretical values of hybrid, steady bars. The shift from unvegetated migrating bars to steady vegetated bars seems to show that in these simulations vegetation constitutes a hydraulic forcing, leading to a shift from free bars to forced bars, with a final configuration largely inherited from the initial state. © 2019 John Wiley & Sons, Ltd.

A model study of the combined effect of above and below ground plant traits on the ecomorphodynamics of gravel bars

“…Such balance can vary following a reduction in frequency and magnitude of floods, but also by biotic factors such as the arrival of invasive species with better survival ability and colonisation strategies than native species 54 . Previous modelling studies on alternate bars have shown the role played by flow variability and vegetation characteristics in modulating the transition between vegetated and bare states 33,34,55 , indicating the presence of two stable alternative states on alternate bars, steady-vegetated bars and migrating bare bars. Our results add on these findings, and provide insights on the role played by both above-and below-ground plant traits and their development, as well on the whole flow regime including low flows and floods with different magnitude, duration, and frequency, which are often not considered.…”

“…Despite modelling efforts to include a more detailed description of vegetation-related feedbacks are mounting 32 , 33 , there is still a lack of appropriate models that links environmental factors with plant morphological traits and describes the associated feedbacks. So far, models have mostly treated vegetation as a whole 34 , not distinguishing between above- and below-ground traits, or have focused on spatial and temporal scales that hindered a clear identification of their role 35 , 36 .…”

Effects of vegetation at a bar confluence on river hydrodynamics: The case study of the Arno River at Greve junction

“…This suggests that, in our study area, sediment deposition may not be confined within the separation zone (Best, 1987) and that the enhancement of bar dimensions can be related to the presence of vegetation, which definitely plays an important role in making the bars stable (Calvani, Francalanci, & Solari, 2018;Jourdain, Claude, Tassi, Cordier, & Antoine, 2020), by promoting deposition and thus enhancing their dimensions.…”

Long term high frequency sediment observatory in an alpine catchment: The Arc‐Isère rivers, France

“…Since the end of the 20th century, there has been a relative shortage of coarse particle input, and an excess of fine particles. The resulting imbalance led to erosion of the main channel, aggradation and fixation of gravel bars due to massive fine sediment deposits and riparian vegetation growth (Jourdain et al, 2020).…”