Code of practice for ancillary structures in the sewerage system, Part III: Inverted syphon
1985 Edition

Overview of IS 4111 Part 3 Scope

• Objective: Establishes the criteria for hydraulic computations and design of inverted syphons utilized in sewerage conveyance.
• Core Principle: An inverted syphon functions as a fully pressurized pipe.
• Velocity Head Concept: The velocity head equals the water level difference upstream and downstream of the syphon.

Noteworthy Points:

• Final computational outcomes should adhere to rounding guidelines specified in IS 2-1960.
• The number of significant digits must conform to standard requirements.

Essential Hydraulic Equation:

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

• (H_v) = velocity head (meters)
• (V) = flow velocity (meters/second)
• (g) = gravitational acceleration (9.81 m/s²)

Conceptual Summary Table:

flowchart LR
    Upstream[Upstream Water Level] -->|Head Difference| Syphon[Inverted Syphon (Full Pipe)]
    Syphon --> Downstream[Downstream Water Level]
    Syphon -->|Velocity Head = (V²/2g)| Calculation[Hydraulic Computation]

This section lays the groundwork for hydraulic design and validation of inverted syphons.

Key Definitions in IS 4111 Part 3

• Inverted Syphon: A pipeline segment flowing full under pressure, channeling sewage beneath an obstruction.
• Velocity Producing Head: The driving head difference that generates flow velocity, equal to the difference in water surface elevation upstream and downstream.

Critical Notes:

• Syphons operate under full-pipe pressurized conditions.
• Velocity head calculations depend on water level differences and are vital for hydraulic design.

Velocity Head Formula:

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

• (v) = flow velocity (m/s)
• (g) = gravitational acceleration (9.81 m/s²)

Rounding Protocol:

• Numerical results must be rounded according to stipulated standards to ensure accuracy.

Summary of Design Guidelines in IS 4111 Part 3

Essential Design Flow Rates (Clause 3.1)

• Minimum dry weather flow: Basis for minimum pipe sizing.
• Maximum dry weather flow: Represents typical peak flow conditions.
• Maximum storm flow: Critical for design of syphons and combined sewer systems.

Design Considerations

• Pipe Dimensioning and Arrangement: Ensure accommodation of flow variability without surcharging or overflow.
• Inlet and Outlet Chamber Design: Facilitate smooth transitions and prevent clogging.

Fundamental Hydraulic Formulas

• Flow rate (Q): ( Q = A \times V )
  • (A): Cross-sectional pipe area (m²)
  • (V): Flow velocity (m/s), typically ranging 0.6 to 3 m/s for sewers
• Velocity (V) via Manning's equation: [ V = \frac{1}{n} R^{2/3} S^{1/2} ] where
  • (n): Manning’s roughness factor (0.013–0.015 for concrete)
  • (R): Hydraulic radius (m)
  • (S): Sewer slope

Design Recommendations

• Employ peak dry weather flows for pipe sizing.
• Account for storm flow in combined sewer designs.
• Provide adequate access chambers at junctions and slope transitions.

flowchart LR
    MinFlow[Min Dry Weather Flow] --> PipeSize[Pipe Sizing]
    MaxFlow[Max Dry Weather Flow] --> PipeSize
    StormFlow[Storm Flow] --> SyphonDesign[Syphon Design]
    PipeSize --> Chambers[Inlet/Outlet Chambers]
    SyphonDesign --> Chambers

Refer to annexures for detailed tables and chamber dimensions.

Flow Rate Variation Considerations (IS 4111 Part 3)

Data Parameters (Clause 3.1)

• Minimum dry weather flow (Q_min): Lowest daily flow during dry periods.
• Maximum dry weather flow (Q_max): Highest daily flow in dry conditions.
• Maximum storm flow (Q_storm): Peak flow under storm conditions, relevant for combined or semi-separated systems.

Hydraulic Behavior (Clause 3.2)

• Inverted syphons behave hydraulically as fully pressurized pipes.
• Velocity head equals the difference in upstream and downstream water levels: [ H_v = \frac{V^2}{2g} = \Delta h ] where
  • (V): velocity in the syphon (m/s)
  • (g): gravitational acceleration (9.81 m/s²)
  • (\Delta h): water level difference (m)

Design Recommendations (Clause 3.4.4)

• For systems with large storm flow fluctuations, employing more than three parallel pipes is encouraged to improve flow management.

Material Requirements (Clause 5.1)

• Use cast iron or reinforced pressure pipes conforming to IS:458-1971.
• Cast iron pipes preferred for stream crossings, installed according to IS:3114-1985.

Summary Table of Flow Rates

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

Refer to IS 4111 Part 3 along with IS 458 and IS 3114 for comprehensive design and material details.

Hydraulic Calculation Principles for Inverted Syphons (IS 4111 Part 3, 1985)

Core Points (Clause 3.2)

• The inverted syphon operates as a fully pressurized pipe segment.
• Velocity head ((h_v)) driving flow equals the upstream water level difference: [ h_v = \frac{v^2}{2g} = \Delta H ] where
  • (v) = flow velocity (m/s)
  • (g) = gravitational acceleration (9.81 m/s²)
  • (\Delta H) = water level difference upstream (m)

Design Flow Rates (Clause 3.1)

• Minimum dry weather flow
• Maximum dry weather flow
• Maximum storm flow for combined or partly separate sewers

Material Standards (Clause 5.1)

• Pipes to be either cast iron (preferred for stream crossings) or reinforced pressure pipes per IS 458-1971.
• Installation of cast iron pipes to comply with IS 3114-1985.

Parameter Summary

flowchart LR
    Upstream[Upstream Water Level] -->|ΔH| Syphon[Inverted Syphon Full Flow]
    Syphon --> Downstream[Downstream Water Level]
    Syphon -->|Velocity v| VelocityHead[Velocity Head (v²/2g)]

Consult IS 4111 Part 3 and associated codes for detailed hydraulic design.

Arrangement and Flow Management of Pipes (IS 4111 Part 3)

Highlights (Clauses 3.4, 3.4.1, 3.4.5)

• Pipe Dimensioning:

  • Single pipe advisable when adequate head allows maintaining sufficient velocity.
  • For low head and fluctuating flows, multiple parallel pipes help preserve desired velocities.

• Pipe Layout:

  • Fore-bay design should facilitate sequential operation of pipes.
  • Use distribution weirs to regulate flow division among pipes.

Recommendations Table

Flow Equation (Full Pipe Flow):

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

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

flowchart LR
    Forebay[Fore-bay] --> Weir1[Distribution Weir 1] --> Pipe1[Pipe 1]
    Forebay --> Weir2[Distribution Weir 2] --> Pipe2[Pipe 2]
    Forebay --> Weir3[Distribution Weir 3] --> Pipe3[Pipe 3]

Distribution weirs control flow levels to ensure pipes activate in succession.

Summary: Single pipes are suitable for sufficient head; multiple pipes with controlled flow distribution are preferred for low head and variable flow conditions.

Guidelines for Inlet and Outlet Chambers (IS 4111 Part 3)

Inlet Chamber (Clause 3.5.1)

• Number of channels matches the count of pipes:
  • Channel 'a': Minimum dry weather flow (main sewer continuation)
  • Channel 'b': Difference between minimum and maximum dry weather flow
  • Channel 'c': Stormwater flow
• Weirs are installed between channels at specified elevations to regulate overflow.
• Design velocity within pipes typically set at 1.2 m/s for full flow.
• Entrance head loss should be at least the velocity head (v²/2g), with allowance for additional losses.
• Total elevation drop calculated as pipe length times slope plus head losses.

Outlet Chamber (Clause 3.5.2)

• Pipes converge into a single outlet channel.
• Larger pipes have higher outlet invert levels than pipe 'a' to avoid eddies and sediment deposition.

Important Formula:

[ \text{Entrance Head Loss} \geq \frac{v^2}{2g} ] Where (v = 1.2, m/s)

Design Considerations

• Adequate space must be available for maintenance and inspection.
• Proper slopes and sizes are essential to maintain velocity and reduce losses.
• Outlet invert arrangement prevents solids settling.

flowchart LR
    MainSewer[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 --> OutletChamber --> SingleOutlet

This configuration ensures smooth flow division and merging preventing sediment accumulation.

Construction Guidelines (IS 4111 Part 3, 1985)

Materials (Clause 5.1)

• Pipes for inverted syphons should be either cast iron or reinforced pressure pipes complying with IS:458-1971.
• Cast iron pipes are the preferred choice for stream crossings.
• Installation must conform to IS:3114-1985 for laying cast iron pipes.

Construction Details

• In constrained spaces where ramps cannot be constructed, vertical pipes in access shafts may be used, though this is discouraged whenever possible (Clause 3.5.4).
• Ancillary structures must be built following the Code of Practice for Sewerage System Ancillaries (Clause 4).

Units and Measures

• 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 of Materials and Construction

flowchart LR
    Syphon[Inverted Syphon] -->|Material| CastIron[Cast Iron Pipe]
    Syphon -->|Material| Reinforced[Reinforced Pressure Pipe]
    CastIron --> Installation[Install per IS:3114-1985]
    Reinforced --> Installation
    SpaceLimit[Space Constraints?] -->|Yes| VerticalPipes[Vertical Pipes in Shafts]
    SpaceLimit -->|No| Ramps[Ramp Construction]

Detailed construction instructions are provided in the standard and referenced codes.

Access Provisions: Hatch-boxes (IS 4111 Part 3)

Key Requirements

• Placement (Clause 4.2.1): Hatch-boxes should be installed near bends susceptible to silt build-up to facilitate cleaning.
• Size: Must be large enough to permit rodding and maintenance access.
• If Hatch-boxes Are Omitted: Manholes must be watertight and capable of withstanding internal pressure.

Definitions

• A hatch-box is a chamber at the lowest level in an inverted syphon system designed for pipe cleaning access.

Alternative Access (Clause 3.5.4)

• If ramps are impractical, vertical pipes in access shafts may be used but are not recommended.

Specifications Table

Typical Hatch-box Dimensions

• Width and length: Approximately 600 mm to 900 mm
• Height: Minimum about 1.2 m for personnel entry

flowchart TD
    Bend[Pipe Bend] --> HatchBox[Hatch-box]
    HatchBox --> Manhole[Manhole]
    Manhole --> Access[Access Ramp / Vertical Pipe]

These provisions ensure effective maintenance access and system reliability.

Bypass System Requirements (IS 4111 Part 3)

Bypass Channel (Clause 4.3)

• A bypass channel from the inlet chamber to an adjacent watercourse must be provided.
• This facilitates flow diversion during maintenance or blockage, preventing system failure.

Pipe System Operation (Clauses 3.4 & 3.4.5)

• Pipes in the syphon system are designed to operate sequentially rather than simultaneously.
• Sequential operation is achieved by fore-bay design incorporating distribution weirs that control flow into each pipe.

Design Parameters

• Bypass channel size should accommodate maximum flow to prevent overflow.
• Distribution weirs must be designed to trigger pipes in order as flow increases.

Bypass Design Summary

flowchart LR
    Inlet[Inlet Chamber] --> Forebay[Fore-bay with Distribution Weirs]
    Forebay --> Pipe1[Pipe 1]
    Forebay --> Pipe2[Pipe 2]
    Forebay --> Pipe3[Pipe 3]
    Inlet --> Bypass[Bypass Channel to Stream]

This approach ensures operational continuity and effective flow management.

Measures for Protecting Syphons in Riverbed Environments (IS 4111 Part 3)

Stability and Weight Considerations

• Syphons installed on or below riverbeds must be weighted sufficiently to prevent buoyant uplift when drained.
• This is accomplished by encasing pipes in reinforced cement concrete (RCC) of appropriate thickness.

Protection from Scour and Movement

• RCC encasement safeguards against undermining caused by water currents.
• Positive flexible joints should be used to accommodate potential riverbed shifts.
• In navigable channels, syphon locations must be marked following river authority regulations.

Design Details (Clause 3.5.1)

• Flow channels in the inlet chamber are arranged for minimum dry weather, excess dry weather, and stormwater flows.
• Pipes are designed for a velocity of 1.2 m/s when flowing full.
• Entrance head loss must be at least the velocity head (v²/2g), with additional allowance recommended.
• Total syphon fall equals the pipe length multiplied by slope plus the sum of head losses.

Typical Protection Features

• RCC encasement around pipes
• Granolithic concrete surfacing
• Penstock chase and crane hooks for maintenance access

Summary Table

flowchart LR
    Inlet[Inlet Chamber] -->|Min Dry Weather Flow| PipeA[Pipe a]
    Inlet -->|Excess Dry Weather Flow| PipeB[Pipe b]
    Inlet -->|Storm Flow| PipeC[Pipe c]
    PipeA --> Syphon[Syphon Pipes in Riverbed]
    Syphon --> RCC[RCC Encasement for Protection]
    RCC --> Anchoring[Anchored to Prevent Movement]

These provisions ensure structural integrity and operational safety in riverbed installations.

Standard Layout of Inverted Syphons (IS 4111 Part 3)

Definitions (Clause 2.2)

• An inverted syphon is a sewer section running full under pressure below adjacent sewer lines, with the pipe crown beneath the hydraulic grade line.

Layout Description (Clause 4.1, Fig.1)

• Inlet chamber comprises multiple channels corresponding to pipe flow divisions:
  • Channel 'a': Minimum dry weather flow directed to pipe 'a'
  • Channel 'b': Excess dry weather flow directed to pipe 'b'
  • Channel 'c': Stormwater flow directed to pipe 'c'
• Channels are separated by weirs set at specified elevations to divert flows appropriately.

Hydraulic Design (Clauses 3.5.1 & 3.2)

• Pipes are designed for a velocity of 1.2 m/s under full flow to reduce head loss.
• Entrance head loss should be at least the velocity head (v²/2g).
• Total fall in the syphon is computed as: [ \text{Total fall} = L \times S_a + \sum h_{loss} ] where
  • (L) = length of syphon
  • (S_a) = slope of pipe 'a'
  • (\sum h_{loss}) = cumulative head losses

Design Procedure

• Determine the size and slope of pipe 'a' based on minimum flow and design velocity.
• Calculate total fall including losses.
• Design pipes 'b' and 'c' to accommodate additional flows.

Formula Reference

flowchart LR
    Inlet[Inlet Chamber] -->|Channel a| PipeA[Pipe a: Min Dry Weather Flow]
    Inlet -->|Channel b| PipeB[Pipe b: Excess Dry Weather Flow]
    Inlet -->|Channel c| PipeC[Pipe c: Stormwater Flow]

This layout facilitates effective flow division and pressure maintenance within the syphon system.