Jurnal Publikasi Internasional

8/11/2019 Jurnal Publikasi Internasional

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1stInternational Conference on Managing Rivers in the 21stCentury: Issues & Challenges

The Effects of Vegetations on the Hydraulic Roughness of an Open

Channel Flow

BADRONNISA YUSUF, Lecturer, Department of Civil Engineering, Faculty of

Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor

ABDUL HALIM GHAZALI,Associate Professor, Department of Civil Engineering, Faculty

of Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor

MEGAT JOHARI MEGAT MOHD NOOR, Associate Professor, Department of Civil

Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor

THAMER A. MOHAMED, Associate Professor, Department of Civil Engineering, Faculty


KATAYON SAED, Lecturer, Department of Civil Engineering, Faculty of Engineering,Universiti Putra Malaysia, 43400, Serdang, Selangor

MANAL A. ABOOD, Research Students, Department of Civil Engineering, Faculty of

Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor

FAIZAL MOHD KHAILANI,Research Students, Department of Civil Engineering, Faculty


ABSTRACT

The purpose of this study is to analyze the effects of different type of vegetations to thevelocity and hence the hydraulic roughness of an open channel flow. Laboratory experiments

were explored to investigate the effects of varying characteristics (type, density, distribution)

of vegetations on the hydraulic roughness. Two types of plants namely Cyperus Alternifolius

(CA) and Otellia Alismoides (OA) were selected for this study. Mannings equation was

adopted to determine the value n, which is roughness coefficient for the channel. The

roughness contributed by the plants was determined using soil conservation service (SCS)

method. The results from the study showed that Otellia Alismoidesgave higher resistance to

flow velocity compared to Cyperus Alternifolius. The degree of effects of the vegetations also

depends on the density and distribution of the vegetation.

Keywords:Roughness coefficient, Mannings coefficient, Vegetations, Open channel, flowresistance.

1 Introduction

The solution of open channel flow related

problems such as computation of

backwater curve, flood routing and

channel improvement are related to the

determination of flow resistance value.

The magnitude of the resistance is usuallymeasured in term of resistance or

roughness coefficient and if Manning

equation is used, it is characterize by

Manning Coefficient. In area where flow

occurs through vegetation, the flow

resistance be largely determined by the

vegetation (Wu et al, 1999). Chow (1959)

reported that, increases in Mannings

roughness coefficient due to vegetationcan be as much as higher than that due to

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channels particle size alone, however

(Thorne, 1990) cautioned that vegetation's

effects are complex and depends on many

factors such as vegetation species, location

and density as well as channel's width,

depth and sediment size. The growth ofthick vegetations in channel can reduce

capacity and retard flow (Thorne, 1990,

Kao and Bartfield, 1978) and increases

flow resistance (Bakry et al, 1992). The

average water velocity of cross section is

reducing, because of roughness presented

by the stems and leaves of the plants.

The effects of vegetation vary

depending on the species, distribution,

density and size of vegetation. The

hydraulic roughness of vegetated channelis a function of the stiffness of the plants

growing in the channel, the depth,

velocity, hydraulic radius, plant density

and frontal area of the plant obstructing the

flow (Freeman, 2000). Vegetations that are

totally submerged in wet season give less

resistance to flow compared with

vegetations that are partially submerged

during dry season (Copeland, 2000).

Many studies have been conducted in

previous years to investigate the resistance

to flow provide by the vegetation, using

either artificial or real vegetation,

submerged or unsubmerged, at the

floodplain or at the main channel.

Thompson and Roberson (1976) used

small diameter cylinders to simulate

vegetation roughness elements of

vegetated open channels flow. They

presented analytical method to predict the

effect of flexible vegetation on flowresistance. Kao and Barfield (1978) made

his study to predict the flow hydraulics for

vegetated channels. Simulated dense

vegetation with random flexible blade was

used to determine the hydraulic properties

of small non-submerging depths.

Experimental results were used to

determine the coefficient of blade

resistance and plotted in the terms of blade

width and flow depth Reynolds number,

respectively. Pashe et al (1985) studiedoverbank flow characteristics on the

vegetated flood plains using cylindrical

wooden rods to simulate non-submerged

vegetative roughness. Reza Mahbub and

Suzuki, (1988), investigated flow

retardance effects of vegetation growth

using plastic styrene tapes to replicategrass. Abdelsalam et al (1992) presented

result of a research program conducted

during 15 years to collect and analyze field

data to describe the hydraulic resistance

characteristics of earthen channel infested

with ditch-bank (emergent) and submerged

aquatic weeds, under actual operating

condition. Fathi Maghdam, 1997 obtained

a mathematical model for estimation of

roughness for unsubmerged and flexible

vegetation in floodplains and vegetatedzone of river. Experiments were performed

to show the effects of non- rigidity and

inundation depth of vegetative roughness.

The researcher summarized that the

Mannings value increases proportionally

to the square root of flow depth and

inversely proportional to the mean velocity

for unsubmerged condition. Wu Fu et al

(1999) used artificial roughness, horsehair

mattress, to investigate the variation of

vegetation resistance with stage for

unsubmerged and submerged conditions.

Flow resistance of natural grass, sedges

and willows was studied by Jarvela (2002).

This study was laboratory based and

focused on the effects of local (tropical)

plants namely Cyperus Alternifolius (CA)

and Otellia Alismoides (OA) on the flow

resistance. These types of plants are

commonly found in shallow water bodies,

either on the bank or the bed of thechannel. In humid country like Malaysia

vegetation growth in a natural channel is

quite a common scene all year round

especially in small streams which do not

receive regular maintenance.

2 Resistance Coefficient

The most frequently used equation related

to the determination of flow resistance is

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Manning Equation due to its simplicity

and practicality.

2

1

3

2

SR

n

1V ! (1)

where V is the mean velocity (m/s), n is

the Mannings roughness coefficient, R is

the hydraulic radius (m), and S is the slope

of energy grade line (m/m). The challenges

part in using this equation lies in the

determination of n. Mannings roughness,

n, is highly variable and depends on many

factors such as stage and discharge,

channel irregularity, channel, changes in

channel width and shape, vegetation,

obstruction and channel alignment. Thepublished table of n (eg. Chow, 1959) is

usually used for initial estimate, whereby

the value are tuned and adjusted to reflect

the actual situation of the channel (Wilson,

2002). The roughness contributed by the

vegetation can be determined by using

(Cowan, 1956) procedure as discussed in

(Chow, 1959) and Soil Conservation

Service (SCS) method as described in

French 1994.

n = (no+ n1+ n3+ n4) m5

Where nois the roughness coefficient for a

straight, uniform and smooth channel, and,

n1, n2, n3, n4 is the value added to no for

channel irregularity, changes in channel

shape and size, obstruction and vegetation,

respectively. m5is the correction factor for

meandering of channel.

3 Materials and Methodology

3.1 Open Channel

The experiments were carried out in a

channel of 2400mm length, 500mm width

and 500mm height. The bed is covered

with sand of a slope of 0.1% referring to

the low land of the real river. Water is

pumped from the storage tank to a gravity

tank and then through the open channel.

The flow is controlled by the valve and

flow meter. The channel was divided into

three regions, the first region is thelocation before vegetation (0-50 cm from

the inlet) the second region is within the

vegetation (50-100 cm from the inlet) and

the third region is after the vegetation

(100-140 cm from the inlet). The second

region was where the vegetations were

planted. See Figure 1.

3.2 Vegetations

The vegetations selected for this study

were Cyperus Alternifolius (CA) and

Otellia Alismoides (OA). These types of

plant are quite common in shallow water

bodies around the country. Table 1 gives

the characteristics of these two plants.

3.3 Experimental Works

Vegetation of selected size and height

were initially planted in a poly bag. The

planted vegetations were then inserted and

covered under the sand bed. The velocity

at different depths and points at regions 1,

2 and 3 with and without vegetation were

measured using Streamflo velocity meter.

The results of the mean velocity were then

used in Mannings equation to determine n

for the overall channel. The roughness

contributed by the vegetation itself wasthen calculated. The number of vegetation

and arrangement were varied in order to

see the effects on the velocity and the

roughness coefficient. Vegetation of 5, 9,

14 numbers were distributed in three

different arrangements as shown in Figure

1a-c.

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Table 1 Characteristics of Vegetation.

CA OA

Family Cyperaceae Hydrocharitaceae

Local Name Rumput Kelulut Keladi Air

Propagation Seeds and stem cutting By fruits

Ecologicalfunction

Bank protection, reduce suspendedsolids and chemical pollutants

biodegrades organic pollutants in water

Reduce suspended solids andchemical pollutant; biodegrades

organic pollutants of water

Habitat Wet sites, swamps, ditches, marshy

lake, shores, secondary forest and

paddy filed

Shallow water bodies, lake and

paddy field

Figure 1 a) Channel section with vegetation in Arrangement Type 1 b) Vegetations

Arrangement Type 2 c) Vegetations Arrangement Type.

4 Results and Discussion

The result from the study shows that the

velocity of flow decreased in the present of

vegetations. OA gave higher reduction in

velocity compared to CA. This is due to

the physical characteristics of the OA.

Physical characteristics include the size of

stem, leaf, root and surface area. Thelength of OA used was 24-28 cm and the

diameter of stem was 0.5-1.0 cm. whereas

for CA, the length and the diameter of the

stem measured was 28-73 cm and 0.3-0.6

cm, respectively. The velocity reduction

also depending on the number and

arrangement of the vegetations planted.

For example, for number of OA of 5

(arrangement type 1), 9 (arrangement type

2) and 14 (arrangement type 3), thepercentage reduction of mean velocity

Region 1Region 2Region 3

Inflowoutflow

(a)

Vegetations

Region 2 Region 2

(b) (c)

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compared to the velocity of flow without

vegetation is 48%, 60% and 76%,

respectively. Whereas for CA in the

arrangement and quantities as above, the

reduction of mean velocity is 42%, 49%

and 60%, respectively. The results showthat the higher the flow area covered by

vegetation the higher the reduction in

velocity.

The roughness coefficients, n for

the channel with and without vegetation

were calculated using Mannings equation.

The contribution of roughness by the

vegetation to the overall roughness was

estimated using SCS method. The value of

n was higher at the vegetation and reduced

after the vegetation. The increment of ndepends on the physical characteristics of

the plants, density and distribution of the

vegetation. OA have the higher effects on

the n. The increment of n compared to that

of without vegetation is by 140%, 206%

and 406% for 5, 9 and 14 numbers of

plants respectively. The contribution of

OA to the overall roughness of the channel

is 58%, 67% and 80% depending on the

numbers and arrangement of plant. When

the Cyperus Alternifolius was present the

value of n increased by 106% for 5

numbers of plants, 133% for 9 numbers of

plants and 213 % for 14 numbers of plants.

CA contributed roughness to the overall

roughness of the channel of 52%, 57% and

68% for 5, 9 and 14 numbers of plants

respectively. Figure 2a and 2b show the

values of roughness coefficient, n of the

channel with and without vegetation.

Figure 3 shows the variation of n acrossthe regions with OA and CA, planted at 3

different arrangements. It clearly shows

the roughness increased significantly in the

region 2 where the vegetations were

planted and the increased depending on the

type, quantities and arrangement of the

vegetation.

0.015 0.015 0.015

0.0210.031

0.061

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

1 2 3

Type of Plant Arrangement

RoughnessCoefficient,n

Roughness contribute by the vegetation

Roughness without vegetation

2 (a)

0.015 0.015 0.015

0.0160.020

0.032

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

1 2 3

Type of Plant Arrangement

RoughnessCoefficient,n

Roughness contributed by the vegetationRoughness w ithout vegetation

2 (b)

Figure 2 Channel Roughness coefficient

without vegetation and with vegetation for

a) OA b) CA.

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0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 25 50 75 100 125 150

Distance from inlet (cm)

RoughnessCoeffi

cient,n

Arrangement

Type 1 (OA)

Arrangement

Type 2 (OA)

Arrangement

Type 3 (OA)

Arrangement

Type 1 (CA)

Arrangement

Type 2(CA)

Arrangement

Type 3 (CA)

Figure 3 Overall Channel Roughness Coefficient with OA and CA at Regions 1, 2 and 3.

5 Conclusions

Otellia Alismoides gave higher reduction

in velocity than Cyperus Alternifolius. The

higher the flow area covered by vegetation

the higher the reduction in velocity.

Roughness coefficient calculated at three

regions showed that the value of n was

higher at the vegetation (Region 2) andreduced after vegetation (Region 3). The

roughness coefficient depends on the type,

quantities and arrangement of the

vegetation. When the quantities of

vegetation are increased the roughness

coefficient also increased. Otellia

Alismoides has a higher effect on the

roughness of the channel contributed about

58% -80% to the overall roughness of the

channel depending on the quantities and

arrangement of the vegetations while

Cyperus Alternifolius contributed 52%-

68% to the overall channel roughness.

Vegetations also effect velocity

distributions of flow because of their

physical structure. The velocity

distributions seemed to be affected more

by the leaves than by the stem of

vegetation.

6 References

1. Abdelsalam M.W, Khattab, A.A.,Khalifa, Bakry, M.F., (1992). Flow

Capacity Through Wide and

Submerged Vegetation Channels.

Journal of Irrigation and Drainage.

ASCE. Vol 118, No. 5.

2. Bakry, M.F., Gates T.K., Khattab A.F.(1992). Field Measured Hydraulic

Resistance Characteristics in

Vegetation Invested Canals. Journal of

Irrigation and Drainage Engineering.

ASCE.Vol.118 No: 2.

3. Chow V.T. (1959). Open ChannelHydraulics. Mc Graw Hill.

4. Fathi Maghdam, M., Koewen, N.,(1997). Nonrigid, Nonsubmerged

Vegetative Roughness on Flood plains.

Journal of Hydraulics Engineering.

ASCE. Vol.123 No.1.

5. Freeman, G.E., Rahmeyer, W.J.,Copeland, R.R. (2000). Determination

of Resistance Due to Shrubs and

Woody Vegetation. Coastal and

Hydraulic Laboratory, US Army Corps

of Engineers. ERDC/CHL TR-00-25.

6. French, R.H. (1994). Open ChannelHydraulics. Mc-Graw-Hill Book Co.,

New York.

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7. Jarvela, J. (2002). Flow Resistance ofFlexible Vegetation, A Flume Study

with Natural Plants. Journal of

Hydrology, 269, 44-54.

8. Kao D.T.Y. and Barfield B.J. (1978).

Prediction of Flow Hydraulics forVegetated Channels. Transaction of the

ASAE. 489-494.

9. Pasche, E., Rouve, G., (1985).Overbank Flow with Vegetatively

Roughened Flood Plains. Journal of

Hydraulic Engineering, Vol. 111, No

9.

10.Reza Mahbub A.K.M. and SuzukiShigeyoshi, (1988). Flow Retardance

in Open Channels due to Artificial

Flexible Vegetation. IrrigationEngineering and Rural Planning No

13. 5-17.

11.Hompson G. T. and Roberson, J.A.(1976). A Theory of Flow Resistance

for Vegetated Channels. Transaction of

the ASAE. 19(2). 288 293.

12.Thorne, C.R., (1990). Effects of

Vegetation on Riverbank Erosion andStability. Vegetations and Erosion,

Wiley, 125-144.

13.Wilson, C.A.M.E., Horritt, M.S.,(2002). Journal Hydrological

Processes, Vol 16.

14.Wu F.C., Shen H.W., Chou Y.J.(1999). Variation of Roughness

Coefficient for Unsubmerged and

Submerged Vegetation. Journal of

Hydraulics Engineering.

ASCE.Vol.125 No.9.

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