Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Effects of submerged rigid vegetation on sediment transport under unidirectional currents, waves andthe combined wave-current flows
Hao Wang1, Sha Lou 1,2*, Xiaosheng Zhou1, Larisa Dorzhievna Radnaeva3，Elena Nikitina3
1

Department of Hydraulic Engineering, Tongji University, Shanghai, China

2 Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, Shanghai, China
3

Laboratory of chemistry of natural systems, Baikal Institute of Nature Management of Siberian branch of the Russian Academy of sciences,
Ulan-Ude, Republic of Buryatia, Russia

ABSTRACT
Vegetation has great influence on flow structure and sedimenttransport, which will further affect the pollution diffusion and the ecosystem. Laboratory experiments were carried out in this paper toinvestigate the effects of submerged rigid vegetation on sedimenttransport under waves, currents and combined wave-current flows.
The flow structure and sediment concentrations affected by thesubmerged rigid vegetation were measured by using Acoustic Doppler
velocimetry (ADV) and Optical Backscattering turbidity sensor (OBS).
The effects of vegetation on the flow velocities, turbulence kinetic
energy (TKE) and sediment distributions were analyzed. According to
the results, it was founded that flow velocity was reduced byvegetation due to the obstruction of vegetation stem, and the velocity

gradient was the largest near the top of the canopy. TKE increased at
the bottom under the unidirectional flow, which was mainly caused bywake turbulence around vegetation stems. Under the regular waves
and combined wave-current flows, more than one inflection points
were found in the vertical profiles of TKE, because of the combined
effects of shear turbulence (canopy-scale) and wake turbulence (stemscale). The near bed sediment concentration was higher in case withdenser vegetation. In flow with vegetation, the bed shear stress could
no longer be used as an index to judge the incipient motion of sediment,
but the wake turbulence around the vegetation stem was the main
influencing factor.
Keywords: Submerged rigid vegetation, Flow structure, Sedimentconcentration, Turbulent kinetic energy, Wake turbulence
wake turbulence in the canopy, the sediment deposition increased.
Yang and Nepf (2018) found that with the increase of vegetation
density, wake turbulence in canopy increased, which led to thedecrease of critical velocity of sediment incipient motion and theincrease of sediment transport rate on bed surface.
The interactions among vegetation, flow and sediment is
influenced by both of vegetation features and hydrodynamicconditions. In this paper, experimental tests were carried out to analyze
the effects of different vegetation features (i.e., height and density) onflow velocity, turbulent energy and sediment concentration undercurrents, waves and the combined wave-current flows. It aims toexplore the influence of submerged vegetation on the flow structureand sediment transport. The results can provide theoretical basis andtechnical support for environmental protection of estuarine and coastal
wetlands.

INTRODUCTION
Wetland is an important part of natural ecosystem, which hasgreat environmental functions and economic benefits, and plays an
irreplaceable role in flood control, waterlogging reduction, runoff
regulation, environmental purification and soil erosion reduction.
(Shang et al. 2018; Wang et al. 2020). Aquatic vegetation commonly
exists in the wetland ecosystem, which can affect the flow velocity,sediment transport, as well as the pollutant diffusion. Therefore, it isof great significance to study the flow structure and sediment transport
under the influence of vegetation, which can provide useful
information for wetland management.
Under the influence of vegetation density, water depth and therelative water depth (i.e., the ratio of vegetation height to water depth),
the flow velocity, turbulence intensity and Reynolds stress of the flow
with vegetation were significantly different from those without
vegetation (Tang et al. 2007). Kouwen (1969) considered that the flow
above the top of the canopy obeyed logarithmic distribution, but notwithin the canopy. In addition, it had been suggested that there was avelocity transition zone near the top of vegetation, where the flow
resistance had an inflection point and the velocity changed greatly(Stephan and Gutknecht, 2002). In vegetated flow, the average kinetic
energy of the eddy generated by the flow passing through thevegetation was converted into turbulence energy, which increased theturbulence intensity of the flow (Nepf et al.1997; Nepf,1999).
However, due to the small scale of the eddy, the turbulence diffusion
intensity in the canopy was relatively weak. On the one hand,vegetation increased the overall flow resistance, resulting in thedecrease of flow velocity and the shear stress on the bed surface; Onthe other hand, it also promoted the turbulence of flow, especially atthe bottom of the stem, thus affecting the initiation and transportationof sediment. Liu and Nepf (2016) found that when the wake turbulencein the canopy exists, the sediment deposition decreased and thesediment under the bed eroded. On the contrary, when there was no1.Theoretical backgroundFor unidirectional flow, instantaneous velocity, u(t), is the sumof average velocity (Uc) and turbulent velocity (u’), as shown in Eq.
(1). For regular waves, the instantaneous velocity along the wave
propagation direction at each moment can be expressed as Eq. (2).

u (t ) = U c + u 'u (t ) = U mean + U w sin( t ) + u '(1)(2)

Where θ is the frequency of the wave; t is time; Uw is the wave orbital
velocity.
Eq. (2) can be used to calculate the instantaneous velocity
fluctuation in the longitudinal direction. Similarly, the lateral andvertical velocity fluctuations, v’ and w’, can be calculated in the sameway, so that the turbulent kinetic energy, TKE, can be calculated byEq. (3).

2842