Poster: Channel dynamics and floodplain formation of a dynamic meandering river: insights from flume experiments (original) (raw)

"1. Introduction Rivers have distinctive patterns such as multi-channel braided and single-channel meandering. Why these different river patterns emerge is only qualitatively understood. Many fundamental fluvial processes are relatively well understood, but their combined effect on river patterns remains hypothetical. Braided rivers emerge when banks are weak, whereas meandering rivers emerge for stronger banks (Kleinhans 2010). Vegetated banks and cohesive floodplains provide bank strength, which affects the channel width-depth ratio. Rivers self-organize their morphology through interactions between channels, floodplain and vegetation. In order to predict the migration and bank/floodplain development of rivers, we try to understand these interactions and to understand how this ultimately leads to different river patterns. The objective of this study is to obtain insights on interactions between channel dynamics, floodplains and vegetation by conducting flume experiments. 2. Effect of channel dynamics The development of alternate bars and meander bends can be explained from simplified analytical physics, i.e. linear stability analysis. The analytical solution can be divided in a bar theory and a bend theory, which was tested in several experiments. 2.1 Bar and bend theory According to the linear stability analysis, meandering channels can theoretically arise from a straight channel (Blondeaux and Seminara, 1985). The unstable river bed leads to formation of alternate bars when width-depth ratios are low, while mid-channel bars form for high width-depth ratios. The alternate bars induce alternate erosion of the banks and deposition at the opposite side of the channel. In experiments starting with a straight channel, we indeed observed the development of alternate bars, bending of the channel and lateral expansion towards maximum amplitude followed by chute cutoff and channel straightening. 2.2 Upstream perturbation The dynamics of a meandering river are determined by the nature of the bend instability. Theoretically, this instability could be absolute or convective. Lanzoni and Seminara (2006) found that bars and channel curvature for meandering rivers mostly convect in one direction, usually downstream due to low width-depth ratios. This implies that, to sustain meandering dynamics, the upstream boundary must continuously be perturbed, so that instabilities continue to propagate from the upstream boundary in downstream direction. In an experiment we showed that a static perturbation led to low amplitude meanders that were static in the sense of no lateral migration, while an experiment with a transverse moving boundary produced series of scroll bars and infrequent bend cutoffs in downstream migrating sinuous meanders (Van Dijk et al., 2012). 3. Effect of floodplain construction and destruction Channel dynamics determined the development of meander bends in the channel, while lack of floodplain formation resulted in chute cutoffs. We tested the effect of vegetation as well as cohesive fines on floodplain construction and destruction. 3.1 Vegetated floodplains Bar and bend theory and empirical relations showed that bank stability is important for the development of a meandering river. Bank strength should be sufficient to keep low-width depth ratios, in order to prevent the channel widening and braiding. Riparian vegetation has the effect of hydraulic resistance and bank strength and it increases bar sedimentation. Prior experiments showed that the addition of vegetation (Alfalfa) on a braided river resulted in bank stabilization and the development of a single-thread channel (Tal and Paola, 2010). The addition of vegetation has also been shown to result in sedimentation of fine material in chutes and decreased the number of cutoffs (Braudrick et al., 2009). We performed experiments in which vegetation was uniformly distributed on the floodplain of a meandering channel. This resulted in bank stabilization and the development of meander bends that were tighter compared to a control experiment without vegetation (Van Dijk et al., submitted b). Furthermore, we tested the effect of vegetation seeds that are distributed by the flow during floods. The vegetation developed on lower areas and hydraulic resistance of the stems resulted in flow diversion, which resulted shallow water depths over bars and floodplains and decreased channel dynamics. Vegetation in the experimental setup was successful in bank stabilization, but the development of new vegetation is limited to continuous manual distribution of seeds and not dependent on the feedbacks between channel dynamics and floodplain formation. 3.2 Cohesive floodplains Bank erosion by bend migration and cutoff is balanced by deposition of sediment forming new floodplains. The process of floodplain shaving and channel extension results in local differences between erosion and deposition (Lauer and Parker, 2008). Overbank deposits are an important floodplain forming process that balances these local differences. We conducted an experiment where the initial bed consisted of poorly sorted sand. The experiment had a simple hydrograph of bankfull discharge and discharge exceeding bankfull. During the higher discharge slightly cohesive fines were added in the feed, so that these fines could deposit in the lows of the floodplain. The addition of fines in the experiment and the simple hydrograph led to an experimental meandering river with higher sinuosity and less chute cutoffs. The fines deposited in the lows of the point bar in the inner bend and formed a levee/ splay on the outer bank where flow diverted from the channel to the floodplain (Van Dijk et al., 2013). The cohesive floodplain in this experiment decreased bank erosion and chute incision compared to the experiment without the addition of fines. 4. Conclusions The formation of meanders is controlled by feedbacks between channel dynamics and floodplains. We conclude that channel dynamics are controlled by the bankfull conditions, while overbank flow is important in constructing and destructing floodplains. Results show that: • In-channel morphology depends on the width-depth ratio that determines the initiation of alternate bars. • With a static perturbation bend growth lead to low amplitude bend and are static in lateral migration. • Experiments show that when upstream perturbation is dynamic a richer morphology than hitherto is produced, including cycles of meander growth and chute cutoff. • Riparian vegetation stabilizes banks, sustaining low width-depth ratios, which results in local erosion and tight bends. • The interaction of flow exceeding bankfull discharge is important in developing floodplains that balance bank erosion. • Cohesive sediment results in distinctive floodplains that sustain dynamic meandering, whereas vegetation increases hydraulic resistance and reduces channel dynamics. "