Abstract
AbstractSide 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. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
A side channel system is a term for a two-channel system that is connected at both ends, in which the side channel conveys much less discharge than the main channel
A situation with an equal discharge in both branches is unstable due to a small transverse bed slope. These results show that the transition between a dominant side channel and a dominant main channel occurs for similar length ratios for a sand-bed river such as the Wabash River, a sand-gravel river such as the Sacramento River and a gravel-bed river such as the river Ain
The stability of the bifurcation and the location of the transition depend on the magnitude .@Á=@y/ and the intensity .1=f . // of the transverse bed slope effect (Equation (A3)), which agrees with previous studies (Bolla Pittaluga et al, 2003, 2015)
Summary
A side channel system is a term for a two-channel system that is connected at both ends, in which the side channel conveys much less discharge than the main channel. Side channels often suffer from aggradation or degradation, which results in the need for regular maintenance. This is both expensive and can deteriorate the targeted ecosystem, and a side channel without the need for intensive maintenance is desirable. It is unclear whether such a maintenance-free side channel system can exist. A better understanding of the mechanisms that influence morphodynamic changes of side channel systems is needed
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