Vegetated mixing layer around a finite‐size patch of submerged plants: 1. Theory and field experiments

Abstract

[1] Dynamics of a vegetated mixing layer around a patch of submerged plants was examined theoretically and experimentally in a lowland river. Theoretical analysis explored analogy with hydrodynamic mixing layers and introduced equations that describe expansion of the layers and alterations in mean flow. Field experiments provided empirical data for validation of the theory and examination of the effects of population density on the mixing layer dynamics. Detailed measurements assessed the structure of turbulent flow, plant morphology, biomechanical properties, bending, and streamlining due to interaction with the flow. The theory was compared with the field experiments and an agreement was concluded. It was found that near the upstream edge of the patch, the vegetated mixing layer expands similar to the canonical mixing layers, although its statistics scale on the velocity differential characteristic for the stabilized part. Downstream mean momentum flux reduces in magnitude and reverses in direction, while turbulence became dominant. This was observed to stabilize the velocity differential and increase the mean velocity of the layer that result in its limited growth described by a logarithmic function. The population density of vegetation was found to control the flow. In dense vegetation, the analogy with mixing layers resembled best, while in sparse vegetation, the flow behaved similar to the boundary layers. This theory can be used for quantitative assessment of seasonal effects in natural vegetated rivers because the population density of vegetation varies during the vegetation season. The companion paper focuses on the structural properties of turbulence in the vegetated shear layers.

High-resolution numerical modelling of flow—vegetation interactions

“…Following Sukhodolova and Sukhodolov (2012), a value of 0.09 has been used here. The change in vortex thickness through time, as measured using the Q and λ 2 criterion (Fig.…”

Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

“…As a consequence of the 2D model approach, it is not possible to derive the stream velocities in and above the vegetation separately. Also, detailed 3D processes are expected to occur around finite submerged vegetation patches [15,64]; vertical circulation patterns and expansion of mixing layers are, however, not captured by the presented 2D model. Skimming flow results in flow separation within and above the canopy, with reduce flow within the canopy and a boundary layer developing above the canopy [65].…”

Turbulence in Rivers

“…The size of the eddies generated by individual boundary protuberances scales, typically, with the dimension of the protruding elements. Examples of boundary singularities and irregularities in rivers causing eddy generation include sediment clusters or boulders protruding from the riverbed [submerged (Buffin-Bélanger and Roy 1998;Buffin-Bélanger et al 2000;Franca 2005b) or emerging (Tritico and Hotchkiss 2005)], high relative roughness riverbeds (Baiamonte et al 1995;Katul et al 2008), vegetation patches (Tanino and Nepf 2008;Siniscalchi et al 2012;Sukhodolova and Sukhodolov 2012;Ricardo 2014), wood debris or remains of organic elements (Shields et al 2004;Blanckaert et al 2014).…”