Code of practice for ancillary structures in the sewerage system, Part III: Inverted syphon

IS 4111 Part 3 - Scope Overview

• Purpose: Defines requirements for hydraulic calculations and design of inverted siphons used in water conveyance.
• Key Concept: An inverted siphon behaves as a pipe running full under pressure.
• Velocity Head: The velocity head in the siphon equals the difference in water levels just upstream and downstream.

Important Notes:

• Final test or calculation results must be rounded off per IS 2-1960.
• Number of significant digits in results should match the specified values in the standard.

Relevant Formula (Hydraulic Calculations):

[ H_v = \frac{V^2}{2g} ] Where:

• (H_v) = velocity head (m)
• (V) = velocity of flow (m/s)
• (g) = acceleration due to gravity (9.81 m/s²)

Summary Table (Conceptual):

flowchart LR
    A[Upstream Water Level] -->|Head Difference| B[Inverted Siphon (Full Pipe)]
    B --> C[Downstream Water Level]
    B -->|Velocity Head = (V²/2g)| D[Hydraulic Calculations]

This scope sets the foundation for hydraulic design and testing of inverted siphons in water systems under IS 4111 Part 3.

IS 4111 Part 3: Definitions - Key Points

The standard defines terms critical for hydraulic design of inverted syphons and related structures. Key definitions include:

• Inverted Syphon: A pipe running full under pressure, conveying flow beneath an obstruction.
• Velocity Producing Head: The head difference causing flow velocity, equal to the difference in water levels upstream and downstream of the syphon.

Important Notes:

• The syphon operates under full pipe flow conditions.
• Velocity head is calculated based on water level differences, critical for hydraulic calculations.

Typical Formula for Velocity Head (h_v):

[ h_v = \frac{v^2}{2g} ] Where:

• (v) = velocity of flow (m/s)
• (g) = acceleration due to gravity (9.81 m/s²)

Rounding Rules:

• Numerical values in the standard follow specific rounding off rules to maintain precision.

If you need detailed tables or hydraulic formulas (e.g., flow capacity, pressure losses), please specify!

IS 4111 Part 3: Design Considerations Summary

Key Design Parameters (Clause 3.1)

• Lowest flow rate (dry weather) during design period — for minimum pipe sizing.
• Highest flow rate (dry weather) during design period — for average peak flow.
• Maximum storm flow rate — critical for syphon and combined system design.

Design Considerations

• Pipe Size & Arrangement: Must accommodate flow variations ensuring no surcharge or overflow.
• Inlet/Outlet Chambers: Detailed design ensures smooth flow transition and prevents clogging.

Typical Formulas (General Sewer Design)

• Flow rate (Q): ( Q = A \times V )
  • ( A ) = cross-sectional area (m²)
  • ( V ) = velocity (m/s), typically 0.6–3 m/s for sewers.
• Velocity (V): Manning’s equation for gravity flow: [ V = \frac{1}{n} R^{2/3} S^{1/2} ] where,
  • ( n ) = Manning’s roughness coefficient (0.013–0.015 for concrete),
  • ( R ) = hydraulic radius (m),
  • ( S ) = slope of the sewer line.

Design Tips

• Use peak dry weather flow for pipe diameter selection.
• Design for storm flow capacity in combined systems.
• Provide access chambers at junctions and changes in slope.

flowchart LR
    A[Lowest Flow Rate] --> B[Pipe Size]
    C[Highest Flow Rate] --> B
    D[Storm Flow Rate] --> E[Syphon Design]
    B --> F[Inlet/Outlet Chambers]
    E --> F

For detailed tables and chamber dimensions, refer to IS 4111 Part 3 annexures or related parts of IS 4111.

IS 4111 Part 3 - Variation of Rates of Flow: Key Points

Fundamental Data (Clause 3.1)

• Lowest dry weather flow (Q_min): Minimum flow during the day in dry weather.
• Highest dry weather flow (Q_max): Maximum flow during the day in dry weather.
• Maximum storm flow (Q_storm): Peak flow during storm conditions for combined or partially separate systems.

Hydraulic Calculation (Clause 3.2)

• An inverted syphon behaves as a full pipe under pressure.

• Velocity head for syphon flow is based on the difference in water levels upstream and downstream.

  [ H_v = \frac{V^2}{2g} = \Delta h ]

  Where:

  • (V) = velocity in syphon pipe (m/s)
  • (g) = acceleration due to gravity (9.81 m/s²)
  • (\Delta h) = difference in water levels (m)

Design Suggestions (Clause 3.4.4)

• For large stormwater flow fluctuations in combined sewers, use more than three pipes to improve flow control.

Materials (Clause 5.1)

• Use cast iron or reinforced pressure pipes (IS:458-1971).
• Cast iron preferred for stream crossings, installed per IS:3114-1985.

Summary Table: Flow Rates for Design

Diagram: Inverted Syphon Flow Concept

flowchart LR
    Upstream[Upstream Water Level]
    SyphonPipe[Inverted Syphon Pipe (Full Flow)]
    Downstream[Downstream Water Level]
    Upstream -->|Head difference (Δh)| SyphonPipe --> Downstream

Note: Use IS 4111 Part 3 along with IS 458 and IS 3114 for detailed design and material specifications.

IS 4111 Part 3 (1985) — Hydraulic Calculations for Inverted Syphons

Key Points from Clause 3.2:

• An inverted syphon behaves as a full pipe under pressure.
• The velocity head (hv) causing flow through the syphon equals the difference in water levels upstream of the syphon.

Hydraulic Formula:

[ h_v = \frac{v^2}{2g} = \Delta H ] Where:

• (v) = velocity of flow in the syphon (m/s)
• (g) = acceleration due to gravity (9.81 m/s²)
• (\Delta H) = difference in water levels upstream (m)

Design Considerations (Clause 3.1):

• Lowest flow rate during dry weather (design period)
• Highest flow rate during dry weather (design period)
• Maximum storm flow rate for combined or partially separate systems

Material Specifications (Clause 5.1):

• Pipes for inverted syphons:
  • Cast iron pipes (preferred for stream crossings)
  • Reinforced pressure pipes (per IS 458-1971)
• Installation per IS 3114-1985 (for cast iron pipes)

Summary Table: Hydraulic Parameters for Inverted Syphon

flowchart LR
    A[Upstream Water Level] -->|ΔH| B[Inverted Syphon Pipe (Full Flow)]
    B --> C[Downstream Water Level]
    B -->|Velocity v| D[Velocity Head hv = v²/2g]

For detailed design, refer to IS 4111 Part 3 along with IS 458 (

IS 4111 Part 3: Pipe Arrangement and Flow Management

Key Points from Clauses 3.4, 3.4.1 & 3.4.5

• Pipe Sizing:

  • Use a single pipe if sufficient head exists to maintain good velocity.
  • For low head & varying flow, use multiple pipes in parallel to avoid low velocity issues.

• Pipe Arrangement:

  • Fore-bay design must ensure successive activation of pipes.
  • Achieved by distribution weirs controlling flow to each pipe.

Design Considerations

Formula for Flow in Pipe (Assuming full flow):

[ Q = A \times V = \frac{\pi d^2}{4} \times V ]

Where:

• (Q) = flow rate (m³/s)
• (A) = cross-sectional area (m²)
• (d) = pipe diameter (m)
• (V) = velocity (m/s)

Fore-bay & Distribution Weir Concept

flowchart LR
    FB[Fore-bay] --> DW1[Distribution Weir 1] --> Pipe1[Pipe 1]
    FB --> DW2[Distribution Weir 2] --> Pipe2[Pipe 2]
    FB --> DW3[Distribution Weir 3] --> Pipe3[Pipe 3]
    style DW1 fill:#f9f,stroke:#333,stroke-width:1px
    style DW2 fill:#f9f,stroke:#333,stroke-width:1px
    style DW3 fill:#f9f,stroke:#333,stroke-width:1px

• Distribution weirs control flow levels, ensuring pipes activate one after another as flow increases.

Summary: Use single pipe for good head; multiple pipes with distribution weirs for low head and variable flow to maintain velocity and avoid stagnation.

IS 4111 Part 3 - Inlet and Outlet Chamber Design Summary

Inlet Chamber (Clause 3.5.1)

• Channels: Number equals pipes in the syphon system:
  • Channel 'a': Minimum dry weather flow (main sewer continuation).
  • Channel 'b': Difference between minimum & maximum dry weather flow.
  • Channel 'c': Storm water flow.
• Weirs: Set at elevations to control overflow between channels.
• Velocity in pipes: Design for 1.2 m/s full flow.
• Head loss at entrance: Minimum of velocity head ( \frac{v^2}{2g} ); more allowance recommended.
• Pipe sizing & slope: [ \text{Total fall} = \text{Length} \times \text{Gradient of pipe 'a'} + \text{Head losses} ]

Outlet Chamber (Clause 3.5.2)

• Pipes merge into a single outlet channel.
• Invert levels: Larger pipes have higher outlet invert than pipe 'a' to prevent eddies and solids accumulation.

Key Formula:

[ \text{Entrance head loss} \geq \frac{v^2}{2g} ]

• (v = 1.2, m/s) (design velocity)
• (g = 9.81, m/s^2)

Design Considerations:

• Provide adequate space for maintenance access (Clause 3.5).
• Ensure gradients and pipe sizes maintain velocity and minimize head loss.
• Outlet invert levels arranged to prevent sedimentation.

flowchart LR
    A[Main Sewer] -->|Min Dry Weather Flow| Channel_a
    Channel_a --> Pipe_a
    Channel_a -- Weir --> Channel_b
    Channel_b --> Pipe_b
    Channel_b -- Weir --> Channel_c
    Channel_c --> Pipe_c
    Pipe_a & Pipe_b & Pipe_c --> Outlet Chamber --> Single Outlet Channel

This diagram shows flow division and merging in inlet/outlet chambers per IS 4111 Part 3.

IS 4111 Part 3 (1985) - Construction Key Points

Materials (Clause 5.1)

• Inverted Syphon Pipes: Use cast iron pipes or reinforced pressure pipes as per IS:458-1971.
• For stream crossings, cast iron pipes are preferred.
• Installation method follows IS:3114-1985 for laying cast iron pipes.

Construction Specifications

• Where space is limited and ramps can't be constructed, vertical pipes in access shafts may replace ramps (Clause 3.5.4). However, this is discouraged if avoidable.
• Ancillary structures must follow the Code of Practice for Sewerage System Ancillaries (Clause 4).

Units and Definitions (SI Units)

• Force: Newton (N) = 1 kg·m/s²
• Pressure/Stress: Pascal (Pa) = 1 N/m²
• Energy: Joule (J) = 1 N·m
• Power: Watt (W) = 1 J/s

Summary Table: Pipe Material & Construction

flowchart LR
    A[Inverted Syphon] -->|Material| B{Cast Iron Pipe}
    A -->|Material| C{Reinforced Pressure Pipe}
    B --> D[Installation as per IS:3114-1985]
    C --> D
    E[Space Restricted?] -->|Yes| F[Vertical Pipes in Access Shafts]
    E -->|No| G[Ramps Construction]

For detailed construction practices, refer to IS 4111 Part 3 and related IS codes mentioned.

IS 4111 Part 3: Hatch-boxes and Access Provisions

Key Points from Clauses:

• Hatch-box location (4.2.1):

  • Positioned near bends prone to silt deposits for easy rodding access.
  • Must be of adequate size to allow pipe cleaning.
  • If omitted, manholes must be watertight and withstand internal bursting pressure.

• Definition (2.1):

  • Hatch Box is a chamber at the lowest level of an inverted syphon system for pipe cleaning.

• Access alternatives (3.5.4):

  • If ramps aren't feasible, vertical pipes in access shafts can be used but are not recommended.

Specifications & Design Considerations:

Typical Hatch-box Size (approximate):

• Width & Length: 600 mm to 900 mm
• Height: Sufficient for personnel entry, typically 1.2 m minimum

Diagram: Hatch-box location near bends

flowchart TD
    A[Pipe Bend] --> B[Hatch-box]
    B --> C[Manhole]
    C --> D[Access Ramp / Vertical Pipe]

Summary: Hatch-boxes ensure maintenance access at silt-prone bends, sized for rodding, with manholes designed for watertightness if hatch-boxes are absent. Access ramps are preferred over vertical pipes for syphon chambers.

IS 4111 Part 3: Bypass Arrangements Key Points

1. Bypass Requirement (Clause 4.3)

• A bypass channel must be provided from the inlet chamber to a nearby stream.
• Purpose: To prevent breakdowns by allowing flow diversion during maintenance or blockage.

2. Pipe Arrangement (Clauses 3.4 & 3.4.5)

• In pipe syphon systems, pipes should operate successively, not simultaneously.
• Achieved by designing the fore-bay with distribution weirs that control flow to each pipe.
• This ensures gradual engagement of pipes, avoiding hydraulic shocks.

3. Design Considerations

• Size and arrangement depend on:
  • Flow capacity
  • Successive activation of pipes
  • Maintenance access via bypass

Typical Design Approach for Bypass:

Conceptual Flow Arrangement:

flowchart LR
    A[Inlet Chamber] --> B[Fore-bay with Distribution Weirs]
    B --> C1[Pipe 1]
    B --> C2[Pipe 2]
    B --> C3[Pipe 3]
    A --> D[Bypass Channel to Stream]

Summary: Provide a bypass channel for emergency flow, design fore-bay with distribution weirs for sequential pipe operation, ensuring smooth flow and system reliability.

Protection of Syphon in Riverbeds (IS 4111 Part 3)

Key Specifications:

• Weight & Flotation:
  Syphons on/under riverbeds must have enough weight to prevent flotation when empty.

  • Achieved by surrounding pipes with Reinforced Cement Concrete (RCC) of suitable thickness.

• Protection from Undermining & Movement:

  • Protect syphon from scour and shifting river beds.
  • Use positive flexible joints where bottom movements are possible.
  • Mark syphon position in navigable channels as per river authority.

Design Considerations (Clause 3.5.1):

• Flow Channels in Inlet Chamber:

  • Channel 'a': Minimum dry weather flow
  • Channel 'b': Excess dry weather flow
  • Channel 'c': Storm water flow

• Velocity:
  Design pipes for velocity v = 1.2 m/s when full.

• Head Loss at Entrance:
  Minimum loss = velocity head = ( \frac{v^2}{2g} )
  (Recommend allowance for more than this.)

• Slope & Fall:
  Total fall = Length × slope of pipe 'a' + sum of head losses.

Typical Protection Detail (Fig. 1 Summary):

• RCC encasement around pipes
• Granolithic concrete for exposed surfaces
• Penstock chase and crane hooks for maintenance

Formula Summary:

flowchart LR
    A[Inlet Chamber] -->|Min Dry Weather Flow| B[Pipe a]
    A -->|Excess Dry Weather Flow| C[Pipe b]
    A -->|Storm Flow| D[Pipe c]
    B --> E[Syphon Pipes in Riverbed]
    E --> F[Protected by RCC Encasement]
    F --> G[Anchored to

Typical Inverted Syphon Layout (IS 4111 Part 3)

Key Definitions (Clause 2.2)

• Inverted Syphon: Sewer section below adjacent stretch, running full under pressure (crown below hydraulic grade line).

Layout (Clause 4.1, Fig.1)

• Constructed with inlet chamber having multiple channels corresponding to pipes:
  • Channel a: Minimum dry weather flow → Pipe a
  • Channel b: Excess dry weather flow → Pipe b
  • Channel c: Storm water → Pipe c
• Weirs separate channels, set at specific water elevations to divert flow accordingly.

Hydraulic Design (Clause 3.5.1 & 3.2)

• Velocity in pipes designed at 1.2 m/s (full flow) to minimize head loss.
• Entrance loss head ≥ velocity head:
  [ h_{loss} \geq \frac{v^2}{2g} ]
• Total fall in syphon: [ \text{Total fall} = L \times S_a + \sum h_{loss} ] where:
  • (L) = length of syphon
  • (S_a) = slope of pipe 'a' (minimum dry weather flow pipe)
  • (\sum h_{loss}) = sum of entrance and other head losses

Design Steps Summary

• Determine size/slope of pipe 'a' based on minimum flow and velocity 1.2 m/s.
• Calculate total fall considering pipe length and slope plus losses.
• Design pipes 'b' and 'c' for additional flows accordingly.

Formula Summary

flowchart LR
    A[Inlet Chamber] -->|Channel a| B[Pipe a: Min Dry Weather Flow]
    A -->|Channel b| C[Pipe b: Excess Dry Weather Flow]
    A -->|Channel c| D[Pipe