PhD-Research on meandering rivers (2009-2013)

I am a geomorphologist interested in the conditions which leads rivers to meander through their floodplain. Therefore, I did several small-scale experiment, where I systematic test the effect of bank strength for different sediment mixture. Further, in a large facility (6x11m) experiment are conducted were the interaction is studied between water flow, sediment transport, erosion and floodplain formation. In experiments of 2010 we showed that important condition for dynamic meandering is a disturbance upstream, which is published in Journal of Geophysical Research - Earth Surface. In experiments of 2011 we produced different channel patterns by just the addition of cohesive fines in the sediment feed, which we published in Earth Surface Processes and Landforms. The poster presentation on this work was reworded with the Outstanding Student Poster (OSP) award. We still have to publish some work on our experiments. This will mainly contain the effect of vegetation and the distribution of vegetation. Furthermore, we submitted a paper showing our basic setup and scaling rules for these experiments.

Meandering rivers - feedbacks between channel dynamics, floodplain and vegetation (2013)

Utrecht Studies in Earth Sciences (Vol. 35) - Universiteit Utrecht

ABSTRACT - Rivers have distinctive channel patterns such as multi-channel braiding and single-channel meandering. Why these different river patterns emerge is only qualitatively understood. Yet, we have not been able to retain dynamic meandering in laboratory experiments. The main objective of this thesis was to develop experimental settings in the laboratory that lead to dynamic meandering, in order to determine how rivers self-organize their morphology through interactions between channels, floodplain and vegetation. I unraveled these interactions in a series of controlled flume experiments.

I found that classical similarity scaling as used in engineering scale models does not work well because the channel width-depth ratio that determines bar formation is a dependent parameter in self-formed channels. Therefore, I relaxed the scaling rules. Systematic small-scale tests showed that there is a narrow range of suitable conditions for continuous meandering regarding sediment mobility and bank strength. Initially, in a straight channel with a low width-depth ratio alternate bars develop and evolve into meander bends. An upstream perturbation is required to initiate bar formation, but the experiments showed that a static perturbation led to low amplitude bends. In contrast, a transversely moving inlet point caused high-amplitude dynamic meandering with scrolls bars and infrequent chute cutoffs.

Floodplain development further defined the resulting river pattern. In experiments with only channel sediment, overbank flow led to chute cutoffs that terminated meander bend development and that were the onset of a braided river with multiple parallel channels. Bank erosion by bend migration is balanced by deposition of sediment forming new floodplains and cutoffs, as also confirmed by numerical modeling. I showed that vegetation and fine sediment on the floodplain decreased the number of chute cutoffs, allowing meander bends to grow further. The fine sediment adds cohesion to the floodplain and increased bank strength, while plants increased bank strength and roughness.

The combination of flume experiments and numerical model results shows that channel dynamics are controlled by bankfull discharge conditions, but overbank flow is highly important in constructing and destroying the floodplain. The composition of the floodplain, e.g. cohesion of the banks and the presence of vegetation, determines whether a braided or a meandering river pattern develops.

Keywords: River patterns, Meandering river, Braided river, Bank strength, Floodplain construction and destruction, Riparian vegetation, Chute cutoffs, Flume experiments, Scaling, River morphodynamics

This PhD-work was closely related to the PhD-work of Wietse van de Lageweg. He studied the architecture of these meandering rivers, as described in his dissertation:

Morphodynamics and sedimentary architecture of meandering river.

Utrecht Studies in Earth Sciences (Vol. 40) - Universiteit Utrecht

ABSTRACT: Full preservation of ancient meandering channels is uncommon: most preserved architectural elements are fragmented due to later erosion and cutting of channels and bars. This makes it challenging to relate preserved architectural elements to the original dimensions at the time of sedimentation. Quantitative information on the three-dimensional geometry and composition of these basic building blocks of reservoirs and aquifers formed by meandering systems is essential for exploration and production purposes and potentially an invaluable source of information about ancient climate. In this thesis the relation between the morphodynamics of meandering rivers and the resulting stratigraphic architecture is explored and quantified in three dimensions. This led to a scale-independent inverse relation that uses stratigraphy to predict the original meandering river dimensions that is of considerable use in interpreting fluvial successions and in reservoir engineering studies.

Channel dimensions need to be scaled correctly to produce meaningful experimental stratigraphy. In particular, any vertical distortion of the channels modifies the resultant stratigraphical and sedimentological architecture, as exemplified by the dependence of the transverse bed and stratigraphical architecture predictors on channel depth. The transverse bed slope of the experimental lateral accretion surfaces has similar values as the targeted natural prototype rivers and agrees well with theory. Also, the formation of experimental stratigraphy for meandering rivers depends on the deepest channel cuts and the order of occurrence of these channel cuts, which is consistent with the prediction from theory.

Three key archimetric (i.e. quantitative, non-dimensional, architectural dimensions) parameters are identified to enhance the three-dimensional characterization of the external geometry and internal stratigraphy of meander belts. First, meander belt width-to-thickness ratios are generally between 100 and 200, which is consistent with reported values of natural meander belts. Second, the internal stratigraphical architecture of meander belts largely consists of fragmented deposits: 95% of the stratigraphic sets are thinner than the mean channel. Third, prediction of the slope of lateral accretion deposits is possible based on the mean channel depth, a typical grain size and channel curvature. This indicates that a few fairly basic morphological (i.e. channel) and sedimentary parameters provide insightful quantitative predictors for the three-dimensional sedimentary architecture of meander belts, which is complementary to well and seismic observations.

Most fluvial systems are subject to changes in forcing. For example, aggradation rates and discharge are extremely variable. This thesis demonstrates that moderate floods (≈50 years recurrence time) and aggradation rates (<10 mm/yr) are unable to alter the stratigraphy and preservation of channel deposits. Floods mainly elongate the morphology and deposits while the channel depth and bar aggradation are similar to smaller floods. Most of the stratigraphical variation within meander belts is found laterally and not vertically. Reworking and fragmentation of deposits predominantly occurs in the center of a meander belt by chute and neck cutoffs. Although aggradation is unable to affect the average stratigraphic set thickness, it increases the overall stratigraphical variability and amplifies the lateral differences in stratigraphic architecture: the aggradation signal is predominantly preserved close to the margins of a meander belt.

Keywords: Rivers, morphology, stratigraphy, sedimentology, experiments, modelling, preservation, archimetrics, floods, aggradation