Flow behaviour due to floodplain roughness along riparian zone in compound channels
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Date
2015
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Universiti Teknologi Malaysia
Abstract
Main channel and floodplain vegetation generally influence the flow behavior in the flooding rivers or compound channels. The hydraulics of vegetated compound channels is complex and still not fully understood. The aim of the research was to enhance the knowledge on the effects of two-line staggered arrays with closed-spacing emergent vegetation or roughness elements along the riparian zone on the non-mobile bed compound straight and meandering channels flows. The experiments were carried out in the hydraulics laboratory of Faculty of Civil Engineering, Universiti Teknologi Malaysia. The experimental investigations were focused on an asymmetrical compound straight channel and a straight, vertical floodplain-walls meandering channel with a sinuosity of 1.54. In addition, the Shiono and Knight Method (SKM) was examined to simulate the transverse distributions of velocity in non-vegetated and vegetated compound straight channels. The physical processes experimentally investigated were stage-discharge, flow resistance, three-dimensional turbulent flow structures, boundary shear stress, momentum transfer and drag force for selected flood flow depths in both compound channels. The densest riparian vegetation increased 6.1% and 32% of flow depth in compound straight and meandering channels, respectively. It also generated the largest flow resistance in both channels. The secondary currents movements were limited due to the presence of floodplain vegetation and the current strength in meandering channel was stronger than in straight channel. The floodplain vegetation also lowered boundary shear stress and a maximum of 77.8% boundary shear stress drop was observed for the densest vegetation case in compound meandering channel. Momentum transfers between main channel and floodplain were quantified as transverse Reynolds and apparent shear stresses. Peak stresses were found in the shear layer zones and increased with density of riparian vegetation. The vegetative-induced drag force FD was calculated using force balance approach and the result showed that it increased with flood flow depth. In addition, the smallest riparian vegetation spacing produced the largest drag coefficient CD in both non-mobile bed compound channels. The hydraulic simulations showed that SKM was able to produce well prediction of the depth-averaged velocity in non-vegetated and vegetated compound straight channels. The findings also revealed that compound meandering channel flow was more complex since its structures were spatially varied along the meander due to channel geometrical properties
Description
Thesis (Ph.D (Civil Engineering))
Keywords
Floodplain management, Riparian plants