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Hydraulic Design of Inverted Siphon

By: Syed Ahmad Amin Shah / On: Feb 04, 2019 / Design of
Inverted Siphon

The economy of the Province of Khyber Pakhtunkhwa is largely agrarian and more than 70% of the adult population is engaged in the agricultural activities for supporting the livelihood of their families.

Shamozai distributary clefts from Machai Branch Canal at RD: 51+000, Machai canal is a part of upper Swat canal system off-taking from Amandara Headworks on River Swat and operated under the control of Dargai subdivision Malakand. Shamozai Disty carries a design discharge of 77 Cusecs, which irrigates valuable lands of the surrounding area. Two minors Likpani minor and Dehri minor further branched out from Shamozai distributary at RD: 42+000 and RD: 48+000 respectively. Due to deferred upholding on account of inadequate Operation & Maintenance, funding constraints, the canal banks, slopes prism, bed & relevant structures have badly deteriorated and in some reaches canal section has been silt up badly, which is causing water shortage for farmers at tail reaches in peak period (June, July, August). At RD: 38+500 right side of Shamozai distributary a scape channel has been provided by the Swabi SCARP to allow excess flow from siphon to flow in to drain or in case of siphon closure but due to corrosive effect of water and canal siltation the siphon is not performing its function for which it is intended and causing much of the flow to escape, which is a major source of water shortage at tail reaches. In order to utilize this excess of water an additional siphon is proposed along the existing syphon to cut the root cause of the water shortage.

Hydrology of Shamozai distributary

This section covers the aspects covering availability of flows at the project for irrigation as well as the estimation of design floods with different return periods. Suspended sediment discharge and bed load has also been estimated, as canals are operated under constant regulations and usually closed during floods or heavy rains. Stream gauges are installed by the Irrigation department at various locations along the Shamozai distributary are:

Discharge Data for Shamozai distributary as per Design

S. No

RD

Discharge

Project

1

0+000

0+000

77

Swabi SCARP

2

0+000

7+500

75

Swabi SCARP

3

7+500

7+800

74

Swabi SCARP

4

7+800

9+500

73

Swabi SCARP

5

9+500

11+000

68

Swabi SCARP

6

11+000

14+000

67

Swabi SCARP

7

14+000

15+500

66

Swabi SCARP

8

15+500

18+200

65

Swabi SCARP

9

18+200

29+000

64

Swabi SCARP

10

29+000

29+500

58

Swabi SCARP

11

29+500

30+500

57

Swabi SCARP

12

30+500

31+233

56

Swabi SCARP

13

31+233

33+400

55

Swabi SCARP

14

33+400

36+400

54

Swabi SCARP

15

36+400

38+500

53

Swabi SCARP

16

38+500

40+500

51

Swabi SCARP

17

40+500

42+500

47

Swabi SCARP

18

42+500

47+900

34

Swabi SCARP

19

47+900

48+000

17

Swabi SCARP

Max. 10 days mean recorded Discharges (m3/s) of Shamozai disty - (Years; 1998-2014)

Year

10 days mean

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

1998

10 days - I

0.00

0.00

68.67

65.38

69.00

77.00

77.00

50.00

66.40

66.00

59.00

50.00

10 days -II

0.00

0.00

69.35

66.03

69.69

77.77

77.30

50.45

67.06

66.64

59.02

50.46

10 days -III

0.00

0.00

69.49

66.16

69.83

77.92

77.92

50.60

67.20

66.79

59.71

50.60

1999

10 days - I

0.00

0.00

68.66

65.37

68.99

76.99

76.53

49.95

66.39

65.97

58.43

49.96

10 days -II

0.00

0.00

68.80

65.50

69.13

77.14

77.14

50.09

66.52

66.12

59.11

50.09

10 days -III

0.00

0.00

69.42

66.09

69.75

77.84

77.37

50.49

67.12

66.70

59.07

50.50

2000

10 days - I

0.00

0.00

69.62

66.29

69.96

78.07

78.07

50.70

67.32

66.92

59.82

50.70

10 days -II

0.00

0.00

69.69

66.35

70.03

78.15

78.15

50.75

67.39

66.98

59.88

50.75

10 days -III

0.00

0.00

68.30

65.03

68.63

76.58

76.58

49.73

66.04

65.64

58.68

49.73

2001

10 days - I

0.00

0.00

67.61

64.38

67.94

75.82

75.82

49.23

65.38

64.99

58.09

49.23

10 days -II

0.00

0.00

69.05

65.74

69.38

77.43

77.43

50.28

66.77

66.37

59.33

50.28

10 days -III

0.00

0.00

68.36

65.09

68.69

76.65

76.65

49.77

66.10

65.70

58.73

49.77

2002

10 days - I

0.00

0.00

67.67

64.43

68.00

75.88

75.88

49.27

65.43

65.04

58.14

49.27

10 days -II

0.00

0.00

69.25

65.93

69.58

77.65

77.65

50.42

66.96

66.56

59.50

50.42

10 days -III

0.00

0.00

68.41

65.14

68.74

76.71

76.71

49.81

66.15

65.75

58.78

49.81

2003

10 days - I

0.00

0.00

68.55

65.27

68.89

76.87

76.87

49.92

66.29

65.89

58.90

49.92

10 days -II

0.00

0.00

68.27

65.01

68.61

76.56

76.56

49.71

66.02

65.62

58.66

49.71

10 days -III

0.00

0.00

69.51

66.19

69.85

77.95

77.95

50.62

67.22

66.81

59.73

50.62

2004

10 days - I

0.00

0.00

69.30

65.98

69.63

77.71

77.71

50.46

67.01

66.61

59.54

50.46

10 days -II

0.00

0.00

68.12

64.86

68.45

76.39

76.39

49.60

65.87

65.48

58.53

49.60

10 days -III

0.00

0.00

66.08

62.92

66.40

74.10

74.10

48.12

63.90

63.51

56.78

48.12

2005

10 days - I

0.00

0.00

64.72

61.62

65.03

72.57

72.57

47.12

62.58

62.20

55.61

47.12

10 days -II

0.00

0.00

65.69

62.54

66.01

73.66

73.66

47.83

63.52

63.14

56.44

47.83

10 days -III

0.00

0.00

66.87

63.67

67.19

74.99

74.99

48.69

64.66

64.27

57.46

48.69

2006

10 days - I

0.00

0.00

65.95

62.79

66.27

73.95

73.95

48.02

63.77

63.39

56.66

48.02

10 days -II

0.00

0.00

65.29

62.16

65.60

73.21

73.21

47.54

63.13

62.75

56.10

47.54

10 days -III

0.00

0.00

67.20

63.98

67.53

75.35

75.35

48.93

64.98

64.59

57.74

48.93

2007

10 days - I

0.00

0.00

64.51

61.42

64.82

72.34

72.34

46.97

62.38

62.01

55.43

46.97

10 days -II

0.00

0.00

67.27

64.05

67.59

75.43

75.43

48.98

65.04

64.65

57.80

48.98

10 days -III

0.00

0.00

66.53

63.34

66.85

74.60

74.60

48.44

64.33

63.94

57.16

48.44

2008

10 days - I

0.00

0.00

68.01

64.75

68.34

76.26

76.26

49.52

65.76

65.36

58.43

49.52

10 days -II

0.00

0.00

67.19

63.98

67.52

75.35

75.35

48.93

64.97

64.58

57.73

48.93

10 days -III

0.00

0.00

66.39

63.22

66.72

74.45

74.45

48.35

64.20

63.82

57.05

48.35

2009

10 days - I

0.00

0.00

65.92

62.76

66.24

73.92

73.92

48.00

63.74

63.36

56.64

48.00

10 days -II

0.00

0.00

61.96

59.00

62.27

69.48

69.48

45.12

59.92

59.56

53.24

45.12

10 days -III

0.00

0.00

61.31

58.37

61.60

68.75

68.75

44.64

59.28

58.92

52.68

44.64

2010

10 days - I

0.00

0.00

65.92

62.76

66.24

73.92

73.92

48.00

63.74

63.36

56.64

48.00

10 days -II

0.00

0.00

61.96

59.00

62.27

69.48

69.48

45.12

59.92

59.56

53.24

45.12

10 days -III

0.00

0.00

61.31

58.37

61.60

68.75

68.75

44.64

59.28

58.92

52.68

44.64

2011

10 days - I

0.00

0.00

64.37

61.29

64.69

72.19

72.19

46.87

62.25

61.87

55.31

46.87

10 days -II

0.00

0.00

60.51

57.61

60.80

67.85

67.85

44.06

58.51

58.16

51.99

44.06

10 days -III

0.00

0.00

59.87

57.00

60.16

67.13

67.13

43.59

57.89

57.54

51.44

43.59

2012

10 days - I

0.00

0.00

63.98

60.92

64.29

71.75

71.75

46.59

61.87

61.50

54.97

46.59

10 days -II

0.00

0.00

60.14

57.26

60.43

67.44

67.44

43.79

58.16

57.81

51.68

43.79

10 days -III

0.00

0.00

59.50

56.65

59.79

66.72

66.72

43.33

57.54

57.19

51.13

43.33

2013

10 days - I

0.00

0.00

65.92

62.76

66.24

73.92

73.92

48.00

63.74

63.36

56.64

48.00

10 days -II

0.00

0.00

61.96

59.00

62.27

69.48

69.48

45.12

59.92

59.56

53.24

45.12

10 days -III

0.00

0.00

61.31

58.37

61.60

68.75

68.75

44.64

59.28

58.92

52.68

44.64

2014

10 days - I

0.00

0.00

65.85

62.70

66.17

73.84

73.84

47.95

63.67

63.29

56.58

47.95

10 days -II

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

10 days -III

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

max. daily 10 days

0.00

0.00

69.69

66.35

70.03

78.15

78.15

50.75

67.39

66.98

59.88

50.75

mean max. daily 10 days

0.00

0.00

63.52

60.48

63.83

71.23

71.20

46.25

61.42

61.05

54.55

46.25

Design Calculations and Methodology

Design Procedure

Steps required for design of a siphon include the following:

  • Determine what inlet and outlet structures are required, and the type and approximate size of pipe.
  • Make a preliminary layout of the siphon profile (siphon and required inlet and outlet structures) using the existing ground line, the canal properties, and the canal stations and elevations at the siphon ends. This layout should provide pipe requirements of cover, slope, bend angles, and provide pipe submergence requirements at transitions, check and pipe inlets, or control and pipe inlets.
  • Compute the siphon head losses in this preliminary layout. If the head losses as computed are in disagreement with the available head, it may be necessary to make some adjustment such as pipe size or even the canal profile. If the computed losses are greater than the difference in upstream and downstream canal water surface, the siphon will probably cause backwater in the canal upstream from the siphon. If backwater exists, the pipe size should be increased or the canal profile revised to provide adequate head. If the computed losses are appreciably less than the difference in upstream and downstream canal water surface it may be possible to decrease the size of pipe, or the canal profile may be revised so the available head is approximately the same as the head losses.
  • On long siphons where the inlet may not be sealed there is the possibility of blow back and unsatisfactory operating conditions. The inlet should be checked for proper performance and adjustments made if necessary.
  • Determine the pipe class. The pipe class can be determined from the amount of external load and internal head shown on the pipe profile.

But due to siltation and corrosive effect the siphon is no more capable of carrying 54 cusecs discharge and as a result it can only pass 38.35 cusecs of discharge and rest 17 cusecs is passed in to drain through escape channel.

Inverted Siphon

Hydraulic Design of Inverted Siphon

The design calculations are as under:

HYDRAULIC DESIGN OF INVERTED SIPHON (USBR)

Inputs      
Discharge

=

17.00

cusecs
Diameter

=

2.25

ft
Manning's

=

0.011

 
Length

=

330

ft
Entry Loss

=

0.4

 
Exit Loss

=

0.7

 
Bend Loss

=

0.04

 
No of Bends

=

2

 
U/s FSL

=

99.52

ft
D/s FSL

=

98.16

ft
Available Head

=

1.36

ft
Gravity Acceleration

=

32.2

ft2/s
       
       
Outputs      
Area

=

3.9761

ft2
Wetted Parameter

=

7.0686

ft
Hyd Radius

=

0.5625

 
Velocity

=

4.2756

ft/s
Velocity Hd

=

0.2839

ft
Entry Loss

=

0.1135

ft
Exit Loss

=

0.1987

ft
Bend Loss

=

0.0227

ft
Friction Loss

=

0.7118

ft
Total H.Loss

=

1.0468

 
       
Design Levels      
U/s FSL

=

99.52

ft
D/s FSL

=

98.47

ft
       

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