Formation of a cohesive floodplain in a dynamic experimental meandering river

Abstract

Field studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers. However, it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering. This is needed because only then does a dynamic equilibrium exist and channels maintain meandering with low width–depth ratios. Our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering. In this study we present two experiments with a self-forming channel in identical conditions, but to one we added cohesive silt at the upstream boundary. The effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2. The experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels. In contrast, in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel. Overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed: first, overbank vertical-accretion of silt, e.g. levee, overbank sedimentation or splays; and second, lateral point bar accretion with silt on the scrolls and in the swales. The first style led to a reduction in bank erosion, while the second style reduced excavation of chutes. We conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering.

Turning the tide: Growth and dynamics of a tidal basin and inlet in experiments

“…Here we use the redness digital number value larger than a threshold as an indication of water depth. Water depth could have been estimated with an error (1 σ ) of about 1 mm in agreement with a detailed calibration for the same technique and dye in Van Dijk et al [], but time variation in lighting and dye concentration would have caused time‐dependent bias.…”

Effects of mud supply on large-scale estuary morphology and development over centuries to millennia

“…In particular, cohesion reduces the ability to re-erode, resulting in more stable bars and levees and longer and deeper channels. Physical experiments produced similar results for deltas (Hoyal and Sheets, 2009) and for river meandering (Van Dijk et al, 2013). However, the sensitivity of the numerical models to parameters such as erodibility and settling velocity indicate that the value of long-term modelling exercises with the current state of the art is to develop generalisations and trends rather than precise hindcasts and predictions of specific cases.…”

“…In rivers, the formation of cohesive floodplains with mud and vegetation causes river channels to be narrower and deeper than in systems with only sand given otherwise equal conditions (Tal and Paola, 2007;Kleinhans, 2010;Van Dijk et al, 2013;Schuurman et al, 2016). This results from a dynamic balance between floodplain erosion by migration of channels and new floodplain formation by mud sedimentation and/or vegetation development.…”

Morpho‐sedimentary characteristics of proximal gravel braided river deposits in a Froude‐scaled physical model

“…Small‐scale physical models have been shown to be a useful source of this type of quantitative information in both aggradational and non‐aggradational conditions for braiding and other river planforms (e.g. Peakall et al ., ; Moreton et al ., ; Paola et al ., ; Kleinhans & van den Berg, ; Van Dijk et al ., , ; van de Lageweg et al ., ,b).…”

“…This approach is referred to herein as the morpho‐sedimentary characteristics of gravel braided rivers to signal the explicit quantitative linking of the known bar‐scale morphology and morphological changes to the deposit geometry and granulometry. This analysis of deposit geometry during formation complements existing facies and stratal analyses of gravel braided river alluvium from field investigations (Boothroyd & Ashley, ; Bluck, ; Bridge & Lunt, ) and previous analysis from physical models for a range of river types (Moreton et al ., ; Kleinhans & van den Berg, ; Van Dijk et al ., , ; van de Lageweg et al ., ,b).…”