Living landscapes: Muddy and vegetated floodplain effects on fluvial pattern in an incised river

Abstract

Cohesive floodplain sediment and vegetation are both thought to cause meandering river patterns. Our aims are to compare the isolated and combined effects of mud and vegetation on river planform and morphodynamics in the setting of intermediate‐sized valley rivers. We use a numerical model for century‐scale simulation of flow, sediment transport and morphology coupled with riparian vegetation settlement, growth and mortality as functions of species traits on which flow resistance depends. Mud fluxes were predicted by excess shear stress relations in combination with the active layer formulation. We found that valley‐flooding water levels increase with vegetation density, causing a higher braiding intensity rather than meandering tendency. The shear stress during floods carves channels through the muddy floodplain surface. Higher mud concentration, on the other hand, increases floodplain aggradation, reduces the overbank flow frequency and ultimately causes formation of a single‐thread channel. Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation settling mutually enhance floodplain formation. However, mud and vegetation counteract in two ways. First, vegetation enhances floodplain accretion, which ultimately increases plant desiccation for high mud concentrations. Second, vegetation increases the tendency of periodic chute cutoffs in valleys. The chute cutoffs locally reset the landscape and create new windows of opportunity for the vegetation. Surprisingly, in systems with a high mud concentration this causes hysteretic loops of vegetation cover and delayed mud deposition. Ramifications for the interpretation of Palaeozoic fluvial facies are that even rootless vegetation, capturing cohesive mud closer to the river channel to form thicker floodplain on the point bar, can enhance the tendency to meander and, under high mud supply, form stable channels. However, meandering is more unlikely in narrower valley rivers with higher vegetation density. © 2018 John Wiley & Sons, Ltd.

Morphodynamics of alternate bars in the presence of riparian vegetation

“…Moreover, considering transport mechanisms associated with fine sediments would provide a more realistic representation of the accretion of vegetated bars (Allain, ; Kleinhans et al. ) and may provide insights concerning processes and feedbacks associated with fine sediment deposits, grain size sorting and transport processes in the presence of vegetation (Cordier et al. ; Perret et al.…”

Fate of pioneering vegetation patches in a dynamic meandering river

“…A possible mechanism for survival is that the combined flow resistance of multiple patches forces the main flow into the channel and elsewhere on the floodplain. Modelling indicates that multiple flow paths in a complex pattern of vegetation patches exist during floods (van Oorschot et al., ; Kleinhans et al., ). This implies that measuring the inter‐patch distance as a function of proximity to the low‐water channel or the forest is too simplistic since also the groups of patches concur to modify the flow pattern.…”

“…A pioneer plant could engineer its environment through hydraulic resistance, which causes the flow to be focused elsewhere, mainly in the channel. The resulting local flow stress reduction implies a favourable environment for the establishment of new individuals of the same and other species (van Oorschot et al., ) and leads to local sedimentation that may further reduce inundation stress (Kleinhans et al., ). However, in order to have these effects, the plant should not be flushed out, which requires rooting (Kleinhans et al., ).…”

Distinct patterns of bank erosion in a navigable regulated river

“…The complexity of the processes involved during floodplain formation (Nanson and Croke, 1992;Kleinhans, 2010;Kleinhans et al, 2018) limits the use of uniform bank erosion coefficients for predictive purposes, even considering complex hydrodynamics . The complexity of the processes involved during floodplain formation (Nanson and Croke, 1992;Kleinhans, 2010;Kleinhans et al, 2018) limits the use of uniform bank erosion coefficients for predictive purposes, even considering complex hydrodynamics .…”