Fine‐sediment erosion and sediment‐ribbon morphodynamics in coarse‐grained immobile beds
Author(s)
Trevisson, Michele
Eiff, Olivier
Type
Journal Article
Abstract
In rivers, fine sediments are often transported over immobile coarse grains. With low sediment supply, they tend to aggregate in longitudinal ribbons. Yet, the long-term evolution of such ribbons and the influence of immobile grains on the erosion of fine sediments are still not well understood. Flume experiments without sediment supply were therefore performed to investigate the erosion of an initially uniform fine-sediment bed covering an immobile bed of staggered spheres through topographic and flow measurements. The topographic measurements yielded the spheres' protrusion above the fine sediment (P) and revealed long-lived ribbons with ridges and troughs. The ridges are the main long-term sediment source as the troughs are quickly eroded to a stable bed level resulting from the spheres' sheltering. The ridges stabilize with a spacing of 1.3 effective water depths, their number resulting from the integer number of wavelengths fitting into the effective channel width which excludes side-wall accumulations. The ridges' erosion is damped by the local upflow of secondary current cells, which displaces the strongest sweep events above the bed. The upflow intensity is controlled by the ridges' height for low P, while for high P by the lateral roughness heterogeneity. The trends in erosion rates over ridges and troughs are similar and characterized by the following sequence of four regimes with increasing P: a drag sheltering, a turbulence-enhancement, a wake-interference sheltering, and a skimming-flow sheltering regime. The critical P levels at the transitions are independent of the flow above the canopy, depending only on the geometrical configuration of the immobile bed.
Date Issued
2022-11-01
Date Acceptance
2022-10-28
Citation
Water Resources Research, 2022, 58 (11)
ISSN
0043-1397
Publisher
American Geophysical Union (AGU)
Journal / Book Title
Water Resources Research
Volume
58
Issue
11
Copyright Statement
© 2022 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
License URL
Publication Status
Published
Article Number
e2021WR031837
Date Publish Online
2022-11-02