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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Sediment management and morphodynamics : Bed load transport and bed topography evolution in the open channel with a
Bed load transport and bed topography evolution in the open channel with a
Author : X.F. YAN(1), W.H.O. Wai(2), & C.W. Li(3)
ABSTRACT
In many actual circumstances, aquatic vegetation patches are only partially distributed on the riverbed. This discontinuous
vegetative blockage induces spatial non-uniformity of water conveyance and sediment transport along the river. The drag
force arising from the vegetation elements significantly decreases the flow velocity within the vegetation region (VR),
however, leads to the increase in velocity the neighboring open water region (NOWR). The associated near-bed velocity,
responsible for the sediment erosion and deposition on the riverbed, turns out to be large for NOWR and small for VR,
respectively. On the other hand, flows in the two flow transitional regions downstream of the leading and trailing edges
gradually vary due to the flow convergence and divergence in the longitudinal direction. The bed load transport and erosion
and deposition effects, therefore, are spatially non-universal under such partially-distributed vegetation patch configuration.
In this study, clean water scour tests are carried out to investigate the effect of flows over a half-channel-width vegetation
patch on sediment motion and bed topography evolution. Rigid polystyrene cylinders are used to mimic vegetation
elements. Non-uniform particles with typical grain size distributions of bed-load-transport dominant rivers are laid on the
bed. The bed load transport rate, bed topography, and sorting effect are evaluated. Phenomena including non-uniform
sediment transport, sand dune, bed armoring and surface sediment sorting are identified. The sediment mixture
characterized by the median diameter and geometric standard deviation in the bed load transport process varies a lot. In
the stage of high bed load transport, coarser sediments are transported downstream rather than finer sediments which are,
to some extent, transversely transported. The bed on the non-vegetated side is strongly eroded and scoured. However, on
the vegetated side the bed remains constant except for that just downstream of the leading edge of the vegetation patch. A
two-stage bed channel forms, as compared to the compound channel. Downstream fining effect with a long sediment bar
was observed downstream of the trailing edge of vegetation patch.
File Size : 539,710 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Sediment management and morphodynamics
Date Published : 18/08/2015
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