Open Channel Flow with Three-Layered Vegetation: Effect on the

Velocity Distribution of Flow

Xiaonan Tang

, Jiaze Cao, Yutong Guan, Yihong Liu, Kangping Xiong, Shilin Niu and Sen Guo

Department of Civil Engineering, Xián Jiaotong-Liverpool University, 111 Renái Road, SIP, Suzhou, China

Keywords: Vegetated Flow; Triple-Layered Vegetation; Downstream Velocity Distribution; Velocity Profile.

Abstract: Aquatic vegetation is widespread in rivers and influences their hydraulic characteristics. Due to individual

differences from a biological point of view, the vegetation of different heights always coexists in nature. The

attention on vegetation flow studies has shifted from the previous single-layer vegetation to more complex

cases such as double- or triple-layer vegetation. Although only a limited number of studies on two-layer

vegetation flow have shown that different vegetation heights have a significant effect on flow structure under

partially submerged conditions, the effect of multi-layer vegetation on flow is unclear. In this regard, we

conducted a novel experiment with triple-layered vegetation with all vegetation heights (10, 15 and 20 cm)

under fully submerged conditions. A micro propeller velocimeter was used to measure the velocity at various

locations in the downstream cross-section of the channel, including the positions behind short and tall plastic

dowels. The measured results showed that the vertical velocity distribution was strongly influenced by

vegetation height and its distribution. The observed data also showed differences in velocity directly behind

vegetation and in the area behind the vegetation gap. Typically, the velocity profile has a similar profile, with

almost a constant velocity from the bed to the height of about 0.75 of the short vegetation height and then a

slight increase to the height of the middle vegetation. Afterwards, the velocity rises fast to the water level,

where the reflection can be observed. This novel experiment reveals the effect of multiple layers of vegetation

on water flow. It would also contribute to the significance of this vegetation configuration and further research

on open channel flow in complex vegetation environments.

1 INTRODUCTION

Vegetation is an essential component of natural river

ecosystems, which can be exploited in river

engineering to meet hydraulic and ecological

requirements (Naiman et al. 1993; Curran & Hession,

2013; Rowiński et al., 2018). Previous experimental

and analytical studies have shown that vegetation can

affect vertical flow distribution, fluid resistance,

Reynolds stress and lateral flow variation in open

channels with single-layered vegetation (Follett and

Nepf, 2012; Tang et al., 2011; Tang et al., 2013a;

Tang et al., 2019a, 2019b; Yang et al., 2020; Box et

al. 2021). Recently, several studies have focused on

channel flow with double-layered vegetation, which

is more complex situation that creates strong vortices

in the interaction zone between the vegetation of

different heights (Tang et al. 2018; Singh et al., 2019;

Rahimi et al., 2020a; Tang et al., 2021b). The velocity

https://orcid.org/0000-0002-2434-9341

distribution in channels with multiple layers of

vegetation has received particular attention. Tang et

al. (2022) experimentally investigated the effect of

partially-submerged triple-layered vegetation on the

velocity characteristics of fluids in open channels.

The measured velocity profiles showed that the triple-

layered vegetation added a significant resistance to

the fluid, and the effect of high vegetation on velocity

was more significant than that of the short one.

Nevertheless, the study by Tang et al. (2022) was

based on partially inundated conditions and may not

be suitable for some situations, e.g., researchers in

freshwater ecosystems have found that reusing

inundated vegetation is an advance and feasible

approach to clean up water bodies and address

eutrophication (Chao et al., 2022). In addition, Tang

et al. (2021a) have also found that higher inundation

rates lead to increased velocity profile gradients in

areas of high and low vegetation interactions.

Tang, X., Cao, J., Guan, Y., Liu, Y., Xiong, K., Niu, S. and Guo, S.

Open Channel Flow with Three-Layered Vegetation: Effect on the Velocity Distribution of Flow.

DOI: 10.5220/0012016800003536

In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 287-293

ISBN: 978-989-758-639-2; ISSN: 2975-9439

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

287

Therefore, it is of great importance to understand the

effect of multiple layers of vegetation on water flow

under fully submerged flow conditions.

In this study, three layers of vegetation, a set of

short, medium, and tall dowels, were placed orderly

at the bottom of an inclined flume, and the experiment

was set to a flow depth of 26 cm with all dowels

submerged. Velocities were measured at different

locations across the channel using a propeller

velocimeter. This research was particularly focusing

on the downstream section, since which is the area

most hydrological issues occur, such as flooding,

water pollution, and eutrophication (Scheumann et

al., 2011).

2 EXPERIMENTAL APPARATUS

AND SETTING

In order to mimic vegetation of different heights in

real water environments, this experiment was

conducted at Xi'an Jiaotong Liverpool University

(XJTLU) with a rectangular cross-section of 0.4 m

wide and 0.5 m high, a water depth of 26 cm, and a

bed slope of 0.003 (Tang et al. 2021). A sketch of the

flume is shown in Figure 1, which has a 4.3 m long

vegetated segment, starting 8.4 m from the entrance

to the flume. Three different heights (10, 15, and 20

cm) of round plastic dowels were installed in the

vegetated area, with each having a diameter of 6.35

mm. All dowels were arranged linearly, as shown in

Figure 2. Each row was a staggered arrangement of

short and medium or tall dowels. The spacing

between two adjacent dowels was 31.75 mm.

There were 12 measurement points in the

downstream area of vegetation, as indicated by the

symbol cross (x) in Figure 2. At each location, the

flow velocity at 23 points above the bed was

measured with a micro propeller velocimeter to

obtain the vertical distribution of velocity. The

sampling time for velocity measurements was set to

20 seconds twice to ensure the accuracy of velocity.

Measurements were made at the downstream section

behind the row of the short and tall dowels (10 and 20

cm), respectively. At a flow depth of 26 cm, the

discharge of the open channel was 27.15 l/s; thereby

all short, middle, and high vegetation was completely

submerged.

Figure 1: The sketch of the experimental flume.

Figure 2: The layout of vegetation array and measurement

locations in the downstream zone.

3 RESULTS AND DISCUSSION

3.1 Velocity Profiles Directly Behind

Vegetation

Figure 3 presents the velocity results for group A

(positions 3, 7 and 11, all behind short vegetation),

while Figure 3 illustrates the velocity profiles for

group B (positions 1, 5 and 9, all behind tall

vegetation). Note that u is the streamwise velocity, u

is the shear velocity, and h is the height of short

vegetation, z is the vertical distance above the bed in

all subsequent figures. The velocity profiles

demonstrate a distinct vertical variation.

As shown in Figure 3, in the low zone near the

bed (layer 1) where z/h is less than or equal to 1, the

flow velocity does not change greatly in the vertical

direction. It is noted that although there exist certain

minor decreases in flow velocity at these locations,

the magnitude of the changes is minimal (less than

10%) and can be taken as an allowable margin of

error. In the intermediate region (layer 2: 1 < z/h <

1.5), i.e., the region between the short vegetation and

the middle vegetation, the variation of flow velocity

is similar for the three positions: the flow velocity

gradually increases with height until about z/h = 1.25,

which seems to be a velocity inflection point. This S-

shaped velocity profile in this region has also been

found in studies of double-layered vegetation

(Rahimi et al., 2020b; Tang et al., 2021b), indicating

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288

a steady slope of the curve at about 0.5 (u∕(u

))/(z∕h).

In the upper region (including layers 3 and 4), the

velocity steadily rises to the water surface starting

from a certain distance below the top of the middle

vegetation (about z/h = 1.25). Meanwhile, the range

of velocity variation was greater, and the rate of

change was larger in comparison with the other layers

(layers 1 and 2). In general, the velocity profile of

group A shows a typical -shaped curve with a small

increase in velocity in the lower layer, followed by a

transition at z/h = 1.25 and then a rapid increase up to

the water surface, which is consistent with the

findings of the other two layers of vegetative flow

(Rahimi et al., 2020b; Tang et al., 2021b).

For the velocity profile of Group B (Figure 4), the

overall profile does not differ much from that of

Group A, but the main difference is in the

intermediate layer (layer 2.) The velocity range rate

of Group B is larger in layer 2 than that of Group A,

indicating that the taller vegetation in Group B seems

to penetrate the flow more deeply. In addition, it can

be found that in the lower layer (layer 1), the velocity

of group A is generally smaller than that of group B

with an amplitude of about 29.4%. Moreover, when

the height of the middle vegetation is reached, there

is a transcendence between the two data sets (2.8 vs

2.25 in velocity).

Finally, when the water surface exceeds the

height of the maximum vegetation, the flow velocity

of the channel remains the same, about 4.5-5.5 for

group A and 3.5-3.8 for Group B; thus, we may draw

the following points:

• The vegetation in the water has a decreasing effect

on the flow velocity. The higher the depth from

the vegetation, the faster the velocity of the water

layer. The water layer beyond the vegetation-

occupied area shows the free surface flow and is

less influenced by vegetation.

• For the velocity profile behind the short

vegetation, the velocity beyond the vegetation

height rises rapidly (i.e., the velocity starts to rise

faster when z/h exceeds 1) and finally reaches a

stable value; however, for the velocity behind the

tall vegetation, it begins to increasease rapidly

after z/h = 2. Finally, the same stable value is

reached.

In addition, the side walls of the channel influenced

the flow velocity profile. Behind the tall vegetation

(group B, Figure 4), the velocity data (z/h<2) show

that the fastest velocities are found near the wall (P1)

than at other locations (P5 and P9) closer to the centre

of the channel. Nevertheless, the velocity in the outer

layer (or the free flow, z/h>2) is not influenced by the

wall. For the flow velocities in group A (behind short

vegetation, Figure 3), the effect of the wall seems to

be somewhat different in the upper layers: (a) just

below the top of the tall vegetation (z/h=1-1.8), the

flow velocities near the side wall (P3) are greater than

those near the centre of the channel (P11); (b)

However, above the tall vegetation, the flow

velocities change in the opposite direction, i.e., the

velocities are smaller near the side wall, indicating

that the side wall has certain blocking effect.

However, velocities away from the wall are less

influenced by the wall boundary but are dominated by

the vegetation.

Figure 3: Velocity profiles directly behind the short

vegetation (Group A).

Figure 4: Velocity profiles directly behind the tall

vegetation (Group B).

3.2 Velocity Profiles behind the Gap of

Vegetation

To better understand the lateral variation of velocity

profiles behind vegetation gaps (P2, 4, 6, 8, 10, 12),

these positions were divided into two groups. Group

C includes P2, P6, P10 (i.e., behind the gap between

tall and short vegetation), while group D includes P4,

P8, P12 (i.e., behind the gap between short and tall

vegetation).

According to Figure 5 (group C), in layer 1

(z/h<1), the velocity profiles of P2, P6 and P10 are

almost identical and do not have significant variation.

In layer 2 (1<z/h<1.5), the velocity gradually

increases with depth, showing a position-independent

profile. However, in layer 3 (1.5<z/h<2), the velocity

of P6 is significantly greater than the other two

Open Channel Flow with Three-Layered Vegetation: Effect on the Velocity Distribution of Flow

289

positions (P2 and P10), which are nearly identical. In

the layer near the water surface (z/h>2), all velocities

are close to a single profile. In general, the velocity

profile exhibits a -shape, although the upper end of

the curve may have different growth rates, depending

on the position. It appears that the growth of velocity

is more linear at position (P6) because the sidewall

less influences it.

Regarding the flow velocity profile of group D

(Figure 6), it generally indicates a similar flow

velocity profile to that of group C. However, within

the vegetative layer (z/h<2), the flow velocity of P12

(in the centre of the channel) is smaller than that of

P4 or P8 (which are identical), due to the effect of the

asymmetric distribution of vegetation. Above the tall

vegetation (layer 4, z/h>2), the velocity of P12

becomes larger than that of both P4 and P8, and their

difference becomes smaller. This result may be due to

a certain blocking effect of the sidewalls on P8 and

P4 (more as closer to the wall).

Figure 5: Velocity profiles behind the gap of vegetation

(Group C).

Figure 6: Velocity profiles behind the gap of vegetation

(Group D).

To compare the differences in the velocity profiles

between the two groups, all data for the 6 positions

are shown in Figure 7. It was found that the velocity

profiles have a similar trend. In the bottom layer (z/h

< 1), the velocity remains almost constant, although

P12 (at the centre) has the smallest value. However,

in the intermediate layer (1<z/h<1.5), the velocity

increases gradually with a similar growth rate, and

P12 has the smallest velocity, while P2 (near the

sidewall) has the highest velocity. As the flow depth

increases, the velocity in the upper layer (z/h > 1.5)

increases rapidly until the water surface and

approaches a curve.

Figure 7: Velocity profiles behind the gap of vegetation for

all positions.

In summary, vegetation has a retarding effect on

the flow through the gap of vegetation. The flow

velocity significantly reflects near the edge of the

vegetation, but the velocity is least affected by the

vegetation in the flow area above the tall vegetation.

When z/h<1, the flow velocity is dominated by the

vegetation, resulting in slight vertical variation in

velocity because large drag force of vegetation.

However, in the vegetated area (1 < z/h < 2), the flow

velocity starts to increase gradually (in the densely

vegetated area) and then increases rapidly (in the less

densely vegetated area). When z/h>2, the free flow

area where the velocity changes the most, and finally

the velocity reaches a stable value near the free

surface. According to Figure 6, the velocity profile of

P12 is smaller than the other locations (P4 and P8)

when z/h<1.75, between P4 and P8 in the range

1.75<z/h<2, and larger than P4 or P8 when z/H>2. In

fact, the velocity profiles of P8 and P4 are almost

indistinguishable when z/h<2. At approximately

z/h=2.15, the velocity profile of P8 is slightly larger

than that of P4 but smaller than that of P12. It

indicates that in the area above the tall vegetation

(z/h>2), the velocity profile near the wall is smaller

than that away from the wall. It can also be seen from

Figure 7 that the velocity profile near the wall is

greater than that far from the wall when z/h>2.15,

while in the upper vegetation layer (z/h>1.5), the

water velocity at P6 is greater than at any other

location (e.g., P2 and P10).

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3.3 Comparison of Velocity Profiles at

Various Locations Behind

Vegetation

To investigate how vegetation affects the velocity

profiles at different locations behind the vegetation,

Figure 8 compares the velocity profiles at locations

far from the wall (i.e., P6-P10). Since they are

positioned at a certain distance from the wall, the

effect of the wall can be considered minimal and

negligible here.

In the bottom region of the channel (z/h<1), the

velocity was affected by the three different heights of

vegetation, with a similar pattern of velocity changes:

the velocity remained almost constant below the short

vegetation (although there were some slight

fluctuations, this small difference can be considered

as a type of measuring error in the experiment).

Furthermore, at about the 0.9h height, the velocity

starts to increase rapidly to the top of the short

vegetation. In terms of flow directly behind and

through the vegetation gap, the flow velocity behind

the vegetation gap was greater than that directly

behind the vegetation, although the difference was

small. This result is consistent with the results of

other studies (e.g. Rahimi et al., 2020b).

In the flow zone of z/h>1, the velocity has

increased rapidly and continued to the water surface.

It is inferred that the flow in the intermediate layer

(1<z/h<1.5) is more affected by overlaid vegetation

(middle and tall), and the increase of velocity is

relatively slow. The difference from the previous part

is that with the increase of height, the offset of the

flow velocity offset directly behind the vegetation

gradually decreases or even disappears. For example,

the flow velocity at P8 is greater than that at P7, but

as the height increases, the velocity at P7 gradually

becomes larger than that at P8. Therefore, this

phenomenon may be explained as a result of the

density of vegetation, which gradually shifts to a

regular pattern affected by the wall alone.

Furthermore, the flow velocity was greater at P7

(behind short vegetation) than at P9 (after high

vegetation). The reason may be that in the upper

layer, because P7 is not directly affected by

vegetation (height is above the short vegetation), it

will have a larger flow velocity than P9, which is

directly affected by the middle and tall vegetation.

Meanwhile, this also results in the reduction of

velocity.

Figure 8: Comparison of velocity profiles at various

positions behind the vegetation.

3.4 Comparison of Mean Velocity

Profiles at Various Positions

Behind Vegetation

In order to reveal the changes of averaged velocity

profiles at some typical locations, this section aims to

compare the combined effects of a group of

vegetation on averaged velocity profiles across a

channel section, as shown in Figure 9. BST represents

the averaged profile of P1, 5 and 9 (behind the tall

after short vegetation). BMS denotes the averaged

velocity profile value at P3, P7 and P11 (behind the

short after middle vegetation). SM represents the

averaged value of velocity profiles at P2, P4, P6, P8,

and P10 (behind the gap of tall and short vegetation).

Figure 9 demonstrates significant differences in

the averaged velocity profile at some typical locations

(BST, BMS and SM): The overall averaged velocity

profile is a ‘ ’ type, i.e., the velocity changes small

in the bottom region up to 0.9h, and then increases

quickly near the top of short vegetation (i.e. z/h =1),

where the strong vertical exchange of flow takes

place; then the velocity increases gradually in the

intermediate region (1<z/h<1.5) and continues fast to

the water surface. It was found that when z/h < 1.3,

the flow velocity of BMS is smaller than that of BST

and SM. Nevertheless, when z/h > 1.3, the flow

velocity change of BMS is faster and becomes larger

than that of BST and SM. The velocity profiles of

BMS, BST and SM are similar in the layer below

z/h=1.4, which is the turning point; afterwards, the

velocity of BST is accelerated and finally reaches the

highest value near the water surface (z/h = 2.2), where

the velocity of BMS and SM approaches.

Open Channel Flow with Three-Layered Vegetation: Effect on the Velocity Distribution of Flow

291

Figure 9: Lateral variation of averaged velocity profiles.

3.5 Lateral Variation of

Layer-Averaged Velocity

The layer-averaged and depth-averaged velocities can

be obtained from the measured velocity vertical

distribution. Figure 10 shows the lateral variation of

the layer-averaged and depth-averaged velocities for

a half channel. Note that in Figure 10, z/h=Ud denotes

the depth-averaged velocity, whereas the others

denote the layer-averaged. In general, the velocity at

the position directly behind the vegetation (odd-

numbered position) is smaller than the velocity at the

centre position behind the vegetation gap (even-

numbered position). However, for the locations close

to the sidewalls (P1 and P2) the velocities do not

appear to conform to the above results, indicating the

effect of walls. The velocities at the even-numbered

locations near the sidewalls are slightly smaller than

the velocities at the odd-numbered points. Thus, it

may be inferred that the wall has certain influence on

the flow velocity. In addition, contrary to the previous

results, in the vegetation-covered area (the first to

third layer), there is a tendency for the depth-averaged

velocity to decrease slightly towards the centre of the

channel, although the surface layer (above the tall

vegetation) is almost unchanged in the lateral

direction.

It is noted that in the middle layer (layer 3, 1.5 <

z/h < 2), i.e., in the area between the middle and tall

vegetation, the variation of layered-velocities is

particularly complex and fluctuating. Similar to the

analysis in the previous sections, the layer-averaged

velocities at P3, P7, and P11 are larger than at any

other position. This result may be due to the fact that

all three locations are directly behind the short

vegetation, thus making their flow velocities less

influenced by the middle and short vegetation

upstream (offset only by the tall vegetation). While at

P1, P5, and P9, their layered velocities are

considerably influenced by the tall vegetation

upstream, thus resulting in smaller flow velocities.

Besides, Figure 11 shows that the averaged layer

velocity increased with increasing flow depth,

indicating that the lowest velocity was found near the

bed (layer 1) and the highest velocity in the upper

layer (layer 4). The averaged velocity of each layer is

directly related to the vegetation density in the

corresponding layer. Layer 1 has the lowest velocity,

corresponding to the highest vegetation density,

leading to the lowest velocity. Layer 4 behaves as a

free-flowing layer (not directly influenced by any

height vegetation), leading to the maximum velocity.

Figure 10: Lateral variation of layer- and depth-averaged

velocity.

Figure 11: Averaged velocity (V) of each layer for the half

channel. Note: U is the cross-sectional mean velocity of the

channel.

4 CONCLUSIONS

A novel experiment was conducted to investigate the

effect of fully submerged triple-layered vegetation on

the flow in an open channel. Detailed flow velocities

were measured using micro propeller velocimetry at

different locations across a downstream section of the

channel. The overall velocity profile has shown that

the flow velocity increased with increasing flow

depth, which was significantly different from the

logarithmic law of velocity in open channels without

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vegetation. More specifically, flow velocities were

relatively small and almost constant near the channel

bed up to 0.4 z/h (a position slightly below the top of

short vegetation), and then increased consistently to

1.5 z/h (i.e. the height of the middle vegetation). In

the region above the middle vegetation, the velocity

increased rapidly to the water surface. There are two

distinct reflections: the first one is below the short

vegetation near 0.4 z/h, and the second occurs close

to the top of middle vegetation. Besides, the lateral

variation of mean velocity profile is complex, which

implies that the open-channel flows with completely

submerged multi-layered vegetation are intricate.

More experiments or data would be required for

further understanding.

ACKNOWLEDGEMENTS

This work was partly supported by XJTLU via the

research fund (RDF-16-02-02, REF-20-02-03, and

PGRS2012007) and by the National Natural Science

Foundation of China (11772270).

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