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| Titel |
Hydrodynamic Controls of Immobile Boulders on Bed Load Grain Movement in Mountain Streams |
| VerfasserIn |
Achilleas Tsakiris, Thanos Papanicolaou |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107996
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| Publikation (Nr.) |
 EGU/EGU2015-7725.pdf |
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| Zusammenfassung |
| Understanding the interaction between the turbulent fluid flow and bed load sediment flux in
a river is fundamental for predicting erosion and landscape evolution, managing riverine
infrastructure, and protecting and improving aquatic habitat. To date, this fundamental
understanding is lacking especially in steep mountain streams, where transport occurs
close to the threshold of entrainment and bed load flux is highly intermittent. A
unique feature of mountain streams that warrants special attention, and which is the
focus of this study, is the presence of large, rarely-mobile boulders. Because of their
large size, these boulders may become fully or partially submerged, thus exhibiting
high or low relative submergence, H/dc, respectively, with H and dc denoting
the approach flow depth and the boulder diameter. It is speculated that the type of
submergence affects the dominant vortex topology around the boulders. We test two
hypotheses, namely: (1) that the topology of the vortex structures resulting from the
interaction of the boulders with the approach turbulent flow varies with varying relative
submergence; and (2) that the patterns of motion of mobile sediment around the
boulders are influenced by the vortex structure topology developing for each relative
submergence condition. A series of detailed flume experiments were conducted for a single
boulder mounted atop a flat rough bed under full and partial submergence conditions.
For each condition, detailed flow field measurements were acquired using Particle
Image Velocimetry (PIV). The PIV measurements revealed that the topology of
the developing vortices around the boulder was distinctly different for the high
and low relative submergence conditions. More specifically, the wake of the fully
submerged boulder was dominated by arch structures, which tilted downstream
under the intense ambient flow shear, as well as by a pair of inner vortices resulting
from the roll-up of the secondary vorticity layer on the boulder surface. In contrast,
under low relative submergence conditions, a pair of strong von-Karman vortices
tilting into the downstream direction was observed within the boulder wake. The
different vortex structures identified for the high and low relative submergence
conditions were found to exert a profound influence on the direction of the bed shear
stress vector within the horizontal bed plane. In particular, under the high relative
submergence, the bed shear stress vector was directed towards the boulder centerline,
which promotes the entrapment of mobile sediment within the boulder wake. In
contrast, the von-Karman vortex street was found to deflect the bed shear stress vector
away from the boulder centerline, which would, in turn, enhance the entrainment of
incoming mobile particles away from the boulder wake. The directionality of the bed
shear stress tensor was in accordance with earlier observations of bed load particle
movement patterns under high and low relative submergence conditions. The findings of
this research highlight the profound influence of large, rarely-mobile boulders on
the interaction between surrounding turbulent fluid flow and the bed load grain
movement with significant implications in the management of riverine environments. |
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