Abstract
Over the last two decades, the role of vegetation in the environmental and ecological restoration of surface water bodies has received much attention. In this context, the momentum exchange between the flow through the main channel and the riparian zone is a key mechanism. The primary goal of this study is to investigate the role of bank vegetation density on flow dynamics across the whole channel. This experimental study presents the major findings from a series of flow measurements across a channel having a sloping bank with vegetation at varying densities. The experiments are conducted under the same, uniform flow and fixed bed conditions, for a range of six linear and rectilinear arrangements of incremental streambank vegetation densities. A set of ten velocity profiles is obtained across the test cross-section of the channel, including the riverbank, for each vegetation density. These flow measurements are analyzed to derive roughness coefficients, which are related to the bulk flow velocities through the main channel and the riverbank and discuss the redistribution of flow velocities. An approximate doubling for the estimates of time-averaged boundary shear stress at the main channel, is observed for the case of no to dense vegetation, which enable further discussing implications for the stability of bed surface material. It is found that the vegetation arrangement, in addition to vegetation density, can have a strong impact in modifying the mean flow velocity at the main channel, for low riparian densities (φ < 0.6%).HighlightsFlow dynamics are measured across the whole channel, including the vegetated riverbank.As stem density increases, mean flow velocity in the main channel increases while mean flow at the riverbank decreases.The arrangement of riparian vegetation can be as important as that of the density, in modifying the mean flow field of the main channel, for low riparian densities.Bed shear stresses at the main channel are estimated to increase with riverbank vegetation, reducing the stability of the stream’s bed surface.
Highlights
Riparian and aquatic vegetation can affect natural streams and waterways in many ways
The measurement grid used to assess flow dynamics comprised of three velocity profiles at the main channel and three profiles at the interface between the main channel and sloping bank, capturing the development of the shear velocity layer (Fig. 2a)
The above analysis of time averaged velocity profiles demonstrates that a clear trend exists: streamwise flow velocities are consistently lower across the riverbank with the increase of riparian vegetation, while the reverse situation holds at the main channel
Summary
Riparian and aquatic vegetation can affect natural streams and waterways in many ways. Nepf [5] used an experimental setup with rectilinear arrays of rods, illustrating the link between turbulence intensity within vegetation and hydrodynamic drag; Wilson et al [7] conducted a laboratory study to compare the effects of aquatic vegetation on flow dynamics for the cases where rods with and without front canopy are used to simulate instream vegetation. In these experiments, even though the vegetation density has been locally changed (e.g., by adding canopy), the rectilinear pattern of vegetation array remained unaltered
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