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| Titel |
Streamwise decrease of the ’unsteady’ virtual velocity of gravel tracers |
| VerfasserIn |
Mario Klösch, Philipp Gmeiner, Helmut Habersack |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250150599
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| Publikation (Nr.) |
 EGU/EGU2017-15075.pdf |
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| Zusammenfassung |
| Gravel tracers are usually inserted and transported on top of the riverbed, before they disperse
vertically and laterally due to periods of intense bedload, the passage of bed forms, lateral
channel migration and storage on bars. Buried grains have a lower probability of entrainment,
resulting in a reduction of overall mobility, and, on average, in a deceleration of the
particles with distance downstream. As a consequence, the results derived from tracer
experiments and their significance for gravel transport may depend on the time scale
of the investigation period, complicating the comparison of results from different
experiments.
We developed a regression method, which establishes a direct link between the transport
velocity and the unsteady flow variables to yield an ‘unsteady’ virtual velocity, while
considering the tracer slowdown with distance downstream in the regression. For that
purpose, the two parameters of a linear excess shear velocity formula (the critical shear
velocity u*c and coefficient a) were defined as functions of the travelled distance since the
tracer’s insertion.
Application to published RFID tracer data from the Mameyes River, Puerto Rico,
showed that during the investigation period the critical shear velocity u*c of tracers
representing the median bed particle diameter (0.11 m) increased from 0.36 m s−1 to 0.44
m s−1, while the coefficient a decreased from the dimensionless value of 4.22 to
3.53, suggesting a reduction of the unsteady virtual velocity at the highest shear
velocity in the investigation period from 0.40 m s−1 to 0.08 m s−1. Consideration of
the tracer slowdown improved the root mean square error of the calculated mean
displacements of the median bed particle diameter from 8.82 m to 0.34 m. As in previous
work these results suggest the need of considering the history of transport when
deriving travel distances and travel velocities, depending on the aim of the tracer
study. The introduced method now allows estimating the travel velocities directly
after seeding (representing the velocity of sediment at the bed surface subject to
actual transport), or the longer term transport of sediment, helping to understand the
velocity of sediment transfer in river networks as a basis for catchment-wide river
restoration plans in the course of the project ’HyMoCARES’, which is co-financed by
the European Regional Development Fund within the Alpine Space programme. |
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