side channel dynamics
Pepijn van Denderen is a PhD student in the research group Marine and Fluvial Systems. His supervisor is prof.dr. S.J.M.H. Hulscher from the Faculty of Engineering Technology (ET).
Side channels are small secondary channels that convey much less discharge than the main channel. Side channels are commonly constructed to increase the discharge capacity of a river during peak flow conditions or to increase the ecological value of the river. The aggradation of side channels that are constructed to increase the discharge capacity of a river should be limited. In the last 20 years, more than 20 side channels have been constructed in the Rhine branches. Observations show that large aggradation occurs in the side channels. Therefore, such channels require regular and costly maintenance. The aim of this research is to better understand the mechanisms that drive the morphodynamic development of side channels and thereby, improve the design of side channels and reduce their maintenance needs.
We first look at natural occurring side channels that can be found in, for example, meandering and anabranching rivers. Using a one-dimensional (1D) bifurcation model, we assess the conditions under which side channels generally aggrade or degrade and we estimate the time scale of their morphodynamic development. We apply the model for a wide range of conditions and compare the results to multitemporal aerial images of four side channel systems. There are limitations to using the 1D model to study the development of side channels, but the model can reproduce the general behavior of the side channel development until the sediment that is transported in the main channel as bed load is no longer responsible for the further aggradation of the channel.
We study the development of side channels in more detail by looking at the development of three side channels at Gameren in the river Waal (the Netherlands). Since the construction of the side channels in 1996 and 1999, bed level measurements have been regularly collected. In addition, we took grain size samples of the sediment deposited in the channels and carried out hydrodynamic computations. We relate the bed level changes, the grain size and the hydrodynamic parameters with each other. In two of the three channels primarily sediment is deposited that in the main channel is transported as suspended bed-material load. In the third channel, suspended load is deposited in addition to the suspended bed-material load. The bed level measurements show that the largest aggradation can be expected in years during which the side channel conveys a minimal amount of discharge. With increasing flow frequency of the side channel, the aggradation rate decreases or even degradation can occur. The variation in the hydrodynamic regime and the sediment sorting at the bifurcation of the side channels are therefore both important mechanisms that should be taken into account in estimating the development of a side channel.
We investigate the effect of the hydrodynamic regime and sediment sorting in more detail using a two-dimensional (2D) mixed-sediment morphodynamic model with varying hydrodynamic conditions. We find that the aggradation rate and the sediment size that is deposited in the side channel is related to the discharge in the upstream channel. The lower discharges are responsible for the fining of the side channel bed. The largest aggradation rate occurs during the peak discharges and at the same time the bed coarsens. We find that the results are affected by the transverse bed slope effect, the grain size of the sediment supply in the upstream main channel, the bed roughness, the active layer thickness, the initial bed level in the side channel and structures at the bifurcation.
Based on the measurements and the modeling work, we define three categories of side channels. Our categorization is based on how the sediment that is deposited in a side channel is transported in the main channel. This results in (1) bed load supplied, (2) suspended bed-material load supplied and (3) wash load supplied side channels. For each of the categories different mechanisms are important for estimating the development of a side channel. Based on the characterization, we propose a method to estimate the development for each side channel category. Our characterization can therefore support river managers in the design, operation and maintenance of side channels.
