RIVER RESEARCH AND APPLICATIONS

River. Res. Applic. 24: 673–687 (2008)

Published online in Wiley InterScience

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION IN OPENCHANNEL FLOWS

DONG SOP RHEE,a*,y HYOSEOP WOO,bz BO AE KWONay and HONG KYU AHNax

a Department of Water Resources Research, Korea Institute of Construction Technology, 2311 Daehwa, Ilsanseo, Goyang, Gyeonggi 411-712,

Koreab Korea Institute of Construction Technology, 2311 Daehwa, Ilsanseo, Goyang, Gyeonggi 411-712, Korea

(www.interscience.wiley.com) DOI: 10.1002/rra.1143

ABSTRACT

Vegetation in rivers has important roles in improving and restoring river environment. Other than it adds high aesthetic value torevetments, that it can be used as a levee protection in environmental friendly way. In open channel hydraulics, vegetation oftencauses changes in the flow resistance, usually resulting in the increase of flood stage. Both experimental and numericalresearches have been conducted on flow resistance of vegetation in open channels, however, the researches were basedmostly onthe vegetation found in the region where the researches were conducted, and this restricts the generality of the results.In this study, three Korean natural vegetations, Zoysia matrella (Korean zoysia), Pennisetum alopecuroides (L.) Spreng.

(Korean native vegetation) and Phragmites communis Trin. (Korean reed) were used in flume tests on the effect of vegetation inthe channel on flow resistance. ‘n-VR’ retardance curves were developed for each grass. All the grasses were tested underunmown conditions. Z. matrella was tested under fully submerged condition and other two were tested under both submergedand un-submerged conditions.Resistance coefficient, expressed as Manning’s n, converged to about 0.027 (total roughness) for Z. matrella as VR increased.

Resistance coefficients for other plants were found affected by the states of the plants, that is, whether they were ‘green’ or‘dormant’. Generally, resistance coefficients are higher when plants are ‘green’ than when they are ‘dormant’. This is becausethe resistance coefficient is influenced by the leaf elements of vegetation on the river flow in addition to the stem of vegetation.The interaction between the bending part of vegetation and thewater surface can also increase the resistance coefficient. In termsof water depth, P. communis Trin were found more affected on the resistance coefficients compared to Z. matrella andP. alopecuroides (L.) Spreng. Copyright # 2008 John Wiley & Sons, Ltd.

key words: flow resistance; open channel flow; resistance coefficient; vegetation

Received 31 August 2006; Revised 30 May 2007; Accepted 22 August 2007

INTRODUCTION

Vegetation in rivers is a problem for river management in Korea as well as in other parts of theworld with a growing

interest in river restoration, and is a very important factor in improving and restoring river environments. Vegetation

used for levee protection and revetments in Korea has high aesthetic values as well as being environmental friendly.

Some emergent vegetation plays an important role in improving riparian and aquatic habitats. Low-flow revetment

using live willows may serve both as scour protection of river bank and riparian habitat; this is a good example

illustration of fulfilling two joint actions. Generally, the ecological characteristics of rivers are improved with

vegetation growth, but there are also unfavourable effects on rivers such as reduced conveyance, and increased river

resistance. In open channel hydraulics, vegetation causes changes in the flow resistance and thus usually results in

the increased water level compared with the un-vegetated reaches.

*Correspondence to: Dong Sop Rhee, Department of Water Resources Research, Korea Institute of Construction Technology, 2311 Daehwa,Ilsanseo, Goyang, Gyeonggi 411-712, Korea. E-mail: dsrhee@kict.re.kryResearcher.zSenior Research Fellow.xSenior Researcher.

Copyright # 2008 John Wiley & Sons, Ltd.

674 D. S. RHEE ET AL.

In vegetated river reaches and floodplains, the flow resistance is largely determined by the type, density and

characteristics of vegetation, as well as by flow depth and velocity. A number of studies have been conducted on

flow resistance of vegetation in open channels, both experimentally and numerically (Chow, 1959; Kouwen and Li,

1980; Temple et al., 1987; Fathi-Maghadam and Kouwen, 1997; Darby, 1999; Temple, 1999; Jarvela, 2002, Kirby

et al., 2005). The vegetation types in these studies were often limited on the local vegetation found in the regions

where the researches were conducted. This causes a problem when attempting to use their results in other regions,

since many of the vegetation used in the studies are uncommon and not found in other areas, for example, in Korean

rivers. Sometimes artificial vegetation can be used for flow resistance studies instead of natural ones. Wu et al.

(1999) used rubberized horsehair mattress to simulate bushes and shrubs in floodplain wetlands, and to study

variation in the vegetative roughness coefficients with the flow depth in submerged and un-submerged conditions.

Strong deflection and the bending of test material were disregarded in the study because the test material was stiff

enough. Cylindrical rigid elements were also used to investigate the effect of vegetation resistance at submerged

and un-submerged conditions (Stone and Shen, 2002; Musleh and Cruise, 2006). As it can be assumed that the

characteristics of artificial materials were quite different from natural vegetation, the universal application of the

study results for design purpose should be considered carefully.

The primary objective of this study is to investigate the effect of vegetative roughness on the flow resistance, and

to provide a better understanding of the vegetative roughness in vegetated open channels and rivers, especially for

Korean native aquatic vegetation. Three typical Korean grasses were selected to test the effects of vegetation type,

grass heights, flow depth and flow velocity on the flow resistance. The grasses selected in this study were all very

flexible and easily bent. Simplified flume tests were performed to evaluate the submerged and un-submerged

vegetation resistance under un-harvested conditions. This study did not intend to address the scaling of vegetation

and the complexities of flow in compound open channels, nor did it focus on the biomechanical properties of the

plants.

FLOW RESISTANCE

Hydraulic resistance of open channel flow occurs mainly by the viscous and drag forces over the wetted perimeter.

The roughness components in vegetated open channels are conceptually divided into three components: soil grain

roughness, form roughness and vegetative roughness. In most vegetated open channels, vegetative roughness

characterized by vegetation density, grass heights and type of vegetation dominates the flow resistance of channel

(Temple et al., 1987). This flow resistance is commonly represented by parameters such as the Darcy–Weisbach

friction factor ( f), Chezy’s resistance factor (C) or Manning’s roughness coefficient (n). Among these factors,

Manning’s n is most frequently used in the computation of open channel flows (Wu et al., 1999; Kirby et al., 2005).

In Korea, designers also use Manning’s n for design purpose.

Historically Manning’s roughness coefficient can be selected from simple lists of channel descriptions (Chow,

1959; Henderson, 1966). According to Cowan (1956), total roughness of a channel can be divided into its

component parts. He developed a procedure for estimating the effects of the roughness factors to determine the

value of n. The value of n is calculated by (Arcement and Schneider, 1984)

n ¼ nb þ n1 þ n2 þ n3 þ n4ð Þm (1)

where nb is a base value of n for a straight, uniform and smooth channel in natural materials, n1 correction factor for

surface irregularities, n2 value for variations in the shape and size of a channel cross section, n3 value for

obstructions, n4 value for vegetation and flow conditions and m correction factor for meandering of a channel.

From an early stage, research on vegetative flow resistance showed that Manning’s n varies systematically with

the product of mean flow velocity and the hydraulic radius (VR) for a given type of vegetation, and that the

relationship is practically independent of channel slope and shape. Further experiments found that vegetation

species having similar characteristics of density, rigidity, grass heights and the level of submergencewould produce

similar n-VR curves (Kirby et al., 2005). In fact, this n-VR relationship is equivalent to a n-Re relationship, since

the product VR is proportional to the Reynolds number Re, whose conventional definition in open channel flow is

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 675

given by (Chow, 1959)

Re ¼ VR

n(2)

The kinematic viscosity v of water does not vary significantly under normal conditions. The Manning equation

has been shown to be applicable for the case of fully turbulent flow (Henderson, 1966). Sometimes, the friction

factor f is used instead of Manning’s n when Re< 104, where the flow regime is likely to be laminar to transitional.

Generally, Manning’s n value decreases as VR increases. The decrease of n results from the increase of plant

bending and the submergence. Minimum Manning’s n value occurs when the product VR is large enough to force

the grasses to be entirely flattened. The friction factor f, however, decreases as VR decreases when flow is laminar or

transitional, namely, when Re< 104 (Chen, 1976; Green, 2005).

In case of large flows or floods, the flow depth is usually much greater than the grass height so that the thickness

of boundary zone by vegetative growth tends to be a minimum and thus the vegetative part in the passing flow

becomes negligible. Consequently the resistance coefficient tends to converge to a constant (Temple et al., 1987).

Likewise, the effect of vegetation can be considered more important in small-scale rivers or watercourses and

floodplains in large rivers, for which the use of a constant roughness coefficient can cause inaccuracy in their

estimated flood level.

VEGETATION SELECTION

In this study, three Korean native vegetation types, Zoysia matrella (a Korean zoysia), Pennisetum alopecuroides

(L.) Spreng. (a Korean pennisetum) and Phragmites communis Trin. (a Korean reed) were used in the flume

experiment for the effect of vegetation on the channel flow resistance. As shown in Figure 1, the selected plants are

representative vegetation types in different elevation zones in typical Korean river cross-sections. Z. matrella

represents the vegetation in ‘levee’ zones, where the most vegetation types are rather short and stiff.

P. alopecuroides (L.) Spreng. represents the vegetation in ‘flood plain’ zones, where the most vegetations have

flexible stems and intermediate height. P. communis Trin. represents the vegetation in ‘low-flow channel’ zones,

where the most vegetation grows very high and has flexible stems (Figure 2).

The three vegetations were tested under unmown conditions. Z. matrella was tested fully submerged and the

others were tested under submerged and un-submerged conditions. Table I summarizes the characteristics of the

vegetations used in this study.

P. alopecuroides (L.) Spreng. and P. communis Trin. were tested for ‘green’ and ‘dormant’ states. ‘Green’ state

means that the tested grasses are fully green and in active growth state. ‘Dormant’ state means that the tested grasses

are inactive and starting to be wilted or dead. Experiments on the target species were performed in order for two

years periods. First, experiments for the ‘dormant’ state were conducted in autumn. This is because in autumn, the

plants are about to bewithered after the active growth during the summer. Targeted vegetation types were planted in

the experimental flume and the temporary planting space just before the plants went withered. After the ‘dormant’

state experiment, the experiments for ‘green’ grasses were performed similarly in the new experimental flume in

next summer.

Figure 1. Conceptual diagram for locations of selected vegetation in river reach. This figure is available in colour online at www.interscience.wiley.com/journal/rra

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

Figure 2. Plants tested (a) Zoysia matrella, (b) Pennisetum alopecuroides (L.) Spreng. and (c) Phragmites communis Trin.

Table I. Characteristics of experimental vegetation

Common name Scientific name Height ofplants (cm)

Average density(stemsm�2)

Korean golden zoysia Zoysia matrella 6–10 743 (series A)Korean pennisetum Pennisetum alopecuroides (L.) Spreng. 35–45 198 (series B)

206 (series D)Reed Phragmites communis Trin. 70–80 161 (series C)

154 (series E)

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

676 D. S. RHEE ET AL.

Figure 3. Outdoor experimental flume for test series A–C

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 677

EXPERIMENTAL SETUP

Experiments for the quantification of hydraulic resistance were conducted in two flumes located in the outdoor

hydraulic laboratory at the Korea Institute of Construction Technology. The first flume was 1.0m wide and 18m

long (Figure 3). The sidewalls and the bottom of the flume were made of concrete blocks with concrete linings. The

water discharge was measured by a rectangular weir located at upstream of flume. Three series of tests (series A–C)

were performed for Z. matrella and ‘dormant’ P. alopecuroides (L.) Spreng. and P. communis Trin. The

measurement reach, located at 4.0m from the entrance channel section, was 6.5m long, and made as 0.2m deeper

in depth compared to upper and downstream reaches. It was filled with a loamy soil to transplant test grasses. The

bed depth was then adjusted to other reaches after planting. The stage measurements were made with a digital point

gauge which has an accuracy of 0.1mm. A 3.0m long mixed grass zone with zoysia and other short native grasses

was located at 1.0m from the entrance channel section to provide a flow transition. Total 60 experimental runs

(Table II) were carried out with the flow rate and the downstream-end depth varied for each type of vegetation. The

tested flow depths covered a broad range so that the vegetation could experience both the submerged and

un-submerged conditions (except for Z. matrella; this type of plant is usually submerged in normal flow conditions).

All the flows were subcritical, and minor variations in the bed surface were neglected.

The second flumewas 3.0mwide and 30m long (Figure 4). The sidewalls and the bottom of the flumewere made

of the same materials as the first channel. The water discharge was measured by a suppressed weir located at the

upstream end of the flume. Two series of tests (series D–E) were performed for ‘green’ P. alopecuroides (L.)

Spreng. and P. communis Trin. The measurement reach, located at 5.0m from the entrance channel section, was

18m long, and made as 0.2m deeper in depth compared to upper and downstream reaches. It was filled with a

loamy soil to transplant test grasses. The bed elevation was also adjusted after planting. The stage measurement was

Table II. Experimental conditions for each test series

A B C D E

Discharge (m3 s�1) 0.01–0.08 0.02–0.10 0.02–0.10 0.03–0.36 0.03–0.36Flow depth (cm) 7.4–17.2 20.8–32.1 20.5–41.1 12.0–35.3 10.6–44.2Velocity (m s�1) 0.14–0.46 0.07–0.41 0.07–0.40 0.03–0.39 0.02–0.37Froude number 0.17–0.36 0.05–0.27 0.04–0.26 0.02–0.22 0.01–0.20Type of vegetation Zoysia Pennisetum Reed Pennisetum ReedState of vegetation Green Dormant Dormant Green GreenManning’s n (total) 0.029–0.043 0.046–0.098 0.054–0.097 0.057–0.195 0.066–0.172

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

Figure 4. Outdoor experimental flume for test series D–E

678 D. S. RHEE ET AL.

made with a point gauge with an accuracy of 0.5mm. The bottom of the upstream reach before the measurement

section from the entrance section of the channel was covered with gravel whose mean particle diameter d50 was

about 11.4mm; again, this allowed for flow transition. Total 90 experimental runs (Table II) were carried out under

various conditions. Broader range of tested flows provided both the submerged and un-submerged conditions.

Minor variations in the soil surface were also neglected. It is noted that parts of stems and leaves of tested grasses

were bent as a result of flow condition changes. Damages to the vegetation and underlying bed, however, were

imperceptible or negligible.

Hydraulic characteristics for each experimental run were determined through a detailed examination of

experimental flow profiles or a series of cross sections along experimental flume. The cross-sectional area Awas

calculated as the average flow depth multiplied by flume width. The wetted perimeter P was calculated in a similar

way. The discharge Q was evaluated using weirs located at the upstream end of each flume. The average

cross-sectional velocity V and the hydraulic radius R were determined as Q/A and A/P, respectively.

DATA ANALYSIS

The value of Manning’s n was evaluated for each experimental run with the data obtained from the flume

experiments (water depth or stage, cross-sectional area, mean velocity, hydraulic radius). In the experimental

flumes used in this study, bottom slope was not adjustable. Instead, to evaluate Manning’s n for each measurement

section, the friction slope was used as follows:

Sf ¼ � d

dxh þ V2

2g

� �(3)

in which h¼water surface elevation, g¼ the acceleration of gravity and Sf ¼ friction slope. Manning’s equation is

expressed in the metric system as

V ¼ 1

nR2=3S1=2 (4)

in which S¼ friction slope.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

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HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 679

From Equations (3) and (4), Manning’s n for each measurement section between the adjacent cross sections is

given by Kirby et al. (2005)

n ¼ R4=3

V2

d

dxh þ V2

2g

� �����1

2

" #1=2

(5)

where R ¼ R1 þ R2ð Þ=2¼ average hydraulic radius for the section, V ¼ average mean flow velocity defined

similarly and superscript 1 and subscript 2 refer to upstream and downstream cross-sections, respectively. The

Manning’s n values for each measurement section were arithmetically averaged to obtain the final representative

value of the resistance coefficient for each experimental run. From the calculations, Manning’s n values for total

roughness of each tested grass were obtained.

The blockage effect of the vegetation itself was considered after the calculation of total roughness. To consider

this blockage effect, an adjustment should be made in the cross-sectional area occupied by the vegetation when

calculating the roughness coefficients. The parameter that measures the channel blockage by vegetation is often

referred as the blockage factor B. In this study, the blockage factor B was estimated as a ratio between total

cross-sectional area and the projected area of the tested plants. In addition, the blockage effect by the vegetation can

be considered as a kind of n3 in the Cowan’s procedure. However, the n3 values are not considered general, because

the density of vegetation is fixed for each test series, A–E (Table I).

After the blockage effects are deducted from the total n values, the vegetation roughness factor n4 is calculated by

subtracting the bed roughness to develop the n-VR retardance curve for each tested grass. Therefore, the final

roughness values for the vegetation alone are calculated by

n4 ¼ n � n3 � nb (6)

EXPERIMENTAL RESULTS AND DISCUSSIONS

From the result of each experimental run, the resistance coefficient for the vegetation n was calculated.

The calculated n values are plotted against the product VR to develop the n-VR retardance curve for each tested

plant.

Zoysia matrella (Korean golden zoysia)

As mentioned above, Z. matrella was tested only under submerged condition, because Z. matrella is relatively

short and stiff and usually submerged for normal flow conditions. Figure 5 illustrates the n-VR retardance curve of

the total roughness for Z. matrella. This seems to be a good example of a typical retardance curve in which

Manning’s n value decrease as VR increases. As the product VR continues to increase, it can be seen that the curve

of Manning’s n approaches to an asymptotic constant value of about 0.027, which is equivalent to total Manning’s n

value for flood plains with short grass, as suggested by Chow (1959). This implies that the resistance coefficients for

short grass may take similar values irrespective of the species of short grass. Although the value of the water depth

to vegetation height ratio ranges from 1.3 to 3.1, this asymptotic value can be easily defined on Figure 5. This

indicates that the resistance coefficient of short grass will be similar if the water depth on a flood plain is twice of the

height of short grass.

If the blockage effects are considered (Figure 6) and the bed roughness is deducted from total n values, the curve

of Manning’s n should approach to an asymptotic value of about 0.007 as shown in Figure 7, which is also

equivalent to the vegetation roughness values n4 suggested by USGS (Arcement and Schneider, 1984). They

suggested the vegetation roughness values as 0.001–0.010 for small amount of vegetation, which means that dense

growths of flexible turf grass, such as Bermuda grass or weeds growing where the average depth of flow is at least

two times the height of the vegetation. This also supports the fact that the resistance coefficient of short grasses is

similar to each other irrespective of the species of short grass.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

Figure 5. ‘n-VR’ retardance curve of the total roughness for Zoysia matrella

680 D. S. RHEE ET AL.

Pennisetum alopecuroides (L.) Spreng. (Korean pennisetum)

Figure 8 illustrates the n-VR curves based on data with a range of downstream-end depths for P. alopecuroides

(L.) Spreng. The downstream-end depth of the measurement reach was controlled by the tail-water gate operation,

to test under both submerged and un-submerged conditions. As shown in this figure, it seems that the resistance

coefficients have a strong relationship when VR> 0.02, while the data on resistance coefficients for VR< 0.02 are

scattered because plants are either bent or not bent depends on the downstream-end depth. This means that if

VR> 0.02, flexible plants having an intermediate height like P. alopecuroides (L.) Spreng. are only influenced to a

Figure 6. ‘n-VR’ retardance curve for Zoysia matrella after considering the blockage effect

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Figure 7. ‘n-VR’ retardance curve of the vegetation roughness for Zoysia matrella

Figure 8. ‘n-VR’ retardance curves of the total roughness for Pennisetum alopecuroides (L.) Spreng.

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 681

limited degree by variation in water depth, so for design purpose the same retardance curve can be applied to both

submerged and un-submerged conditions.

All data for ‘dormant’ and ‘green’ states are presented together in Figures 8–10. Comparing the trends for these

two data sets, it is found that ‘green’ resistance coefficients are always larger than ‘dormant’ ones. This seems to be

because the rigidity of ‘green’ plants is larger than ‘dormant’ plants, as ‘dormant’ plants are bent more easily,

hence, the flow resistance in river decreases when the plants are dormant, as in autumn and winter. It is noted that

the ‘green’ state roughness is about 80–110% higher than ‘dormant’ state if vegetation roughness is considered

only.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

Figure 9. ‘n-VR’ retardance curves for Pennisetum alopecuroides (L.) Spreng. after considering the blockage effect

682 D. S. RHEE ET AL.

As shown in Figure 10, after the blockage effects and bed roughness are deducted from the total n values, the

Manning’s n values for ‘green’ state converge to a constant value of about 0.012, and this is equivalent to the

vegetation roughness value n4, as suggested by USGS (Arcement and Schneider, 1984). They suggested the values

as 0.010–0.025 for medium amount of vegetation, turf grass growing where the average depth of flow is from one to

two times the height of the vegetation. The values of the water depth to vegetation height ratio ranged from 0.6 to

1.3 in this case. Similarly, the Manning’s n values for ‘dormant’ state converge to a constant value of about 0.001,

which is equivalent to the vegetation roughness values n4 suggested by USGS for small about of vegetation.

Figure 10. ‘n-VR’ retardance curves of the vegetation roughness for Pennisetum alopecuroides (L.) Spreng.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

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Figure 11. ‘n-VR’ retardance curve of total roughness for ‘dormant’ Phragmites communis Trin.

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 683

Equations (7) and (8) give the relationship between n4 and VR for green and dormant P. alopecuroides (L.) Spreng.:

green : n4 ¼ 0:0022=VR þ 0:012 (7)

dormant : n4 ¼ 0:0011=VR þ 0:001 (8)

Phragmites communis Trin. (reed)

Retardance curves of the total roughness for P. communis Trin. are presented in Figures 11 and 12. The

downstream-end depth of measurement reach was also controlled by tail-water gate operation for these

experimental runs. As shown in these figures, the ‘dormant’ n-VR curve is quite different from the ‘green’ ones. The

‘dormant’ retardance curve has a similar tendency to the results for P. alopecuroides (L.) Spreng. If VR> 0.025, it

seems that the resistance coefficients converge to one curve irrespective of the downstream-end water depth. That

implies that the tested reeds affect the flow resistance in a similar way to Korean pennisetum. On the contrary, as

shown in Figure 12, the ‘green’ retardance curves showed different patterns each other. The resistance coefficients

of these experimental runs are strongly influenced by the downstream-end water depth condition (in which

‘uncontrolled’ means no tail-water gate operation to maintain water level; water flows freely and tends to be

shallow). Manning’s n increases as water depth increases, when VR> 0.07; each retardance curve begins to

converge into one curve such as ‘dormant’ one. This is due to the fact that the tested ‘green’ reeds were in growing

stage, and the stems and leaf elements of the vegetation were distinct, and only the stem-part affects flow resistance

when the flow depth is relatively shallow. As the flow depth increases, the leaf part of the vegetation also begins to

affect the flow resistance simultaneously with the stem part. At the beginning of ‘dormant’ state, plants usually

begin to be dormant after a full growth, so the tested ‘dormant’ reeds have many leaves compared to the ‘green’

ones. Therefore, it is concluded that the leaf part of all tested vegetation plays an important role in the flow

resistance of vegetation. This result is consistent to the previous studies that the variation of vegetation resistance

with water depth is usually due to the vertical variation of biomass distribution and a strong shear layer is developed

at the top of submerged canopy when the vegetation becomes submerged (Nepf and Vivoni, 2000; Wilson et al.,

2003; Defina and Bixio, 2005). Also the interaction between the surface-floating leaves of bending vegetation and

the water surface tends to increase the magnitude of the resistance coefficient.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

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Figure 12. ‘n-VR’ retardance curves of the total roughness for ‘green’ Phragmites communis Trin.

Figure 13. ‘n-VR’ retardance curves for ‘dormant’ Phragmites communis Trin. after considering the blockage effect

684 D. S. RHEE ET AL.

After the blockage effects are considered (Figures 13 and 14) and bed roughness is deducted, as shown in

Figures 15 and 16, theManning’s n values of the vegetation roughness for ‘dormant’ and ‘green’ states converge to a

constant value of about 0.005 and 0.013, respectively. The usage of the value 0.013 however, should be limited,

because no reasonable asymptotic values are not found when the data for VR> 0.07 are considered only, so the data

for VR> 0.05 are used to develop a regression curve. From the value point of about VR> 0.05, it is observed that

the reeds are significantly deflected by the water flow. These values are also equivalent to the vegetation roughness

values n4 suggested by USGS for small and medium amount of vegetation. Equations (9) and (10) give the

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

Figure 15. ‘n-VR’ retardance curves of the vegetation roughness for ‘dormant’ Phragmites communis Trin.

Figure 14. ‘n-VR’ retardance curves for ‘green’ Phragmites communis Trin. after considering the blockage effect

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 685

relationship between n4 and VR for green and dormant P. communis Trin., respectively:

green : n4 ¼ 0:0025=VR þ 0:013 (9)

dormant : n4 ¼ 0:0013=VR þ 0:005 (10)

It is also noted that the applicability of the Equation (10) will be limited because limited data were used to

develop the regression curve. Compared with the experimental results of P. alopecuroides (L.) Spreng., the

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DOI: 10.1002/rra

Figure 16. ‘n-VR’ retardance curves of the vegetation roughness for ‘green’ Phragmites communis Trin.

686 D. S. RHEE ET AL.

asymptotic values of this study are very similar. This indicates that these grasses should have similar roughness

values in large floods in summer season.

Application to the river restoration works in Korea

Before the river restoration works started in early 1990s in Korea, it was prohibited to plant vegetation in river

reaches because the vegetation generally reduces the conveyance of river flow, so the possibility of flooding events

increases. Most Korean river engineers usually consider concrete revetements and large ripraps when they design

river management projects. The resistance induced by the natural vegetation in the river reach is estimated

empirically based on the experience of senior engineers. In the design of river restoration works, the Manning’s

roughness value n always was selected from the guidelines developed in foreign countries. Because there is no

linkage between those guidelines and the Korean natural vegetation, the engineers always suffered from lack of

reliability when they estimated the flow levels in rivers.

The advantage of the n-VR method is that engineers are already familiar with the use of the curves in spite of the

iterative characteristics of this method. In Korea, VR [m2 s- 1] values are usually about 4–6 for small streams, and

about 18–22 for small to medium rivers. Therefore, the asymptotic values and regression curves developed for the

tested vegetation in this study can be successfully used as guideline values for the river restoration design. And this

result will be a bridge to other criteria developed for the foreign grasses and river environments, so those criteria can

be adapted to Korea in connection with the results of this study.

SUMMARY AND CONCLUSIONS

An experimental study has been conducted using some Korean natural vegetation to investigate the effect of

vegetation on the flow resistance in variable water levels giving both submerged and un-submerged conditions.

Z. matrella, P. alopecuroides (L.) Spreng. and P. communis Trin. were selected as representatives of Korean native

vegetation. New experimental results presented in this paper can be used to evaluate the flow resistance in

connection, combined with the results of previous research that considered the differences in vegetation type.While

further experimentation is needed to extend the applicable range of the n and VR and to increase the target species

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra

HYDRAULIC RESISTANCE OF SOME SELECTED VEGETATION 687

for the evaluation, similarities in vegetation characteristics in terms of the height and rigidity may support the worth

of this study.

The general conclusions can be drawn: (1) the resistance coefficient of Z. matrella approaches an asymptotic

value of 0.027, which is same as the value suggested by Chow (1959). This value can be applicable to other short

grasses; (2) flexible intermediate-height grasses like P. alopecuroides (L.) Spreng. are less influenced by variation

in water depth, therefore, for the channel design purpose the same retardance curve can be applied to both

submerged and un-submerged conditions; (3) the resistance coefficients of ‘dormant’ plants are usually smaller

than those of ‘green’ ones; (4) the resistance coefficients are larger when the flow is affected by both the leaf and

stem part of vegetation than when it is affected only by the stem part of vegetation. The findings of this study give an

extensive understanding to the characteristics of hydraulic resistance about the Korean riverine vegetations. The

asymptotic values and regression curves developed for the tested vegetation can be used as a guideline in the river

restoration design. The result can be used as a bridge to other researches, so the criteria developed for the foreign

grasses and river environments may be adapted to Korean river conditions.

ACKNOWLEDGEMENTS

This study was supported by the research project ‘Development of Multi-functional River Restoration Techniques’

in Korea Institute of Construction Technology. The authors thank Duhan Lee and Hong Koo Yeo for providing

useful comments.

REFERENCES

Arcement GJ, Schneider VR. 1984. Guide for Selecting Manning’s Roughness coefficients for Natural Channels and Flood Plains.United States

Geological Survey Water-supply Paper 2339, USGS.

Chen CL. 1976. Flow resistance in broad shallow grassed channels. Journal of Hydraulics Division 102(3): 307–322.

Chow VT. 1959. Open-channel Hydraulics. McGraw-Hill: New York.

Cowan WL. 1956. Estimating hydraulic roughness coefficients. Agricultural Engineering 37(7): 473–475.

Darby SE. 1999. Effect of riparian vegetation on flow resistance and flood potential. Journal of Hydraulic Engineering 125(5): 443–454.

Defina A, Bixio AC. 2005. Mean flow and turbulence in vegetated open channel flow. Water Resources Research 41: W07006. DOI: 10.1029/

2004WR003475.

Fathi-MaghadamM, Kouwen N. 1997. Nonrigid, nonsubmerged, vegetative roughness on floodplains. Journal of Hydraulic Engineering 123(1):

51–57.

Green JC. 2005. Modelling flow resistance in vegetated streams: review and development of new theory.Hydrological Processes 19: 1245–1259.

Henderson FM. 1966. Open-channel Flows. Macmillan: New York.

Jarvela J. 2002. Flow resistance of flexible and stiff vegetation: a flume study with natural plants. Journal of Hydrology 269: 44–54.

Kirby JT, Durrans SR, Pitt R, Johnson D. 2005. Hydraulic resistance in grass swales designed for small flow conveyance. Journal of Hydraulic

Engineering 131(1): 65–68.

Kouwen N, Li RM. 1980. Biomechanics of vegetative channel linings. Journal of Hydraulics Division 106(6): 1085–1103.

Musleh FA, Cruise JF. 2006. Functional relationships of resistance in wide flood plains with rigid unsubmerged vegetation. Journal of Hydraulic

Engineering 132(2): 163–171.

Nepf HM, Vivoni ER. 2000. Flow structure in depth-limited vegetated flow. Journal of Geophysical Research 28: 28547–28557.

Stone BM, Shen HT. 2002. Hydraulic resistance of flow in channels with cylindrical roughness. Journal of Hydraulic Engineering 128(5):

500–506.

Temple DM. 1999. Flow resistance of grass-lined channel banks. Applied Engineering in Agriculture 15(2): 129–133.

Temple DM, Robinson KM, Ahring RM, Davis AG. 1987. Stability Design of Grass-lined Open Channels, Agricultural handbook, 667. USDA:

Washington, D.C.

Wilson CAME, Stoesser T, Bates PD, Batemann Pinzen A. 2003. Open channel flow through different forms of submerged flexible vegetation.

Journal of Hydraulic Engineering 129(11): 847–853.

Wu F-C, Shen HW, Chou Y-J. 1999. Variation of roughness coefficients for unsubmerged and submerged vegetation. Journal of Hydraulic

Engineering 125(9): 934–942.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. 24: 673–687 (2008)

DOI: 10.1002/rra