International Conference on Advances in Civil and Environmental Engineering 2015 © Faculty of Civil Engineering, Universiti Teknologi MARA Pulau Pinang
DETERMINATION OF OPEN CHANNEL ROUGHNESS COEFFICIENT 1,
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ROSSELI, S.R. *, MOHAMMAD RAZI, M.A. , FAUZI, M.A. , TUTUR, N. , 1 1 MARZUKI, N.A. , RAJA MOHD NOR, R.N.H. 1
Faculty of Civil Engineering, Universiti Teknologi MARA, 13500 Permatang Pauh, Pulau Pinang, Malaysia.
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Faculty of Civil and Environmental Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Batu Pahat, Johor, Malaysia. *Corresponding Author:
[email protected]
Abstract Researches on determining open channel roughness coefficient has been done by Westerners to ease engineers for developing any kind of hydraulic structures. However, the values provided by Westerners through past researches cannot be used for exact reference on any open channels in this country because of different location, soil type and geography. Therefore, this study was carried out to determine roughness coefficient, n value for selected type of channels as a reference for future research that will be done in this country which have similar soil type and geography. The objectives of this study were to determine n value range of selected channel, to compare the obtained n value between past research, to determine states of flow for selected channel using obtained n value and to compare discharge values at other channel by using n values obtained at selected channel. This study was conducted at Parit Karjo and Parit Botak was selected as a comparison channel as this channel was in the same district, Parit Raja. The difference of n value depends on location, channel properties, geometry and flow discharge. The needed data for evaluating the n values are channel cross-section, flow velocity, channel depth and width. The data were obtained from site measurement using current flow meter wood and measuring tape. To classify the soil type for both channels, sieve analysis has been conducted. Data were collected 25 times at every 5 cross sections along the channel. As the result, that the range of n value from 0.048 to 0.987 that had been determined for well-graded sand, gravelly sand with few or no fines open channels in Parit Raja. The states of flow for Parit Karjo channel at obtained n values were turbulent and transitional flow. Keywords: Roughness coefficient, The Manning formula, Open channel.
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1. Introduction Flow profile calculation, velocity and discharge plays important roles in water resource project, flood management planning and the determination of hydraulic effects for river conservation. In many cases, these calculation need adequate selection of channel roughness coefficient [4]. The Manning formula has been widely used to determine the roughness coefficient, n for open channels. The n value is a measure for surface roughness that affect the flow resistance. Small errors of this value can also give effect to the calculations. Although the n value have been practiced in hydraulic and hydrology analysis, the determination is a challenge for engineers because the values cannot be compute equally for all open channels [1]. The roughness coefficient values provided in reference books or past research normally based on study of selected open channels in overseas. High technology also caused the Westerners capable of developing database as ways to ease the determination of n values [5]. There were few researches being done for open channels in this country. The values provided overseas past research cannot be used in this country as exact reference because of different location and geography. Engineers have to determine the roughness coefficient before starting any hydraulic works. Other factors such as surface roughness, vegetation, channel irregularity, channel alignment, silting and scouring, obstruction and discharge also affects the roughness coefficient values [3]. Chow also stated n values between 0.025 and 0.033 for clean open channel, straight and no rifts. The objectives of this study were to determine n value range of selected channel, to compare the obtained n value between past research, to determine states of flow for selected channel using obtained n value and to compare discharge values at other channel by using n values obtained at selected channel.
2. Materials and Methods This study was conducted at Parit Karjo and Parit Botak was selected as a comparison channel as this channel was in the same district, Parit Raja. The determinations of factors affecting the roughness coefficient were limited to discharge, velocity and water area. Sieve analysis experiment has been conducted to obtain the soil classification at channel bank for both channels. The experiment was performed at Geotechnical lab, Department of Geotechnical and Transportation, Faculty of Civil and Environment Engineering, UTHM. This study also was carried out at Parit Karjo channel to obtain data such as depth of flow, cross sectional flow area, wetted perimeter, water surface width, channel width and side slope using equipments from Water Resource lab, Department of Water Engineering and Environment, Faculty of Civil and Environment Engineering, UTHM. The open channel dimensions change relatively due to several factors such as channel roughness, erosion and others. This situation leads to the changes of depth of flow each time of measurement and give effects to water area (A), top with of channel (T) and wetted perimeter (P) value and roughness coefficient (n). Five channel cross sections have been selected which 120 metres apart from one
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cross section to another along Parit Karjo. The repetition of data collections have been done at the same cross-sections to obtain the differences in flow, velocity by and channel geometry. Fig. 1 showed the length section for channel between two cross sections while Fig. 2 showed channel cross section measurement.
Fig. 1. Length section for channel.
Fig. 2. Channel cross section measurement. Eq. (1) showed the Manning formula was used to determine the roughness coefficient, n value [2]: Q=
AR 2 / 3 S 0 n
1/ 2
(1)
where Q = discharge A = water area R = hydraulic radius S0= slope angle n = roughness coefficient Eq. (2) showed Reynolds number equation was used to determine state of flow for both channels. Re =
vL
(2)
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where, L = characteristic length v = velocity of flow γ = kinematics viscosity of water [2]. From v, y and B values obtain from site works, the data were analysed. Equations below were used to analyse the data: i.
Water area, A By referring to Fig. 3, Ai = 0.5(Bi+1 – Bi)(Yi+1 + Yi)
(3)
for i = 1,..., NPT-1 A = ∑Ai ii.
Wetted perimeter, P By refering to Fig. 3, Pi = [(Bi+1 – Bi)2 + (Yi+1 – Yi)2]1/2
(4)
for i = 1,...,NPT-1 P = ∑Pi
B3
B4 Y4 – Y3
Fig. 3. P value calculation method using Pythagoras. iii.
Hydraulic radius, R R=
A = P
A P
i
i
(5)
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Slope angle, S0 If side slope of the channel indicates θ angle from horizontal, hence the slope angles is S0 = tan θ.
y θ Bi+1 – B1 θ = tan-1 v.
(
y ) Bi 1 B1
Discharge, Q (7)
Q = Av vi.
(6)
Roughness coefficient, n Q=
AR 2 / 3 S 0 n
1/ 2
(8)
3. Results and Discussions Table 1 showed the result obtained for this study. For soil classification, Parit Karjo channel is classified as well-graded sand, gravelly sand with few or no fines and manmade channel. The range for n value obtained was from 0.048 to 0.987 by calculating parameters such as A, P, R, S0 and range of Q from 0.086 m3/s to 8.669 m3/s using discharge equation. From the result above, the range of n value obtained was higher than the comparison value 0.025 to 0.033 chosen at early stage of this study. At first, Parit Karjo was estimated as clean open channel, straight and no rifts. The comparison value was obtained by referring to Chow and this value was for clean open channel, straight and no rifts. However, Chow did not mention the soil type. In this study, the determination of state of flow also has been done. States of flow was obtained at n minimum, normal and maximum. At n = 0.048, flow was turbulent, while at n = 0.520 and 0.987 the flow were transitional. As a comparison that had been done at Parit Botak, Q theory values obtained from the range of Q values of Parit Karjo as showed in Table 2. It showed that the range is relevant for open channels in Parit Raja with same soil classification, well-graded sand. There were some differences between Q theory and Q experiment. This caused by A and v values that changed frequently as Parit Botak was also an open channel.
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Table 1. Summary of result from Parit Karjo channel analysis. Well-graded sand, gravelly sand with few or no fines
Soil classification
Range of n
0.048 to 0.987
Range of Q
0.086 m3/s to 8.669 m3/s
n values
Q values
State of flow
n minimum = 0.048
5.855 m3/s
Turbulent flow
n normal = 0.520
0.870 m3/s
Transitional flow
n maksimum = 0.987
0.605 m3/s
Transitional flow
Table 2. Summary of result from Parit Botak channel analysis. Soil classification
Well-graded sand, gravelly sand with few or no fines
Q theory using equation n values from Parit Karjo Q=
AR 2 / 3 S 0 n
Q experiment using equation
1/ 2
Q = Av
n = 0.048
8.631 m3/s
8.482 m3/s
n = 0.520
0.797 m3/s
0.594 m3/s
n = 0.987
0.420 m3/s
0.398 m3/s
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4. Conclusion From this study, we can conclude that the range of n from 0.048 to 0.987 that had been determined is for well-graded sand, gravelly sand with few or no fines and for open channels in Parit Raja. The states of flow for Parit Karjo channel at 3 n values determined earlier are turbulent and transitional flow. The limitations for this study were n values obtained are for well-graded sand, gravelly sand with few or no fines, and only for open channels in Parit Raja. There were several recommendations to overcome any weaknesses for future research. Researcher should obtain data using other suitable method to decrease errors while computing parameters such as depth of channel, velocity, and channel width and compare the obtained n values for future research with several open channels with same soil classification to proof the n values can be used widely. The study about determination of channel roughness coefficient must be continue widely because of the important usage in constructing any hydraulic structure in Malaysia.
Acknowledgement The authors would like to express an acknowledgement to the Faculty of Civil and Environment Engineering, Universiti Tun Hussein Onn Malaysia for providing the facilities and their support to accomplish this study. The author also wishes to acknowledge cooperation given by laboratory technicians to complete this study.
References 1. 2.
3. 4.
5.
Jarrett, R.D. (1985). Determination of Roughness Coefficients for Streams in Colorado. USGS Water Resources Investigation Report. Amat Sairin Demun (1997). Hidraulik Saluran Terbuka dengan Penggunaan Komputer. Johor Darul Ta’zim. Universiti Teknologi Malaysiaμ Penerbit UTM. Chow, V.T. (1973). Open-channel Hydraulics. Singapore. McGraw-Hill Book Co. Robertson, B.A.; Cassidy, J.J.; Rutherfurd, I. (2000). Towards an Australian Handbook of Stream Roughness Coefficients. Journal of Hydraulics Engineering. Sturm, T.W. (2001). Open Channel Hydraulics. United States of America. The McGraw Hill Companies.
