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| Titel |
Bedload fluctuations in a steep macro-rough channel |
| VerfasserIn |
Tamara Ghilardi, Mário J. Franca, Anton J. Schleiss |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089109
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| Publikation (Nr.) |
 EGU/EGU2014-3300.pdf |
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| Zusammenfassung |
| It is known that bedload fluctuates over time in steep rivers with wide grain size distributions,
even when conditions of constant sediment feed and water discharge are met. Bedload
fluctuations are periodic and related to fluctuations in the flow velocity and channel bed
morphology. In cascade morphologies, the presence of large relatively immobile boulders has
a strong impact on flow conditions and sediment transport; their influence on bedload
fluctuations is considered in this research.
Sediment transport fluctuations were investigated in a set of 38 laboratory experiments
carried out on a steep tilting flume, under several conditions of constant sediment and
water discharge, for three different slopes (S=6.7%, 9.9%, and 13%). The impact
of the diameter and spatial density of randomly placed boulders was studied for
several flow conditions. Along with the sediment transport and bulk mean flow
velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps,
which are indicators of the channel morphology, were measured regularly during the
experiments.
Periodic bedload pulses are clearly visible in the data collected during the experiments,
along with well correlated fluctuations in the flow velocity and bed morphology parameters.
Well-behaved cyclic oscillations in the auto-correlation and cross-correlation functions
confirm the periodicity of the observed fluctuations and show that the durations of these
cycles are similar, although not necessarily in phase. A detailed analysis of data time
series and image acquired during the tests show a link between bedload pulses
and different bed states, boulder protrusion, and surface grain size distributions. A
feedback system exists among channel morphology, flow kinematics and sediment
transport.
A phase analysis for the observed variables, based on the identification of bedload cycles
in the instantaneous signal, is performed. The link between the phases of bedload
and each of the morphological parameters show a hysteretic path. The relation
between the phase-averaged bedload and the phase-averaged flow velocity show a
considerable lesser degree of hysteresis. Comparing the phase averaged bedload of the
experiments, it is observed that the shape of bedload cycles is the same for all tested
hydraulic conditions. The cycles present a long duration low sediment transport event
and a shorter peak transport event. This indicates that long periods of sediment
aggradations alternate with short erosion periods, even under constant hydraulic
conditions.
The bedload pulses may be characterized by their amplitude and period as a function of
various boulder spatial densities and diameters. We show that for higher stream power, the
fluctuations decrease, both in cycle duration and in amplitude. The presence of boulders
increases the stream power needed to transport a given amount of sediment, thus decreasing
fluctuations.
KEY WORDS: Bedload fluctuations; Morphological changes; Sediment transport;
Boulders; Steep channel. |
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