Morphodynamic assessment of side channel systems using a simple one‐dimensional bifurcation model and a comparison with aerial images

Abstract

Side channel construction is a common intervention applied to increase a river's conveyance capacity and to increase its ecological value. Past modelling efforts suggest two mechanisms affecting the morphodynamic change of a side channel: (1) a difference in channel slope between the side channel and the main channel and (2) bend flow just upstream of the bifurcation. The objective of this paper was to assess the conditions under which side channels generally aggrade or degrade and to assess the characteristic timescales of the associated morphological change. We use a one-dimensional bifurcation model to predict the development of side channel systems and the characteristic timescale for a wide range of conditions. We then compare these results to multitemporal aerial images of four side channel systems. We consider the following mechanisms at the bifurcation to be important for side channel development: sediment diversion due to the bifurcation angle, sediment diversion due to the transverse bed slope, partitioning of suspended load, mixed sediment processes such as sorting at the bifurcation, bank erosion, deposition due to vegetation, and floodplain sedimentation. There are limitations to using a one-dimensional numerical model as it can only account for these mechanisms in a parametrized manner, but the model reproduces general behaviour of the natural side channels until floodplain-forming processes become important. The main result is a set of stability diagrams with key model parameters that can be used to assess the development of a side channel system and the associated timescale, which will aid in the future design and maintenance of side channel systems.

Morphodynamics of alternate bars in the presence of riparian vegetation

“…Second, as observed in natural river reaches, the mobility of vegetated bars has been observed through the formation of a side channel (van Denderen et al. ), leading to the migration of vegetated bars downstream and to bar coalescence. This process is observed in 19 scenarios with different intensities.…”

“…Thus flow deflection linked to bar advance and associated bathymetric change seems responsible for the erosion of vegetated bars in this configuration. Second, as observed in natural river reaches, the mobility of vegetated bars has been observed through the formation of a side channel (van Denderen et al, 2018), leading to the migration of vegetated bars downstream and to bar coalescence. This process is observed in 19 scenarios with different intensities.…”

Ongoing River Capture in the Amazon

“…Bifurcations are dynamic and can change on timescales of days to millennia as the interactions between discharge, erosion, and sedimentation cause one of the channels to become the dominant or only channel. The stability of the bifurcation is a function of the capacity of each channel to convey the water and sediment supplied to it (Edmonds & Slingerland, ; Fisk, ; Kleinhans et al, ; van Denderen et al, ).…”

Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

“…Typically, the branch located in the outer bend receives less sediment. Two factors have been proposed for the steering: first, the transverse transport of sediment by the secondary flow toward the inner bend (Kleinhans et al., ; van Denderen et al., ) and, second, coarsening of bed material toward the outer bend (Frings & Kleinhans, ; Sloff & Mosselman, ), Interestingly, practical engineering methods disagree with mathematical models about the division of sediment in bends. While it is a common engineering practice to minimize the amount of diverted sediment by diverting water from outer bends (Raudkivi, ), mathematical models predict that most sediment is diverted into the outer bend branch (Kleinhans et al., ; Redolfi et al., ).…”