A Review on Hydrodynamics of Free Surface Flows in Emergent Vegetated Channels

Abstract

This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

Bed Roughness Effects on the Turbulence Characteristics of Flows through Emergent Rigid Vegetation

“…It must be pointed out that riparian vegetation is very varied and may be broadly divided into rigid/flexible, depending on the mechanical properties of the plant, and emergent/submergent, if the flow depth exceeds the height of vegetation or does not, respectively (e.g., [13,[19][20][21][22][23][24][25][26]). Much research has been devoted to the study of flow-plant interaction in recent years, as demonstrated by the growth of scientific publications dealing with this topic.…”

“…Concentrating on emergent vegetation, very recently, Maji et al [26] compiled a comprehensive state-of-the-art, highlighting important research contributions on flow hydrodynamics, including complex interactions between turbulence and vortex structures. Most of the studies have been focused on the investigation of the interaction between flow and vegetation, considering experimental or numerical tests, on a smooth channel bed with staggered-arranged vegetation stems [47,48] or fully and uniformly covered by artificial flexible grass [49,50].…”

Modelling transverse solute mixing across a vegetation generated shear layer

“…The reduction in the mean flow velocity promotes sedimentation, whilst a reduction in contaminant concentration can be achieved through dispersion and bio-chemical degradation. It follows that the detention of contamination, and subsequent bio-chemical degradation, is affected by the reach hydrodynamics (Maji et al, 2020;Persson et al, 1999;Koskiaho, 2003).…”

Flow Resistance in Open Channel Due to Vegetation at Reach Scale: A Review

“…Experimental studies of velocity profiles, like those shown in Figures 3-5, are of much interest; in some cases also the shear stress profiles are studied by means of velocity fluctuant components and Reynolds stress theory [19,[107][108][109][110][111].…”