Bed load transport and topography evolution in the open channel with a partially-distributed vegetation patch

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Xufeng Yan, Wing Hong Onyx Wai, Chi Wai Li

Wednesday 1 july 2015

9:30 - 9:45h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 8B. Sediment - River

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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.