What is a Invert

The invert refers to the lowest internal point of a closed conduit, such as a pipe, culvert, sewer, or channel. In drainage and sewerage systems, it is typically defined as the bottom inner surface of the pipe’s cross-section — the level at which liquid first flows when entering the conduit.

Understanding and accurately referencing the invert is essential in the design, installation, and maintenance of gravity-based drainage systems, where elevation differences are used to drive flow. The term is also commonly used in setting invert levels, which represent the vertical height of the pipe’s internal base relative to a known datum, such as Ordnance Datum (OD) in the UK or sea level.

Purpose and Importance

The invert level serves as a fundamental reference point for:

  • Designing pipe gradients to ensure adequate flow velocity and self-cleansing capability.
  • Establishing hydraulic profiles within a drainage network.
  • Determining pipe and manhole depths for excavation and installation.
  • Coordinating connections between new and existing systems.
  • Setting out structures such as inspection chambers, flow controls, outfalls, and treatment facilities.

Without consistent and accurate invert data, it is difficult to ensure that flow paths are continuous and that water will move through the system by gravity as intended.

How Inverts Are Measured

The invert level is measured as the vertical distance from a fixed reference point — such as finished ground level or a survey benchmark — to the bottom inner surface of the pipe. It is usually recorded in millimetres or metres and may be denoted as:

  • Invert level (IL) – e.g., IL = 1.85 m AOD (Above Ordnance Datum)
  • Invert depth – the distance from cover level to the pipe base

In practical applications, invert measurements are taken using laser levels, total stations, or digital measuring rods during site surveying and drainage layout.

Use in System Design

In gravity drainage systems, maintaining proper invert levels is crucial to ensuring that flow moves downstream without backflow, surcharging, or stagnation. Key design considerations include:

  • Pipe slope (gradient): The change in invert level over a given length determines flow velocity.
  • Manhole step-downs: Each manhole or inspection chamber requires careful attention to inlet and outlet invert levels to maintain continuity.
  • Backfall avoidance: Incorrect inverts can result in backfall, where flow runs in the wrong direction or stagnates.
  • Hydraulic calculations: Invert levels are used in modelling to determine capacity, surcharge points, and flood risk.

Invert vs. Soffit

It is important to distinguish the invert from the soffit, which is the highest internal point of a conduit’s cross-section (the inside top of the pipe). The difference between the invert and soffit gives the internal height or diameter of the pipe.

This distinction is particularly important when assessing clearance, pipe capacity, or headroom in culverts or large-diameter sewers.

Real-World Applications

In the field, invert levels are used extensively by:

  • Civil engineers to design and coordinate pipework systems
  • Surveyors to set out construction works and verify compliance
  • Contractors during trench excavation, bedding, and pipe laying
  • Drainage inspectors to assess pipe condition and identify flow issues
  • Hydraulic modellers to define boundary conditions for simulations

In maintenance, invert levels are also referenced when evaluating silt accumulation, pipe deformation, or damage below the flow line.

Conclusion

The invert is the lowest point inside a pipe or conduit and serves as a vital reference in the design and operation of gravity drainage systems. By establishing the invert level, engineers and contractors ensure that water moves efficiently through sewers, drains, and culverts under the influence of gravity. Accurate measurement and documentation of inverts are essential for proper hydraulic performance, system connectivity, and long-term reliability of underground infrastructure. Whether designing new developments or rehabilitating old systems, understanding and managing invert levels is fundamental to effective drainage engineering.