STUDY OF FLOW CHARACTERISTICS
IN A NON-SYMMETRICAL COMPOUND CHANNEL : SMOOTH FLOOD PLAIN
PHANG HOCK KIM
ti , ýNp, l A YSIq
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G NIMPS
FACULTY OF ENGINEERING
UNIVERSITY MALAYSIA SARAWAK
2003
TA 151 P535 2003
Universiti Malaysia Sarawak Kota Samarahan
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BORANG PENYERAHAN TESTS
Judul: Study of flow characteristics in a non-symmetrical compound channel: smooth flood plain.
5ES1 FE, NGA. 11Ati: 2000 - 2003
Saya PHANG HOCK LIM
(HURUF BESAR)
mengaku membenarkan tesis im disimpan di Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dengan syarat-syarat kegunaan seperti berikut:
1. Hakmilik kertas projek adalah di bawah nama penulis melamkan penulisan sebagai projek bersama dan dibiayai oleh UNIMAS, hakmiliknya adalah kepunyaan UNIMAS.
2. Naskhah salinan di dalam bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran berhilis daripada penulis.
3. Pusat Khidmat Maklumat Akademik, UNIMAS dibenarkan membuat salinan untuk pengajian mereka.
4. Kertas projek hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar yang dipersetujui kelak.
5. ' Says membenarkan/tidak membenarkan Perpustakaan membuat salinan kertas projek im sebagai bahan pertukaran di antara institusi pengajian tinggi.
6. s" Sila taodakan ( ,()
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SULIT (Mengandungi maklumat yang berdarjah keselamatan stau kepentingan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972).
TERHAD (Mengandungi maldumat TERHAD yang telah ditentukan oleh organisasi/
badan di mana penyelidikan dijalankan).
TIDAK TERHAD
K" NGAN PENULIS)
Alamat tetap: 126, BERATOK VILLAGE,
94200 KUCHING, SARAWAK,
(Nama Penyelia )
Tarikh: 15 MARCH 2003 Tarikh: a9 //-
o 3-
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CATATAN .
..
Peteng yang Wak berkeeaao.
Jika Kertas Prsjek isd SULIT alas, TERHAD, sBs lamrirkas swat daripda pIbakk berlmasa/
srgardsasi berkenae ienpn meeyertakaa sekali tempb kertas prsjek. Td perle dEmbukm sebagai SULIT alas TERHAD.
Disahkan oleh
ANDATANGAN PENYELIA)
Dr. NABIL BESSAIH
APPROVAL SHEET
The following final year project report:
Title : Study of flow characteristics in a non-symmetrical compound channel : Smooth Flood Plain.
Name : Phang Hock Lim
Matric no. : 4585
had been read and checked by :
.e y- 4.2 3 -. ä 3
ssociate Prof. Dr. Nabil Bessaih Date
Supervisor
STUDY OF FLOW CHARACTERISTICS IN A NON-SYMMETRICAL COMPOUND
CHANNEL :
SMOOTH FLOOD PLAIN
P. KMIDMAT MAKLUMAT AKADlMIK UNIMA:
0000118386
PHANG HOCK LIM
Tesis Dikemukakan Kepada
Fakulti Kejuruteraan, Universiti Malaysia Sarawak Sebagai Memenuhi Sebahagian Daripada Syarat
Penganugerahan Sarjana Muda Kejuruteraan Dengan Kepujian ( Kejuruteraan Sivil ) 2003
Specially Dedicated to :
Nicole Tan Hui Ling and family My family
To all who had guided, helped, taught,
advised and motivated me along the way.
ii
ACKNOWLEDGEMENT
The author would like to express his profound appreciation and gratitude to the following persons whom either direct or indirectly helped to complete his project.
1. Associate Prof. Dr. Nabil Bessaih, project supervisor, for his guidance, invaluable advice and comments, and also encouragement throughout this project.
2. Civil Engineering Program, Facvulty of Engineering, Universiti Malaysia Sarawak in providing all laboratory facilities and materials.
3. All my friends and family who have always been there whenever needed.
lll
ABSTRAK
Pada beberapa tahun kebelakangan ini, kajian tentang aliran air dalam sungai yang mempunyai dataran banjir adalah subjek yang selalu dipertimbangkan. Akan tetapi, masih terdapat kesusahan untuk mengira isipadu aliran dalam sungai yang mempunyai dataran banjir. Kaedah yang biasa digunakan untuk mengira isipadu aliran di sungai yang berbentuk mudah adalah kurang tepat apabila aliran air melebihi tebing sungai. Ini adalah sangat merbahaya jika isipadu aliran air kurang dikira Jika isipadu aliran air lebih dikira, maka rekabentuk yang terhasil adalah terlebih juga. Dalam kajian ini, ciri-ciri aliran air di dalam sungai yang berdataran banjir dan berbentuk tidak simetri telah dikaji melalui eksperimen yang dijalankan. Satu saluran yang diperbuat daripada kayu dan
mempunyai nisbah lebar, B/b =9 telah dibina. Hasil kajian menunjukkan kaedah yang biasa digunakan seperti Kaedah satu saluran, Kaedah pembahagian saluran menegak dan Kaedah pembahagian saluran melintang yang tidak menimbangkan kesan interaksi momentum adalah kurang sesuai untuk mengira isipadu aliran air. Kaedah ubahsuai
pembahagian saluran didapati adalah kaedah yang paling sesuai untuk mengira isipadu aliran air dalam sungai yang berbentuk mudah. Kesan interaksi momentum di antara dataran banjir dan saluran utama akan menyebabkan kehilangan tenaga keupayaan aliran dan mengurangkan isipadu aliran sebanyak 4% hingga 20%.
iv
ABSTRACT
The study of flow in compound channel sections has been the subject of considerable research in recent years. There is still difficulty in estimating discharge capacity for rivers with compound cross-sectional shape. Method appropriate to simple cross-sectional shapes are less accurate when applied to overbank flows and may lead either to under-estimation of discharge capacity, which is dangerous, or to over- estimation of discharge capacity which leads to over design. In this study, the flow characteristics of non-symmetrical compound channel with smooth flood plain were
studied by experimental investigations. A wooden compound channel with with width ratio, B/b =9 was built purposes. The results show that conventional methods such as Single channel method, Vertical divided channel method and Horizontal divided channel method which do not consider the momentum interaction effect are less accurate to predict the discharge. The Weighted divided channel method was found to be the best method to predict discharge for simple compound channel. The momentum interaction between flood plain flows and main channel flows increases head losses and reduce the discharge capacity of 4% - 20%.
V
CONTENT
JUDUL
DEDICATION
ACKNOWLEDGEMENT ABSTRAK
ABSTRACT CONTENT
LIST OF TABLES LIST OF FIGURES
LIST OF NOTATIONS
CHAPTER 1: INTRODUCTION 1.0 Introduction
1.1 Objective
CHAPTER 2: LITERATURE REVIEW 2.0 Introduction
2.1 Single channel method ( SCM )
2.2 Divided channel method (DCM ) 2.3 Apparent friction method
2.4 Coherence method ( COH ) 2.4.1 Region 1
2.4.2 Region 2 2.4.3 Region 3 2.4.4 Region 4
2.4.5 Choice of region 2.4.6 Tolerences
2.5 Weighted Divided Channel Method (WDCM)
2.6 Compound channels with asymmetric (non-symmetrical) floodplains
i ii iii
iv
V vi viu
ix
xi
1
2
3 4 5 6
11 14 15 16 16 17 17 18 21
V1
CHAPTER 3: METHODOLOGY 3.0 Introduction
3.1 Test rig
3.2 Current meter
3.3 Electronic point gauge
3.4 Experiment arrangement 3.5 Velocity measurement
3.6 Discharge calculation 3.7 Manning's n
CHAPTER 4: ANALYSIS OF RESULT
4.0 Introduction 4.1 Manning's n
4.2 Channel's bed slope, s 4.3 Influence of flow depth 4.4 Momentum interaction
4.5 Influence of width ratio, B/b 4.6 Surfer plot
4.7 Comparison of different method
4.8 Weighted divided channel method ( WDCM )
CHAPTER 5: CONCLUSION
CHAPTER 6: RECOMMENDATION
REFERENCES
25 25 28 29 30 32 33 34
35 35 35 36 38 39 40 41 43
45
47
48
APPENDICES 51
LIST OF TABLES
TABLE PAGE
1 Main geometrical and flow characteristics of experiments analyzed. 10 2 Summary of experiments undertaken on the SERC-FCF: Phase A. All
dimensions are in metres: see Fig 2a for nomenclature. For the asymmetric channel, B= half tatal width at flood plain elevation.
The numbers of results refer to above-bank stage-discharge measurements. 22 3 Error percentage with different method for B/b=9.41
4 Error percentage with different method for B/b=6.6.42 5 Error percentage with different method for B/b=4.6.42 6 Trial and error result for epsilon, 4.43
viii
LIST OF FIGURES
FIGURE PAGE
I Compound channel geometry. 2
2 Two typical subdivisions of a compound channel. 6
3 Variation of apparent friction factor with relative depth and width ratio in
the Flood Channel Facility experiments. 9
4 Dependence of the apparent friction factor on the flood plain Reynolds
number and the width ratio ( HRW=HR Wallingford, 1992; KD=Knight &
Demetriou, 1983; see Table 1) 10
5 Sample test results from FCF: plot of DISADF ( ratio of measured
discharge to sum of zonal calculated discharges ); also coherence COH to
same scales; test 02, averages of 3.12
6 Variation of main channel mean velocity with relative depth. 19 7 Variation of floodplain channel mean velocity with relative depth. 19
8 Comparison of observed and predicted main channel velocity for
asymmetric floodplains. 23
9 Comparison of observed and predicted floodplain velocity for asymmetric
floodplains. 23
10 Comparison of observed discharges with values predicted by the WDCM
for asymmetric floodplain. 24
11 Test rig cross-section view. 26
12 Plan view for test rig, pump and water tank arrangements. 26 13 Water flow into the channel from a wooden tank. 27 14 Water flow back to the iron tank through a collecting tank. 27
15 The collecting tank. 27
16 Data been collected by using a miniature current meter. 28 17 The electronic point gauge to measure flow depth. 29
18 Pump. 30
19 Control valve. 30
20 Water flow with the relative width B/b = 4.6.31
ix
21 Water flow with the relative width B/b = 6.6.31 22 Water flow with the relative width B/b = 9.0.32
23 Point for getting velocity. 32
24 Relative depth versus the discharge. 36
25 Changes of velocity in floodplain and main channel. For B/b = 9.37
26 Variation of main channel mean velocity and floodplain mean velocity with
relative depth. 38
27 Variation of main channel discharge and floodplain discharge with relative
depth. 39
28 Stage discharge relationship with different B/b. 39 29 Surfer plot for the channel B/b =9 with stage of 67.6 mm. 40
X
LIST OF NOTATIONS
A
ARF B
b
COH
DISADF
DISDEFBF fa
FP H h
MC n
P Q R Re R2
Area
Aspect ratio factor
Flood plain width to centre of main channel Half width of main channel
Coherence
Adjustment factor, measured discharge / basic calculation Discharge deficit as a proportion of bankfull flow
Apparent friction factor Flood plain
Flow depth
Depth of main channel Main channel
Manning's n
Wetted perimeter Discharge
Hydraulic radius
Reynolds number Correlation
S
Tm V
W
Y
ý
P
Channel's bed slope Average shear stress
Velocity
Semi top width of main channel Stage
Epsilon value Fluid density
xi
CHAPTER 1
INTRODUCTION
1.0 Introduction
Rivers represent one of mankind's most important environmental assets. River flows are often affected by man's activities and therefore require careful management for water supply, waste disposal, flood alleviation and power generation as well as amenity uses. River engineering design must ensure the optimum use of a valuable resource and flood protection.
The study of flow in compound channel sections has been the subject of considerable research in recent years. Two-stage or compound river geometry ( Fig. 1) ensure of reasonable depth at low flows and flood plains provide conveyance for floods. The term `compound' covers channel cross-sections having berms or flood plains that come into action at high flows but normally are dry.
There is however difficulty in estimating discharge capacity in rivers of a compound cross-sectional shape. Method appropriate to simple cross-sectional shapes are less accurate when applied to overbank flows and may lead either to under-estimation of discharge capacity,
I
which is dangerous, or to over-estimation of discharge capacity which leads to over design and wastage of resources.
ýB
._. -. ý
S3 H
LhL/ / 'S'- i
`_ 1
. -ý
2b
S2
Fig. 1: Compound channel geometry.
1.1 Objective
The main objective of this project is to study the flow characteristics in a non- symmetrical compound channel with smooth flood plain.
2
CHAPTER 2
LITERATURE REVIEW
2.0 Introduction
The apparently simple problem of determine the discharge capacity of a compound channel under uniform flow conditions has proved to be difficult, especially in a non- symmetrical channel. Sellin (1964), first identified the modification of the velocity distribution and the resulting changes in the discharge capacity caused by the turbulent interaction between the main channel and the floodplain. Since that time, work has proceed on methods to quantify the momentum exchange process.
There are several traditional hydraulic methods to predict the discharge capacity for a straight compound channel as below:
a) Single channel method ( SCM )
b) Vertical divided channel method (DCM-V )
c) Horizontal divided channel method ( DCM-H )
3
All the methods mentioned above assumed that there is no interaction between floodplain and main channel. The discharge capacity predicted will not accurate when applied to overbank flows and may lead either to under-estimation of discharge capacity, which is dangerous, or to over-estimation of discharge capacity.
Many experimental studies have been carried out addressing various aspects of the problem, ranging from the boundary shear distribution to the structure of turbulence in compound section and various methods as well as empirical formulas have been proposed for discharge calculation as below:
a) Apparent friction factor method b) Coherence method ( COH )
c) Weighted divided channel method (WDCM )
2.1 Single channel method ( SCM )
This method is counting for whole channel without considering the interaction between floodplain and main channel. Area and the wetted perimeter for the compound cross section will be calculated without divided it into small parts.
4
2 .2 Divided channel method ( DCM )
Compound channels have traditionally been analysed by dividing the compound cross- section into relatively large homogeneous sub-area which are easier to analyse. This approach will be termed the divided channel method (DCM).
Compound channel divided into three zones: main channel, left floodplain and right floodplain as shown in Fig. 2(a). Typically, these divisions are made by drawing straight vertical lines at the edges of the main channel which are assumed to be shear-free and as a result , not included in the wetted perimeter. This treatment of a compound channel assumed that there is no interaction between the subdivided area despite the existence of mean velocity discontinuities at the assumed internal boundaries. The ratio of the observed discharge in the main channel, Q&,,, to the calculated discharge, QQ. assuming that there is no interaction
, is plotted against relative depth. When temporary floodwalls are present, a ratio near unity is obtained for the trapezoidal channel. The presence of floodplains next to the edges of the main channel dramatically reduces the main channel discharge.
F v
NH
B
Mý
I---N
b
ýh
Fig 2(a) : Vertical divisions of a compound channel.
5
ý a
Fig 2 (b) : Horizontal divisions of a compound channel.
2 .3 Apparent friction factor method
Myers (1978), questioned the use of shear-free vertical divisions and showed, using boundary shear stress measurement, that an apparent shear force must be present at these boundary to produce a balance between the gravitational and boundary resistance forces. This approach has been expanded by Wormleaton et al. (1982), Knight et aL(1982), Baird and Ervine (1984), and Wormleaton and Merrett (1990), who have developed various empirical relationships expressing the apparent shear stress as a function of various geometry and flow variables.
The apparent shear stress on the imaginary vertical interface between main channel and flood plain may expressed as
T. = 1/2 pcfi, AV2 = 1/8 pfaAV2
(1)
6
Where p is the fluid density
AV is the difference of mean velocity in the two subsections
fa is an apparent friction factor analogous to the familiar Darcy-Weisbach friction factor (fa =4ca)
Assuming uniform flow and considering the balance of forces along the flow direction in
the main channel leads to :
T.. 2y + T. P. = pgS. A.
(2)
Where So is the bottom slope
Am , P. are the area and wetted perimeter ( excluding the interface ) of the main channel, respectively
Tm is the average shear stress on the main channel boundary
If T. can be estimated, Eq. (2) may used to evaluate the main channel discharge for a given flow depth, whereas the flood plain discharge could also be obtained by a similar procedure ( Radojkovic and Djordjevic, 1985; and Wormleaton and Merrett, 1990 ).
The apparent friction factor fa should in principle depend on the geometry of the cross section and on some Reynolds number(s) as well as on the boundary roughness(es). For a smooth typical section, dimensional considerations lead to an expression of the form
f. = fp ( B/b, y/h, b/A, Sm, Sf, Re Rcr) (3)
7
Where Rey, and Ref are the main channel and flood plain Reynolds numbers, respectively
as :
Reo=4RsV® p/µ Ref=4RfV. p/µ
Where R. and Rf are the respective hydraulic radii.
To explore the dependence of fa on the parameters shown in Eq. (3) the experimental results from series 1,2,3,7,8,10 conducted at the facility were analysed (HR. Wallingford, 1992).
The boundaries were smooth in all cases expect for the flood plains in series 7. In addition, and in order to detect in particular the Reynolds number dependence, data of small-scale experiments presented by Knight and Demetriou (1983) were also considered. The analysis was based on the reported value of T. on the vertical interface, calculated from the measured data; the velocity difference between subsection was obtained not from the measured value but from the velocities estimated by the Manning formula for each subsection, assuming n=0.010 for the smooth boundary (in accordance with the single channel value reported by Myers and Brennan, 1990) and n=0.020 for the rough boundary. This approach was preferred to allow the evaluation of f, in a predictive sense, i. e. without the need for measuring the velocities of the subsections.
Fig 3 shows the dependence of fa on the relative depth for various width ratios and section geometries. It is seen that fa remains nearly constant for 0.3 < y/h < 1.0, its value depending clearly on B/b, as noted earlier by Christodoulou (1992). Remarkably, the results of series 2 and 7 essentially coincide despite the different roughness, implying that the effect of roughness can be successfully incorporated in the value of the mean velocities estimated by
8
Series +1
2
OA
113
Q2
LÜ
fm
05
X
L
I
h 'ý.
ý
03 oA
du 10
0 7
.
"
fý "
ä.
ý"O- _ý - -- z. ý t; " ? Q'
I.
Z1, b'a- ,v
-n- -º - e^ -- ---a
L. -
ý 02
Bib 6.61
4.2 22 4A 4A
42
04 ae
as
toy/h
Fig 3: Variation of apparent friction factor with relative depth and width ratio in the Flood Channel Facility experiments.
proper selection of Manning's n. It is also seen that for small y/h the apparent friction factor increases appreciably in all cases suggesting a Reynolds number influence for low overbank flows. After several trials it was found that fa can be best correlated to the flood plain Reynolds number Ref, as shown in Fig 4. This figure includes data from both large scale (HR Wallingford,
1992) and small scale (Knight and Demetriou, 1983) experiments. Despite the inevitable experimental scatter, it is clear that f8 tends to a constant value depending on B/b for large Ref and increases for lower towards a limiting line on the (Ref, fa) plane independent of B/b is seen to
9
play the role of the relative roughness K, /d of pipe flow; even the variation of the ultimate value
of fa ( for large Re, ') with B/b is nearly logarithmic.
+ HRW I
4A
0
ßl
70
"i\
"""i " KD 1
" 2
" 3
0011
Fig 4: Dependence of the apparent friction factor on the flood plain Reynolds number and the width ratio ( HRW= HR Wallingford, 1992; KD=Knight & Demetriou, 1983; see Table 1)
Reference B b h Sm S1 b/h B/b Q y/h Re Ref(10 )
(m) (m) (m) (I/s) (104)
Hr Wallingford (1992)
Series 1 10 1.50 0.15 1 0 10 6.67 208-1014 0.060-0.67 6.8-32.6 0.41-22 Series 2 6.3 1.50 0.15 1 1 10 4.2 212-1114 0.043-0.92 11.8- 0.26-42
63.4
Series 3 3.3 1.50 0.15 1 1 10 2.2 225-835 0.053-1.0 22.9- 0.34-36.6 75.6
Series 7 6.3 1.50 0.15 1 1 10 4.2 216-543 0.04-1.01 12.7- 0.12-25 31.5
Series 8 6.0 1.50 0.15 0 1 10 4.0 185-1103 0.053-1.0 9.5-57.4 0.32-43 Series 10 6.6 1.50 0.15 2 1 10 4.4 237-1092 0.053-0.86 12-54.7 0.35-33.8 Knight &
Demetriou (1983)
Series 1 0.608 0.152 0.076 0 0 2 4 4.9-29.4 0.12-1.02 2.5-12.7 0.53-10.7
Series 2 0.456 0.152 0.076 0 0 2 3 5.0-23.4 0.15-0.96 3.1-12.3 0.68-9.0 Series 3 0.304 0.152 0.076 0 0 2 2 5.2-17.1 0.12-0.97 4.3-11.2 0.49-5.7
Table I: Main geometrical and flow characteristics of experiments analyzed.
10
