Mixing layer development in compound channel flows with submerged and emergent rigid vegetation over the floodplains

Dupuis, Victor; Proust, Sébastien; Berni, Céline; Paquier, André

doi:10.1007/s00348-017-2319-9

Cited by 52 publications

(54 citation statements)

References 37 publications

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“…Lateral profiles of mean velocities and turbulence quantities have been measured at different x-stations for each transition. As shown in our previous study (Dupuis et al, 2017), the properties of the compound channel mixing layer (width, dimensionless velocity difference λ, etc.) change across the water column.…”

Section: Mixing Layer Dynamicsmentioning

confidence: 60%

“…In our previous study (Dupuis et al, 2017), we observed that for the two flows with uniform roughness CM and CW, the normalised profiles of longitudinal velocity and turbulence quantities were self-similar when going downstream. The velocity was normalised in the form (U − U 2 )/(U 1 − U 2 ) and, to take into account the mixing layer asymmetry, the lateral coordinate was normalised by the subsection mixing layer width, i.e.…”

Section: Normalised Lateral Profilesmentioning

confidence: 76%

“…We showed in Dupuis et al (2017) that this velocity is a fair estimate of the convection velocity of the coherent structures. Length scales L 22 at the interface and at z f = H f /2.…”

Section: Coherent Structuresmentioning

confidence: 76%

“…The transition occurs between a rough bed, representing a submerged dense meadow, and an array of emergent cylinders, representing a woodland with emergent trees. As pointed out in Dupuis et al (2017), compound channel flows with either a bed roughness or a cylinder array on the floodplain feature, for the same total discharge, different cross-sectional distributions of longitudinal momentum, of secondary currents and of turbulent quantities. In addition, if the mixing layer develops self-similarly all along the flume for both roughness types, its growth rate significantly differs from one type to another.…”

Section: Introductionmentioning

confidence: 89%

“…1b), corresponding to different floodplain land occupations: (1) floodplains covered by meadow along the whole flume length (configuration denoted CM for Compound channel with Meadow), (2) floodplains covered by wood along the whole length (CW: Compound channel with Wood), (3) floodplains covered by wood in the upstream half of the flume and by meadow in the downstream half (CWM: Compound channel with a longitudinal transition from Wood to Meadow) and (4) floodplains covered by meadow in the upstream half and by wood in the downstream half (CMW: Compound channel with a longitudinal transition from Meadow to Wood). The first two flow configurations (CM and CW) were investigated in detail in Dupuis et al (2017). Figure 2a shows a picture of configuration CWM.…”

Section: Experimental Setup and Methodologymentioning

confidence: 99%

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Compound channel flow with a longitudinal transition in hydraulic roughness over the floodplains

et al. 2017

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Transitions from submerged dense vegetation (meadow) to emergent rigid vegetation (wood) and vice versa are modelled using plastic grass and vertical wooden cylinders. For a given roughness transition, the upstream discharge distribution between main channel and floodplain (called subsections) is also varied, keeping the total flow rate constant. The flows with a roughness transition are compared to flows with a uniformly distributed roughness over the whole length of the flume. Besides the influence of the downstream boundary condition, the longitudinal profiles of water depth are controlled by the upstream discharge distribution. The latter also strongly influences the magnitude of the lateral net mass exchanges between subsections, especially upstream from the roughness transition. Irrespective of flow conditions, the inflection point in the mean velocity profile across the mixing layer is always observed at the interface between subsections. The longitudinal velocity at the main channel/floodplain interface, denoted U int , appeared to be a key parameter for characterising the flows. First, the mean velocity profiles across the mixing layer, normalised using U int , are superimposed irrespective of downstream position, flow depth, floodplain roughness type and lateral mass transfers. However, the profiles of turbulence quantities do not coincide, indicating that the flows are not fully self-similar and that the eddy viscosity assumption is not valid in this case. Second, the depth-averaged turbulent intensities and Reynolds stresses, when scaled by the depth-averaged velocity U d,int exhibit two plateau values, each related to a roughness type, meadow or wood. Lastly, the same results hold when scaling by U d,int the depth-averaged lateral flux of momentum due to secondary currents. Turbulence production and magnitude of secondary currents are increased by the presence of emergent rigid elements over the floodplains. The autocorrelation functions show that the length of the coherent structures scales with the mixing layer width for all flow cases. It is suggested that coherent structures tend to a state where the magnitude of velocity fluctuations (of both horizontal vortices and secondary currents) and the spatial extension of the structures are in equilibrium.

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Section: Mixing Layer Dynamicsmentioning

confidence: 60%

Section: Normalised Lateral Profilesmentioning

confidence: 76%

“…We showed in Dupuis et al (2017) that this velocity is a fair estimate of the convection velocity of the coherent structures. Length scales L 22 at the interface and at z f = H f /2.…”

Section: Coherent Structuresmentioning

confidence: 76%

Section: Introductionmentioning

confidence: 89%

Section: Experimental Setup and Methodologymentioning

confidence: 99%

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Compound channel flow with a longitudinal transition in hydraulic roughness over the floodplains

et al. 2017

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Mixing layer and coherent structures in compound channel flows: Effects of transverse flow, velocity ratio, and vertical confinement

Proust

Fernandes

Leal

et al. 2017

Water Resources Research

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Turbulent mixing layers associated with streamwise uniform and nonuniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise nonuniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth‐averaged mean flow. This study first aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study second aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds‐stresses increase with local velocity ratio for a fixed confinement and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi‐two‐dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow toward the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi‐two‐dimensional structures.

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Flow behaviour in a multi‐layered vegetated floodplain region of a compound channel

Barman

Kumar

2022

Ecohydrology

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Riparian vegetation plays a crucial role in determining the flow behaviour in the channel. The effect of flow on the slope and main channel varies based on the size, type, and density of floodplain vegetation in a compound channel. Though real field vegetation distribution is non‐uniform, most studies mainly concentrate on uniformly distributed vegetation with fixed vegetation height. This paper attempts to address this issue through laboratory studies as it was not explored properly. Flow properties like velocity, Reynolds shear stress, turbulence intensities, and turbulent kinetic energy behave differently in heterogeneous vegetated channel compared with the homogeneous vegetated channel. These flow properties in the slopes and main channel section are more pronounced for uniformly distributed vegetation than non‐uniform distribution. The multi‐layered/varying vegetation height showed higher velocity, turbulent intensity, and turbulent kinetic energy than single‐layered/constant vegetation height on slopes and main channel sections. Integral scales (Taylor and Euler) analysis showed greater magnitude for uniform multi‐layered vegetation than non‐uniform multi‐layered vegetation. Further, the study of octant analysis reveals that internal outward and internal inward interaction events dominate the slope floodplain interaction section, which is not seen in no‐vegetation cases. These studies give a better perspective to understand flow in heterogenous vegetation and move closer to real field scenarios.

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Mixing layer development in compound channel flows with submerged and emergent rigid vegetation over the floodplains

Cited by 52 publications

References 37 publications

Compound channel flow with a longitudinal transition in hydraulic roughness over the floodplains

Compound channel flow with a longitudinal transition in hydraulic roughness over the floodplains

Mixing layer and coherent structures in compound channel flows: Effects of transverse flow, velocity ratio, and vertical confinement

Flow behaviour in a multi‐layered vegetated floodplain region of a compound channel

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