An experimental study of flow through rigid vegetation

Abstract

[1] Better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation. Instead of spatial averaging, this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels. Velocity data was collected with a one dimensional laser Doppler velocimeter under emergent and submerged flow conditions. The effects of dowel arrangement, density, and roughness were also examined. The results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it, however the boundaries are marked by inflection points. The strongest vortices and turbulence intensities can be found there, especially in the region immediately downstream of a dowel. These results support the idea that the flow in the region near the bed and at the top of the dowel array is very unstable leading to the formation of coherent structures and are areas of significant mass and momentum exchange.

Model Versus Nature: Hydrodynamics in Mangrove Pneumatophores

“…In the case of uniform emergent vegetation, reduced current velocities are found to be approximately constant over the water depth, except in the near-bed region where additional bed shear causes a local velocity peak (Liu et al, 2008;Nepf, 1999;Yager and Schmeeckle, 2013). The velocity within emergent vegetation monotonically decreases with the vegetation density (ad = Nd 2 , wherein N is the number of stems per unit area and d the stem diameter), the water depth-to-stem diameter ratio (H/d) and the surface roughness (CD) of the vegetation elements (Liu et al, 2008;Nepf, 1999). The turbulence intensity, expressed as the square-root of the turbulent kinetic energy divided by the flow velocity, k 1/2 /u, is constant over depth outside the bed-shear layer, similar to the velocity profile (Liu et al, 2008).…”

“…Mazda and Wolanski, 2009;Nepf, 2012;Tempest et al, 2015). In the case of uniform emergent vegetation, reduced current velocities are found to be approximately constant over the water depth, except in the near-bed region where additional bed shear causes a local velocity peak (Liu et al, 2008;Nepf, 1999;Yager and Schmeeckle, 2013). The velocity within emergent vegetation monotonically decreases with the vegetation density (ad = Nd 2 , wherein N is the number of stems per unit area and d the stem diameter), the water depth-to-stem diameter ratio (H/d) and the surface roughness (CD) of the vegetation elements (Liu et al, 2008;Nepf, 1999).…”

“…Above a submerged canopy with height hc, a characteristic logarithmic velocity profile develops for depth ratios H/hc > 1.25 (Liu et al, 2008;Nepf and Vivoni, 2000), with an inflection point close to the top of the canopy. Often Kelvin-Helmholtz vortices are generated in this canopy shear layer, giving rise to a dynamic mixing layer and a local peak in the turbulence profile (Chen et al, 2013;Liu et al, 2008;Raupach et al, 1996).…”

“…The velocity within emergent vegetation monotonically decreases with the vegetation density (ad = Nd 2 , wherein N is the number of stems per unit area and d the stem diameter), the water depth-to-stem diameter ratio (H/d) and the surface roughness (CD) of the vegetation elements (Liu et al, 2008;Nepf, 1999). The turbulence intensity, expressed as the square-root of the turbulent kinetic energy divided by the flow velocity, k 1/2 /u, is constant over depth outside the bed-shear layer, similar to the velocity profile (Liu et al, 2008). While turbulence intensity increases with the presence of sparse emergent vegetation, a critical vegetation density exists beyond which the turbulence decreases because of the reduction in velocity with increased density (Nepf, 1999).…”

“…Often Kelvin-Helmholtz vortices are generated in this canopy shear layer, giving rise to a dynamic mixing layer and a local peak in the turbulence profile (Chen et al, 2013;Liu et al, 2008;Raupach et al, 1996). As with emergent canopies, the elevation of the logarithmic velocity profile with respect to the top of the submerged vegetation, and consequently the penetration depth of the turbulence into the vegetation, depends on the vegetation density, the relative height of the vegetation (H/hc) and the surface roughness of the vegetation elements (Liu et al, 2008;Lowe et al, 2005;Nepf and Vivoni, 2000).…”