A canal syphon is a type of inverted syphon used in irrigation systems to transport water across obstacles like rivers, valleys, or other water bodies. It operates by siphoning, conveying water under pressure from a higher elevation to a lower one, without significant loss of head or water pressure. The term "long barrel syphon" or "long syphon" refers to these structures when they span considerable distances, such as crossing large valleys or wide rivers.
The design of a long barrel syphon involves several critical considerations, including hydraulic principles, structural integrity, and environmental/economic factors.
Design Considerations for Long Barrel Syphons:
1. Purpose and Functionality
- Continuity of Supply: The primary purpose is to maintain a continuous water supply to agricultural lands for efficient irrigation and crop production.
- Obstacle Crossing: Canal syphons are designed to convey water under obstacles like rivers, valleys, roads, or railway lines.
- "Depressed Sewers": In sewer systems, inverted siphons are also known as "depressed sewers" or "sag pipes" because the pipeline dips below the general sewer line to pass under obstructions, acting as a low-head pressure pipe.
- Common Materials: Canal syphons can be constructed from various materials, including steel, concrete, plastic, and composite materials.
- Advantages: Each material has its advantages; for instance, steel is high-strength and durable, concrete is robust and long-lasting, plastic is lightweight and flexible, and composite materials balance strength, durability, and cost-effectiveness.
- Case Studies:
- The All-American Canal in California uses a large steel syphon.
- The Nile River Syphon in Egypt uses concrete.
- The Murray River Syphon in Australia employs a composite material syphon.
- In the Warland Reservoir project, three DN600 ductile iron pipes were chosen.
- For Warley Moor Reservoir, SDR17 plastic pipework was used due to slope stability concerns, offering flexibility for differential settlement and lighter weight for installation.
- The West Hallington Reservoir project utilized DN700 pipes fabricated from coated mild steel.
- Precast Pipes: Precast RCC pipes can be economical for smaller discharges, while circular or horseshoe-shaped conduits are suitable for larger discharges. RCC pipes may be used for discharges up to 3 cumecs, with a minimum diameter of 0.9m.
- Head Loss: Minimizing head loss is crucial for efficient performance and maintaining water pressure and flow rates. The head loss equation involves factors like friction factor, syphon length, flow velocity, and diameter. Estimated losses at entry, exit, and within the barrel due to friction, elbows, and joints should be calculated and increased by 10 percent for design.
- Flow Rates and Velocity: The syphon must be designed to accommodate varying flow rates and water levels. The pipe diameter should ensure a self-cleaning velocity to prevent silting.
- A minimum velocity of 3 ft/s and a minimum 6-inch diameter are typical guidelines for inverted siphons.
- For a canal syphon example, a velocity of 2.67 m/sec was calculated, which was within the recommended range of less than 6.00 m/sec.
- The waterway required is calculated by dividing the full supply discharge by the velocity in the barrels, which must not exceed the permissible velocity.
- Multiple Barrels: For varying flow conditions, two or more pipes (barrels) are often laid in parallel in inverted siphons. This allows individual pipes to come into action successively as the flow increases (e.g., one for minimum flow, another for average, and a third for storm water), ensuring adequate velocity to prevent silting even at low flows. The Warland Reservoir project utilized three DN600 pipes to provide redundancy and facilitate individual testing.
- Pressure: The syphon must withstand internal pressure and external loads. Inverted siphons operate under positive upstream head pressure and are positioned below the hydraulic grade line.
- Water Seal: A water seal of 1.5 times the change in velocity head, with a minimum of 150 mm, should be provided over the crown of the barrels at the start to prevent air from entering.
4. Structural Integrity and Durability
- Material Selection and Corrosion Protection: Materials must withstand environmental conditions and internal pressure, and measures to prevent corrosion are essential for longevity. Corrosion-resistant pipe and manholes are recommended.
- Loadings: Barrels are designed to resist various loads, including self-weight, superimposed loads, surcharge, full internal water pressure, soil reaction, uplift pressure, and earth pressure from the sides. The combination of loads resulting in maximum stresses must be carefully considered.
- Uplift Pressure: Syphon barrels must be checked for self-flotation, especially during construction or partial completion when a flood might occur. Extra dead load (e.g., lean cement concrete, masonry) should be added on top of the barrel if necessary to counteract uplift. Uplift calculations are performed for conditions where the drainage channel is full and the carrier channel is dry, and vice-versa. A factor of safety of 1.2 against uplift is recommended.
- Scour and Foundation: The syphon barrel should preferably be kept perpendicular to the drainage channel and founded below the anticipated/design scour level. The top of the syphon should be at least 300 mm to 750 mm below the drainage channel bed level to provide an overburden cushion against abrasive damage.
- Cut-offs: Cut-offs are provided under the entrance and exit ends of the barrel and extend under wing walls to manage exit gradient and scour. The depth of cut-off is typically 1.5 times the normal scour depth below full supply level.
- Longitudinal Analysis: For long barrels, longitudinal analysis is performed, especially when loose soil or various soil types are present at the foundation level. This analysis considers the barrels as beams on elastic foundations.
- Partition Walls: In multi-cell barrels, partition walls generally do not experience significant lateral pressure when all barrels are running. However, they are designed to accommodate differential head if an adjacent barrel is empty. Minimum thickness for RCC walls is 225 mm and for masonry is 450 mm.
- Joints: For RCC barrels, expansion/contraction joints with water stops are provided not more than 20 m apart; for plain concrete slab and masonry walls, joints are not more than 10 m apart. Collars encircling plain joints are recommended for barrels resting on compressible soils to protect water-stops from vertical shear due to excessive settlement.
5. Environmental and Economic Factors
- Environmental impact assessments should be conducted to minimize the syphon's environmental footprint.
- A cost-benefit analysis helps evaluate economic viability and identify cost-saving opportunities.
- Designs should prioritize sustainability and adaptability to changing environmental conditions.
6. Construction and Maintenance
- Construction Techniques: Common methods include open-cut construction, tunnel boring, and pipe jacking. For longer distances and vertical curves, horizontal directional drilling is an option.
- Challenges: Geotechnical risks, logistical challenges (e.g., accessing remote sites, managing schedules), and environmental concerns (e.g., habitat disruption, water pollution) are significant during construction. The construction of siphon works often requires a significant drawdown of reservoir levels.
- Maintenance Strategies: Regular inspections, cleaning, and flushing are crucial for longevity. Pigging techniques (ice pigging, mechanical pigging) are effective for cleaning force mains and siphons that cannot be cleaned by jet trucks, removing debris and preparing pipes for inspection.
- Access for Maintenance: For long inverted siphons in wide rivers, hatch boxes with vent pipes should be constructed at intervals (e.g., every 100 m) to facilitate rodding and prevent air locking.
- Isolation: In big syphons, stop log grooves can be provided at the upstream entrance and downstream ends to isolate one or more barrels for periodic inspections, repairs, and maintenance.
- Dewatering: For long syphons, providing a sump pit at a suitable location is desirable for ease of dewatering.
7. Solved Example of Design of Long Barrel Syphon
This design example focuses on the hydraulic design of a canal syphon, which is a crucial component in irrigation systems used to transport water across obstacles. The example details calculations for various hydraulic parameters and structural elements related to pressure and flow.
Design Input Data:
- Canal Parameters:
- Full supply discharge of canal: 40.00 Cumec
- Bed width of canal: 18.00 m
- Full supply depth of canal: 2.10 m
- Bed level of canal (C.B.L.) at Downstream (D/S): 250.00 m
- Side slope of canal (s): 1.50 :1
- Free board of canal: 0.75 m
- Drain Parameters:
- Maximum observed flood discharge: 100.00 cumec
- Bank level: 254.00 m
- Bed level: 251.80 m
- Highest Flood Level (H.F.L.): 253.25 m
- Slope: 1/600
Hydraulic Design Steps:
-
Section of the Drainage Channel:
- According to Lacey's formula, P = 4.83 X Q^(1/2).
- For Q = 100, P = 4.83 X 10 = 48.3.
- Provide bed width of the drain at the crossing = 44.50 m.
-
Canal Waterway and Barrel Sizing:
- Bed width of canal = 18.00 m.
- Normal Cross-sectional area of the channel (A) = BD + sD^2 = 18.00 * 2.10 + 1.50 * (2.10)^2 = 44.42 Sq.m..
- Velocity in the normal section = Q/A = 40 / 44.42 = 0.90 m/sec.
- Adopt barrel size:
- Width = 3.00 m
- Wall thickness = 0.30 m
- Height = 2.50 m
- Number of barrels = 2 Nos.
- Reduce the canal waterway from 18.00 m to 6.30 m (This implies 2 barrels of 3.00m width with wall thickness between them, as 2 * 3.00 + 0.30 = 6.30, considering the diagram).
- Velocity through the barrels = 40 / (2 * 3.00 * 2.50) = 40 / 15 = 2.67 m/sec.
- This velocity (2.67 m/sec) is less than the recommended range of 6.00 m/sec, which indicates the size is acceptable.
- Froude Number Check:
- F = V / (gd)^(1/2)
- Where V = 2.67 m/sec, g = 9.81 m/sec^2, d = 2.50 m.
- F = 2.67 / (9.81 * 2.50)^(1/2) = 0.538.
- Since F (0.538) is less than 1, the flow will be subcritical in the barrel, which is acceptable.
-
Head Loss and Bed Levels at Different Sections:
- Width of canal in the flumed portion = 6.3 m.
- Provide 2 in 1 splay in contraction and 3 in 1 splay in expansion transition.
- Length of contraction transition = (11.70 / 2) * 2 = 11.7 m.
- Length of expansion transition = (11.70 / 2) * 3 = 17.55 m.
- Assume Length of the barrels in the flumed portion = 70.0 m (> 68.50 m assumed length is O.K.).
- In the transitions, the side slopes of the section shall be warped from 1.50:1 to vertical.
- At Section 4-4 (Normal Channel Section):
- Area of section = 44.42 Sq.m..
- Velocity = Q/A = 40 / 44.42 = 0.901 m/sec.
- Velocity head = V^2 / 2g = (0.901)^2 / (2 * 9.81) = 0.0414 m.
- R.L. of bed = 250.00 m (given).
- R.L. of water surface = 250.00 + 2.1 = 252.10 m.
- R.L. of Total Energy Line (T.E.L.) = 252.10 + 0.0414 = 252.141 m.
- At Section 3-3 (Entry/Exit of Barrel):
- Provide water depth = 3.00 m (slightly higher than barrel depth to keep ends submerged for siphoning).
- Area of section = 3.00 X 6.3 = 18.9 Sq.m..
- Velocity = Q/A = 40 / 18.9 = 2.116 m/sec.
- Velocity head = V^2 / 2g = (2.116)^2 / (2 * 9.81) = 0.228 m.
- Loss of head in expansion from Section 3-3 to Section 4-4 = 0.3 * (V2^2 - V1^2) / 2g = 0.3 * ((2.116)^2 - (0.901)^2) / (2 * 9.81) = 0.056 m.
- Hence, elevation of T.E.L. at Section 3-3 = 252.141 + 0.056 = 252.197 m.
- R.L. of water surface = 252.197 - 0.228 = 251.969 m.
- R.L. of bed = 251.969 - 3.00 = 248.969 m.
- From Section 3-3 to Section 2-2, area and velocity are constant.
- Head Loss Through Barrels:
- Formula: (1 + f1 + f2 * L/R) * V^2 / 2g.
- f1 = 0.080 (for bell mouthed syphon).
- R = A/P = 18.9 / (22.5 + 23.0) = 18.9 / 11 = 0.682 m.
- f2 = a(1 + b/R) where a = 0.00316, b = 0.10000.
- f2 = 0.00316 * (1 + 0.10000 / 0.682) = 0.00362.
- L = 70.0 m.
- Therefore, loss of head in barrels = (1 + 0.080 + 0.00362 * 70.0 / 0.682) * (2.116)^2 / (2 * 9.81) = 0.526 m.
- Water Elevation and Velocity Profile Table:
- Contraction Transition (Section 1-1 to 2-2):
- Distance 0 m: Water surface 252.720 m, Velocity head 0.041 m, Velocity 0.901 m/s, Area 44.392 Sq.m, Bed level 250.62 m, Depth 2.10 m, Bed width 18.00 m.
- ... (intermediate points)
- Distance 11.7 m: Water surface 252.495 m, Velocity head 0.228 m, Velocity 2.116 m/s, Area 18.900 Sq.m, Bed level 249.50 m, Depth 3.00 m, Bed width 6.30 m.
- Expansion Transition (Section 3-3 to 4-4):
- Distance 0 m: Water surface 251.969 m, Velocity head 0.228 m, Velocity 2.116 m/s, Area 18.900 Sq.m, Bed level 248.969 m, Depth 3.00 m, Bed width 6.30 m.
- ... (intermediate points)
- Distance 17.55 m: Water surface 252.100 m, Velocity head 0.041 m, Velocity 0.901 m/s, Area 44.392 Sq.m, Bed level 250.00 m, Depth 2.10 m, Bed width 18.00 m.
- Contraction Transition (Section 1-1 to 2-2):
-
Invert Level:
- Bed level of drain = 251.80 m.
- Provide 0.30 m thick concrete slab and 0.60 m thick earth fill over the slab.
- Invert level of the concrete = 251.80 - (0.6 + 0.3 + 2.5) = 248.40 m.
- Invert level at the entrance = 249.495 m (same as bed level at section 2-2).
- Invert level at the exit = 248.969 m (same as bed level at section 3-3).
- The invert level of the barrel would be kept at 248.40 m in a length of 44.5 m (under base of drain) and then meet the respective bed levels at the entrance and exit, to obtain a slope of about 1 in 15 in the barrel at either side.
- Length of barrel upstream = 16.00 m.
- Length of barrel downstream = 8.00 m.
-
Pucca Floor:
- Length of pucca floor upstream = 1/2 * 11.7 = 5.85 m, say 6.00 m.
- Length of pucca floor downstream = 3/4 * 17.55 = 13.16 m, say 13.00 m.
-
Uplift Pressures on the Barrel Floor and Pucca Floor:
- a) Static Pressure:
- At bottom of barrel floor:
- Deepest invert level of the barrel = 248.40 m.
- Bottom level of barrel floor = 248.10 m (248.40 - 0.30).
- Assuming sub-soil water level upto bed level (R.L. 250.00 m), max static head = 250.00 - 248.10 = 1.90 m.
- At the downstream end of barrel:
- Floor level at d/s end = 248.969 m.
- Assuming floor thickness 2.00 m, bottom level = 246.969 m (248.969 - 2.00).
- Static head = 250.00 - 246.969 = 3.031 m.
- At the upstream end of barrel:
- Floor level at u/s end = 249.495 m.
- Assuming floor thickness 1.50 m, bottom level = 247.995 m (249.495 - 1.50).
- Static head = 250.62 - 247.995 = 2.624 m.
- At bottom of barrel floor:
- b) Seepage Head:
- Total seepage head = H.F.L. in drain - Bed level of canal = 253.25 - 250.00 = 3.25 m.
- At bottom of barrel floor:
- Total creep length = 1.5 (half barrel span) + 8.00 (barrel length) + 13.00 (pucca floor) = 22.50 m.
- Creep length upto point 'a' (center of first barrel) = 1.5 m.
- Residual seepage head at point 'a' = 3.033 m.
- Total uplift = Static uplift + residual seepage head = 1.90 + 3.033 = 4.93 m or 4.93 t/m^2.
- At the downstream end of barrel floor:
- Total creep length upto end of barrel floor (point b) = 1.5 + 8.0 = 9.5 m.
- Residual seepage head at this point = 1.88 m.
- Total uplift = 3.031 + 1.88 = 4.909 m.
- Required floor thickness (sp.gr.=2.22) = 4.909 / 2.22 = 2.211 m, say 2.20 m.
- Provide 2.20 m thick concrete floor d/s and reduce to 0.90 m at end of floor.
- At the upstream end of barrel floor:
- Total creep length upto end of barrel floor = 1.5 + 16.00 = 17.5 m.
- Residual seepage head at this point = 0.83 m.
- Total uplift = 2.624 + 0.83 = 3.454 m.
- Required floor thickness (sp.gr.=2.22) = 3.454 / 2.22 = 1.556 m, say 1.60 m.
- Provide 1.60 m thick concrete floor u/s and reduce to 0.70 m at end of floor.
- a) Static Pressure:
The structural design of the canal syphon would then follow these hydraulic calculations, considering various loads like self-weight, superimposed loads, internal water pressure, soil reaction, uplift pressure, and earth pressure. The barrels, especially for multi-cell designs, are analyzed for transverse and longitudinal stresses under different conditions, such as dry carrier channel with maximum drainage discharge or full carrier channel with dry drainage. Reinforcement details, such as those shown with 16mm and 12mm bars at various spacings, are also specified.
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