Hydraulic Design of Inverted Siphon

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

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.

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