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Flow depth, velocity, and sediment motions in a straight widened channel with vegetated floodplains

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Abstract

Experiments were conducted in a straight channel that widens from a single channel to a compound channel based on a real navigation channel with a similar shape in a mountainous region of China. Flow entered the single channel and then the compound channel with vegetated floodplains. The channel widened at the interface between the single and compound channels, resulting in different flow depths between the single and compound channels. An equation was proposed to estimate the percentage that the flow depth decreased from the single channel to the compound channel. Vegetation increased the floodplain resistance, causing more water to flow in the main channel and yielding a greater mean velocity in the main channel relative to the nonvegetated case. The velocity reached a maximum and gradually decreased until reaching a constant in the compound channel. Sediment motion (bedload vs. deposition) was determined by the local bed shear and the critical bed shear. Sediment was deposited in the compound channel at the position where the local bed shear was smaller than the critical bed shear. When the floodplains were vegetated, the increase in the main channel velocity enhanced the local bed shear, which surpassed the critical bed shear; thus, deposited sediment was reinitiated and moved downstream, and no deposition was observed.

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Abbreviations

A :

Channel area

A fp :

Floodplains channel area

A mc :

Main channel area in the compound channel

A sc :

Channel area in the single channel

a :

Frontal area of vegetation per unit volume

B :

Compound channel width

B f :

Floodplain width

B m :

Main channel width

B s :

Single channel width

Dr :

Relative flow depth in the compound channel

d :

Stem width

d s :

Particle diameter

d * :

Dimensionless particle parameter

Fr :

Channel Froude number

g :

Local gravitational acceleration

H :

Flow depth in the single channel or in the main channel of the compound channel

H cc :

Flow depth at fully developed flow region in the main channel of the compound channel

H f :

Flow depth at fully developed flow region on the floodplain of the compound channel

H sc :

Mean flow depth at #-3.5 and #-3 in the single channel

h :

Bankfull depth

\(\overline{h}\) :

Mean flow depth

h v :

Grass height

K :

Velocity ratio

N :

Grass density

n :

Manning’s coefficient

Q :

Upstream discharge

Q fp :

Floodplain discharge at fully developed flow region in the compound channel

Q mc :

Main channel discharge at fully developed flow region in the compound channel

R :

Hydraulic radius

Re :

Channel Reynolds number

s :

Bed slope

U d :

Depth-averaged velocity

U fp :

Floodplain velocity at fully developed flow region in the compound channnel

U mc :

Main channel velocity at fully developed flow region in the compound channel

U sc :

Mean single channel velocity at -3.5# and 3# in the single channel

U(x):

Main channel velocity at measurement transects

u :

Measured velocity

u * :

Local shear velocity

u *c :

Critical shear velocity

u *cc :

Local shear velocity in the compound channel

u *sc :

Local shear velocity in the single channel

x, y, z :

Longitudinal, lateral and vertical directions, respectively

δ 0 :

Viscous sublayer thickness

ε r :

Absolute relative error

v :

Kinematic viscosity

ρ :

Flow density

ρ p :

Sediment density

τb :

Bed shear stress

τz :

Spatially averaged shear stress

τzx :

Total shear stress

τzx Rey :

Reynolds shear stress

τzx vis :

Viscous shear stress

τ*c :

Critical Shields parameter

β :

Scale factor

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Acknowledgements

This project was supported by the National Natural Science Foundation of China (51879175), the Fok Ying Tung Education Foundation (171067) and Ministry of Education Chunhui Project (2020-703-3). The longitudinal profiles of flow depths and velocities are all shown in Supporting Information.

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Correspondence to Chao Liu.

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Li, F., Shan, Y., Huang, S. et al. Flow depth, velocity, and sediment motions in a straight widened channel with vegetated floodplains. Environ Fluid Mech 21, 483–501 (2021). https://doi.org/10.1007/s10652-021-09783-9

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  • DOI: https://doi.org/10.1007/s10652-021-09783-9

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