A study of overbank flows in non-vegetated and vegetated floodplains in compound meandering channels
thesisposted on 02.02.2011, 09:13 authored by Zulhilmi Ismail
Laboratory experiments concerning stage-discharge, flow resistance, bedforms, sediment transport and flow structures have been carried out in a meandering channel with simulated non-vegetated and vegetated floodplains for overbank flow. The effect of placing solid blocks in different arrangements as a model of rigid, unsubmerged floodplain vegetation on a floodplain adjacent to a meandering channel is considered. The aim was to investigate how density and arrangements of floodplain vegetation influence stage-discharge, flow resistance, sediment transport and flow behaviours. Stage-discharge curves, Manning's n and drag force FD are determined over 165 test runs. The results from the laboratory model tests show that the placing of solid blocks along some part of the bend sections has a significant effect on stage-discharge characteristics. The change in stage-discharge by the blocks is compared using different arrangements, including the non-vegetated floodplains case. The experimental results show that the presence of energy losses due to momentum exchange between the main channel and the floodplain as well as the different densities of the blocks on a floodplain induce additional flow resistance to the main channel flow, particularly for shallow overbank flows. In general, the results show that the density and arrangement of blocks on the floodplains are very important for stage-discharge determination and, in some cases, for sediment transport rates, especially for a mobile main channel. Also, the correction parameter, a is introduced in order to understand the effects of blocks and bedforms on the force balance equation. By applied the correction factor c; a stagedischarge rating curve can be estimated when the avalue is calibrated well. Telemac 2D and 3D were applied to predict mean velocity, secondary flow and turbulent kinetic energy. Telemac computations for non-vegetated and vegetated floodplain cases in a meandering channel generally give reasonably good predictions when compared with the measured data for both velocity and boundary shear stress in the main channel. Detailed analyses of the. predicted flow variables were therefore carried out in order to understand mean flow mechanisms and secondary flow structures in compound meandering channels. The non-vegetated and two different cases of vegetated floodplain for different relative depths were considered. For the arrangement on a non-vegetated floodplain shows how the shearing of the main channel flow as the floodplain flow plunges into and over the main channel influences the mean and turbulent flow structures, particularly in the cross-over region. While applying vegetated floodplain along a cross-over section confirmed that the minimum/reduction shearing of the main channel flow by the floodplain flow plunging into and over the main channel is observed from the cross-sectional distributions of the streamwise velocity (U), lateral velocity (V), and secondary flow vectors. In addition to that, the vegetated floödplain along the apex bend region shows a small velocity gradient within the bend apex region. However, strong secondary flow in the cross-over section suggested that the flow interaction was quite similar to the non vegetation case in the cross-over section region.
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