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Fill characteristics of abandoned channels and resulting stratigraphy of a mobile sand‐bed river floodplain

Schwendel, Arved ORCID: https://orcid.org/0000-0003-2937-1748, Aalto, Rolf, Nicholas, Andrew and Parsons, Daniel (2019) Fill characteristics of abandoned channels and resulting stratigraphy of a mobile sand‐bed river floodplain. In: Fluvial Meanders and Their Sedimentary Products in the Rock Record. 1 ed. International Association Of Sedimentologists Series, 48 . John Wiley & Sons Ltd., pp. 251-272

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Abstract

Floodplains are diverse sedimentary environments where infill processes of abandoned
channels interact with overbank sedimentation and bank erosion. The result,
particularly in river systems with high suspended load and rapid channel migration,
is a complex three‐dimensional mosaic of deposits with spatial variability in terms of
grain‐size, age, organic carbon content and resistance to erosion. Abandoned channels
represent a significant deposition volume in fluvial systems that can accommodate
large proportions of the equivalent material mobilised during their abandonment.
However, time scales and fill processes vary between different kinds of abandoned
channels and the sediment calibre involved and are not fully understood, particularly
in respect to highly dynamic sand‐bed rivers. This study investigates time scales and
spatio‐temporal patterns of infill of abandoned chute channels and abandoned channel
segments left behind following neck cutoff of meander bends. The study focuses on the
Rio Beni, a large, tropical, sand‐bed river in the Bolivian Amazon basin. Electrical
resistivity ground imaging is used to elucidate the stratigraphy of floodplains and
satellite imagery is employed to investigate contemporary fill processes and rates.
Given suitable bend migration patterns, chute channels may remain stable for several
years but are eventually abandoned and rapidly filled with bed material during a single
flood season. Smaller scroll sloughs can convey coarse bedload across point bars and,
when filled, present stratigraphic bodies similar to chute fills. Abandoned meander
bends tend to develop plug bars at both ends immediately after cutoff. Of these bars,
the downstream plug aggrades at a faster rate due to the often larger diversion angles
with the main channel and efficiently seals off the bend. The subsequent infill of the
channel is a function of hydraulic connectivity and distance to the active channel as
well as rate of lake deposition. Considerable overbank deposition can increase the
spatial sedimentological heterogeneity of these floodplains, which needs to be taken
into account in floodplain evolution models.

Item Type: Book Section
Status: Published
DOI: https://doi.org/10.1002/9781119424437.ch10
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
Q Science > QE Geology
School/Department: School of Humanities, Religion & Philosophy
URI: http://ray.yorksj.ac.uk/id/eprint/3937

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