Velocity Profile and Lateral Distribution in an Open Channel with

Two Distinct Vegetative Zones

Xiaonan Tang

, Yutong Guan and Yuxiang Hu

Department of Civil Engineering, Xi’an Jiaotong-Liverpool University, China

Keywords:

Vegetative flow, Non-flexible vegetation, Riparian environment, Velocity distribution, Layered Vegetation

Abstract: Riparian vegetation has drawn increasing attention because it plays a vital role in the ecological environment

and flow process in river systems. Previous literature on vegetative ﬂow mostly focuses on understanding the

hydraulic feature of uniform single-layered vegetation in a channel. In many rivers, vegetation often grows

unevenly along riversides, on which very few studies have been done. For understanding the effect of

unevenly distributed vegetation on the flow, this paper presents novel experimental results in an open channel

with each side of the bed occupied by different height vegetation. The vegetation was mimicked by dowels

10 cm and 20 cm high and in two flow conditions: fully and partially submerged. A micro ADV (Acoustic

Doppler Velocimetry) was used to measure 3D velocities at various positions. Observed data show that the

averaged velocity profile in the non-vegetative region (free-flow zone) is influenced by neighboring

vegetative regions, where the velocity reflects at some distance below the top of vegetation. A large lateral

velocity gradient exists near the interfacial boundary between vegetative and non-vegetative flow regions,

indicating that a transition layer occurs near the interfacial boundary owing to the momentum exchange.

Moreover, with the increasing flow depth, the zonal velocity in the non-vegetative region decreases slightly

while the zonal velocity increases accordingly in the vegetative region (either short or tall vegetation sub-

region), indicating that the averaged velocity in the vegetative region is affected by the submergence of

vegetation. These ﬁndings on the channel flow with unevenly layered vegetation would benefit riparian

management and the design of ecological and habitat zones in terms of the width and height of vegetation.

1 INTRODUCTION

Riparian vegetation plays a vital role in the water

environment and ecologic systems of rivers.

Vegetation creates a place of habitat with rich

biodiversity for aquatic animals and birds, enhances

water quality, and prevents the erosion of river banks

and beds. In natural rivers, different types of

vegetation grow on the riverside. In practice,

vegetation is intentionally planted for ecological

purposes or engineering requirements. In the

watercourse, riparian vegetation interacts with the

flow and affects the hydraulic features of flow

(Ghisalberti & Nepf, 2006). The existence of

vegetation induces additional flow resistance to flows

because of the drag force of vegetation (Stone &

Shen, 2002), thus altering the velocity and turbulence

structure of flow (Lopz & Garcia, 2001; Nezu &

Sanjou, 2008; Yang et al., 2015; Tang et al., 2021c).

In the literature, many studies have been

undertaken on the mean velocity of flow and the flow

resistance with one-layered vegetation, which is

considered rigid or flexible in either submerged or

emergent conditions (Aberle & Jarvela, 2013; Cheng,

2011; Yang et al., 2020; D’Ippolito et al., 2021).

Vegetation in laboratory studies was mostly

mimicked by artiﬁcial vegetation, either rigid or

flexible (Cheng, 2011; Yang et al., 2020).

Considering the significant role of vegetation in the

river system, researchers have drawn great interest in

understanding vegetation's impact on the sediment

transport and hydraulics of flows at diﬀerent scales

(Curran & Hession, 2013; Chembolu et al., 2019).

The flow interaction with vegetation is complex.

The flow mechanism of vegetation in limited depths

has been interpreted (Okamoto & Nezu, 2013; Nikora

et al., 2013; Hui et al., 2014; Rahimi et al., 2020c),

resulting in various analytical models for predicting

the velocity profile (Nepf, 2012; Tang, 2019a, 2019b)

and the lateral variation of depth-averaged velocity

(Tang & Knight, 2009; Tang et al. 2010, 2011).

Moreover, numerical solutions have been made to

240

Tang, X., Guan, Y. and Hu, Y.

Velocity Proﬁle and Lateral Distribution in an Open Channel with Two Distinct Vegetative Zones.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 240-245

ISBN: 978-989-758-560-9; ISSN: 1755-1315

understand the hydrodynamics of vegetated flows

through numerical models (e.g. Neary, 2003; Stoesser

et al. 2010) and CFD modelling using Ansys

FLUENT (Rahimi et al., 2020 a&b; Anjum &

Tanaka, 2019; Anjum et al., 2018).

In natural rivers, different types of riparian

vegetation co-exist, such as trees, shrubs, grasses.

Tall vegetation is usually emergent whilst short

vegetation is submerged. The flow through different

height vegetation has a complex flow structure.

Recently, some investigators have studied the open

channel flow with the entire bed occupied with

combined tall and short vegetation, so-called doubly

or two-layered vegetation (e.g., Rahimi et al., 2020c;

Singh et al., 2019; Hui et al., 2014; Liu et al., 2008).

Recently, Tang et al. (2019, 2021d) also

experimentally investigated the impact of partial two-

layered vegetation on the velocity and turbulent

structure of the flow. However, very little research

has been performed to understand the impact of

vegetation unevenly distributed over each side of a

channel bed, which often occurs in natural rivers.

In this paper, a novel experiment of vegetative

flow was taken in a tilting water flume. The two sides

of the flume bed were occupied by different layers of

vegetation: one side in one layer, the other in two

layers. 3D velocity at various locations along a

section was measured using ADV (Acoustic Doppler

Velocimetry) to understand the flow characteristics in

the regions with and without vegetation. This paper

presents averaged velocity in three different zones

under partially and fully submerged conditions.

2 EXPERIMENTAL SETTING

The experiment was performed in the 20m long titling

flume of hydraulics laboratory at Xián Jiaotong-

Liverpool University (XJTLU). This rectangular

flume, 0.4m (wide) x 0.5m (high), was adjusted at a

bottom slope (S) in 0.003 (Figure 1)

(Tang & Hu,

2021). The rigid vegetation was mimicked by PVC

cylindrical dowels of 6.35 mm in diameter, with two

heights of 0.2 m (tall) and 0.1m (short). A pre-

perforated PVC plate of 10 mm thick is laid on the

flume bed to hold the dowels. As shown in Figure 1,

the 4.3m-long vegetative session begins 8.4m away

from the channel entrance and has a distinct pattern

of vegetation on two sides of the bed (see Figure 2)

(Tang et al., 2021a). Tall dowels are on the left side

(vegetation region 1), while on the right side

(vegetation region 2) of the flume, tall dowels in two

rows are close to wall B, along with short dowels in

two rows near the free region in the center. All dowels

were linearly distributed with 31.75 mm apart

between the dowels. Therefore, the width of the free

region and each vegetation region is the same, equal

to one-third of the bed width.

Due to the limitation of accessible area, two types

of Nortek micro ADV (downward and sideward)

were used for measuring velocity at different

positions at a cross-section. The downward ADV was

mainly used to measure most 3D velocities in a depth

except close to the water surface, where the velocity

was obtained by the sideward ADV, which was able

to measure the velocity close to the water surface. 60

seconds were set as the sampling time for each

measurement. To ensure reliable data, SNR (signal-

to-noise ratio) and correlation coefficient during the

measurement should maintain at least 10 and 70%,

respectively. The velocity data obtained by ADV was

then processed using WinADV software. The

experiment was undertaken in three flow depths: 9,

14 and 22 cm. The corresponding flow rate is 6.1, 11.1

and 19.49 l/s, which respectively denotes the

following conditions: emergent (all dowels non-

submerged), partial submerged (only short dowels

submerged), and fully submerged (all dowels fully

submerged).

The measuring positions are shown in Figure 2

and coded as follows: BT and BS respectively denote

the measuring positions behind the tall and short

dowels, whereas FR means in the free region (i.e., the

non-vegetative central area). Other notations include

that BST and BSS are denoted as behind and side

away from tall and short dowels, NT denotes the

position next to tall dowels, NS represents the

position next to short dowels, and NST is the location

next to the short and tall dowels.

For the effective comparison, in the figures of the

subsequent section, the height of the short dowel (h)

is used to normalize the vertical distance (z) above the

flume bottom, whereas the cross-sectional average

velocity U is normalized velocities.

Figure 1: The sketch of the experimental channel.

Velocity Proﬁle and Lateral Distribution in an Open Channel with Two Distinct Vegetative Zones

241

Figure 2: The layout of dowels and measurement points in

the experiment.

3 VELOCITY RESULTS

3.1 Averaged Velocity Profiles

The lateral variation of velocity profiles at different

locations (e.g., BS, BT, BST) has been reported in

Tang & Hu (2021). To obtain a broad view of velocity

profiles in each subzone, the averaged velocity is

calculated to reveal its variation, as given in Figures

3-4 for the two experimental depths: partially

submerged flow (H= 14 cm) and fully submerged

flow (H = 22 cm).

In the partially submerged flow (Figure 3), the

observed result reveals that the averaged velocity

profile in the free zone (FZ) is significantly larger

than those in the vegetated zone (VZ1 and VZ2). This

result suggests that the vegetation decreases the water

velocity. Compared with VZ1 and VZ2T, the

averaged velocity in VZ2T is smaller than that in VZ1

despite the same type of tall vegetation, showing that

the effect of velocity of neighbouring zone: The large

velocity in VZ1 is caused by the higher velocity of

neighbouring zone (FZ) compared with the smaller

velocity in VZ2T due to small velocity in the

neighouring zone (VZ2S). This effect also explains

the moderated velocity profile in VZ2S, which is

between the highest velocity zone FZ and the smallest

velocity zone VZ2T. Furthermore, the vertical

variation of averaged velocity is different depending

on whether the vegetation is submerged or not. The

velocity remains nearly constant in emergent

conditions (VZ1 and VZ2T), where the velocity

begins to increase from some distance above the bed

until to the water surface for both the free flow zone

(FZ) and short vegetation zone (VZ2S). It is noticed

that the velocity in VZS2 has a reflecting point near

the top of short vegetation, but the velocity in FZ does

not have. The reflecting point of velocity in

submerged conditions has also been observed in the

literature (Yang et al., 2015; Tang et al. 2021b).

Figure 3: Averaged velocity profiles for H=14 cm.

Figure 4: Averaged velocity profiles for H= 22 cm.

In the fully submerged flow (Figure 4), the

observed averaged velocity profiles are similar to

those in the partially submerged flow. This is that the

averaged velocity profile in the free flow zone (FZ) is

significantly larger than those in the tall vegetated

zone (VZ1 and VZ2T), while the velocity profile in

the short vegetation zone (VZ2S) lies between those

in the free zone and tall vegetation zone (VZ1 and

VZ2T). This result confirms that the vegetation

dramatically reduces the velocity of flow due to the

additional drag force caused by vegetation. In a close

comparison between VZ1 and VZ2T, the averaged

velocity in VZ2T is larger than that in VZ1 in the low

depth (about z/h < 1.6), but they are almost the same

in the upper layer close to the water surface (z/h >1.6),

showing that the velocity of neighbouring zone

affects the flow in the low layer near the bed, but has

a limited impact on the flow of the upper layer close

to the surface. This result demonstrates that the

different impact of the neighboring zone on the flow

of tall vegetation zone exists between the fully and

partially submerged flow conditions.

In the fully submerged flow, the averaged velocity

profile in the short vegetation zone (VZ2S) shows an

‘S-type’ profile, indicating that the velocity has three

VZ1

VZ2S VZ2T

WRE 2021 - The International Conference on Water Resource and Environment

242

inflecting points: two near the top of short vegetation

and one near the edge of tall vegetation, which was

also showed by Rahimi et al. (2020c) and Huai et al.

(2014).

Furthermore, it was found that there exists an

almost constant velocity layer near the bed,

depending on the submergence. The height of the

constant velocity layer is about 1.5h for the tall

vegetation zones (VZ1 and VZ2T), whereas its height

is about 0.75h for the short vegetation zone (VZ2S).

This result is well consistent, showing that the

constant velocity starts to increase at the point below

0.25 height of the vegetation. This point is the first

reflecting point of velocity, mainly affected by the

penetration of surface flow, as obtained by many

researchers (Nepf. 2012; Tang et al. 2021d).

Meanwhile, it should be noted that the vegetation

affects the vertical distribution of velocity in the free

zone (FZ), which does not follow the logarithm of

velocity in open-channel flow.

3.2 Lateral Variation of Depth-mean

Velocity

To study how the vegetation affects the lateral

variation of velocity, the depth-mean velocity (U

)

was computed and presented in Figure 5. In general,

Figure 5 shows that large velocities occur in the

central free zone and decreases from the centre of the

free zone to either edge of the vegetated zone. It was

observed that a large velocity gradient occurs around

either the interface between the non-vegetative zone

(FZ) and vegetative zone (VZ1 and VZ2S). This

result implies that there exists extensive momentum

exchange around the interface between FZ and VZ

zones, which is caused by the large velocity

difference. This finding is similar to the results

observed by Tang et al. (2021b & c).

In addition, as the water depth increases, the

vegetation from partially submerged (H=14cm) to

fully submerged (H=22cm) affects the lateral

variation of velocity. Compared with the partially

submerged condition, the lateral velocity gradient

around the interface is relatively smaller, revealing

that the vegetation causes a relatively small difference

in velocity between vegetative and non-vegetative

zones as increasing flow depth. This implies that the

vegetation causes relatively large additional flow

resistance in the emergent or partially submerged

condition than the fully submerged condition.

Figure 5: Lateral variation of depth-mean velocity (B = 40

cm).

3.3 Zonal Velocity in Different Zones

The averaged velocity of each region, called zonal

velocity (Ui), can be obtained from the lateral

distribution of depth-mean velocity (Figure 5). The

obtained zonal velocity at each region is shown in

Figure 6, which includes three flow depths. Figure 6

shows that the highest zonal velocity occurs in the

free flow zone, the lowest zonal velocity occurs in the

zone of tall vegetation, and the zone of short

vegetation (2S) has the modest zonal velocity. This

result is as expected that the more vegetation, the

smaller the velocity becomes. The relative size of

zonal velocity in each zone will change depending on

the submergence of vegetation. In the emergent case

(H=9 cm), the zonal velocity in the free flow zone is

almost 1.7 times of the cross-sectional velocity (U),

i.e. almost 70% higher than U, whereas the zonal

velocity in vegetated region 1 is about 0.7 times of U,

i.e. 30% smaller than U, and a similar velocity occurs

in vegetation region 2, which is equivalent to region

1 on average. As depth increases, the zonal velocity

in the free region becomes relatively smaller while

the zonal velocity in vegetation 2 increase

accordingly, although the zonal velocity in vegetation

1 does not change very much. To be detailed, the

zonal velocity in the free flow zone decreases from

1.6U to 1.46U when the flow depth (H) rises from 14

cm (partially submerged case) to 22 cm (fully

submerged case), while the zonal velocity increases

from 0.57U to 0.78U in tall vegetation zone (2T) and

from 0.85U to 1.07U in short vegetation zone (2S).

This result reveals the effect of the vegetation

submergence on the zonal velocity: larger

submergence of vegetation leads to a higher zonal

velocity due to relatively smaller flow resistance

inserted.

In addition, because the discharge is the product

of the velocity and area, a large zonal velocity

corresponds to a large discharge for the same sized

region. The zonal velocity in Figure 6 implies that:

Velocity Proﬁle and Lateral Distribution in an Open Channel with Two Distinct Vegetative Zones

243

(1) for a given depth, the free region carries out a

higher portion of discharge compared with that in the

same sized vegetation zone; (2) the short vegetation

zone (2S) has more flows than the same sized tall

vegetation zone (2T), which is enhanced particularly

with the increasing submergence of vegetation.

Figure 6: Comparison of zonal velocity in each region.

4 CONCLUSIONS

The impact of the height and distribution region of

vegetation on the open-channel flow has been

investigated through experiments with two distinct

conditions. The observed results show that the

averaged velocity in each zone is affected by the

distribution/pattern and submergence of vegetation.

A few points may be drawn as follows:

1) The averaged velocity profile in the non-

vegetative zone (FZ) is much larger than that in

the vegetative zone, showing retarding effect of

vegetation. The velocity profile of the fully

submerged flow has shown significant reflecting

points depending on the submergence of

vegetation: the height of reflecting velocity

occurs about z/h=1.5 for tall vegetation zone,

while the reflecting velocity happens at about

z/h=0.75,1.15 and 1.80, as an ‘S-type’ profile.

2) The averaged velocity profile in the non-

vegetated zone (FZ) shows a constant value in a

layer near the bottom, and it does not follow the

logarithm law.

3) There exists a great lateral momentum exchange

in a layer along the interfacial boundary between

vegetative and non-vegetative regions, showing

a large lateral velocity gradient near the interface.

This gradient becomes smaller as increasing flow

depths when the flow changes from partially

submerged to fully submerged condition.

4) The zonal velocity of each region is affected by

the submergence of vegetation. With increasing

flow depth, the status of vegetation changes from

emergent, partially submerged to fully

submerged conditions, the zonal velocity in the

free region becomes slightly smaller, but the

zonal velocity in vegetation region 2 (either short

or tall vegetation sub-region) increase

accordingly. This result will help practitioners in

the planning and management of vegetated

channels

ACKNOWLEDGMENTS

This work received support from XJTLU (RDF-16-

02-02, KSF-E-17, PGRS2012007, REF-20-02-03).

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