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| Titel |
How does sediment affect the hydraulics of bedrock-alluvial rivers? |
| VerfasserIn |
Rebecca Hodge, Trevor Hoey, George Maniatis, Emilie Leprêtre |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128889
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| Publikation (Nr.) |
 EGU/EGU2016-8928.pdf |
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| Zusammenfassung |
| Relationships between flow, sediment transport and channel morphology are relatively well
established in coarse-grained alluvial channels. Developing equivalent relationships for
bedrock-alluvial channels is complicated by the two different components that
comprise the channel morphology: bedrock and sediment. These two components
usually have very different response times to hydraulic forcing, meaning that the
bedrock morphology may be inherited from previous conditions. The influence of
changing sediment cover on channel morphology and roughness will depend on
the relative magnitudes of the sediment size and the spatial variations in bedrock
elevation.
We report results from experiments in a 0.9m wide flume designed to quantify the
interactions between flow and sediment patch morphology using two contrasting bedrock
topographies. The first topography is a plane bed with sand-scale roughness, and the second
is a 1:10 scale, 3D printed, model of a bedrock channel with spatially variable roughness
(standard deviation of elevations = 12 mm in the flume). In all experiments, a sediment pulse
was added to the flume (D50 between 7 and 15 mm) and sediment patches were allowed to
stabilise under constant flow conditions. The flow was then incrementally increased in
order to identify the discharges at which sediment patches and isolated grains were
eroded.
In the plane bed experiments ∼20% sediment cover is sufficient to alter the channel
hydraulics through the increased roughness of the bed; this impact is expressed as the
increased discharge at which isolated grains are entrained. In the scaled bed experiments,
partial sediment cover decreased local flow velocities on a relatively smooth area of the bed.
At the scale of the entire channel, the bed morphology, and the hydraulics induced by it, was
a primary control on sediment cover stability at lower sediment inputs. At higher inputs,
where sediment infilled the local bed topography, patches were relatively more
stable, suggesting an increased impact on the hydraulics and the role of grain-grain
interactions. We draw together these experiments using a theoretical framework to
express the impact of sediment cover on channel roughness and hence hydraulics. |
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