What is a Self-cleansing (velocity)

Self-cleansing velocity refers to the minimum flow velocity in a drainage or sewer pipe that is required to prevent the settlement of suspended solids. When the flow within a pipeline falls below this threshold, particles such as grit, silt, or organic matter begin to deposit, potentially leading to blockages, reduced hydraulic capacity, and increased maintenance requirements. Maintaining adequate self-cleansing velocity ensures that solids remain in suspension and are transported effectively through the system.

This principle is essential in the design and operation of both foul and combined sewer systems, as well as in stormwater networks where debris may enter the flow. A properly engineered system must maintain these minimum velocities during periods of low flow, not just at peak demand.

Importance in Drainage and Sewer Design

Ensuring self-cleansing velocity is achieved and maintained is one of the most important aspects of hydraulic design for below-ground pipework. Failure to do so results in:

  • Sediment build-up in pipelines

  • Foul odours due to stagnant wastewater

  • Increased risk of blockages and overflows

  • More frequent need for jetting and mechanical cleaning

  • Deterioration of pipe material due to corrosive conditions

By ensuring solids are continuously flushed through the system, self-cleansing flow helps preserve the performance, hygiene, and lifespan of the drainage infrastructure.

Typical Self-Cleansing Velocities

The minimum self-cleansing velocity varies depending on the characteristics of the solids and the pipe material. However, accepted guidelines suggest:

  • Foul drainage systems: A minimum of 0.75 m/s is commonly used as a baseline for self-cleansing.

  • Stormwater drains: For pipes with the potential for heavy sediment load (e.g. sand, grit), velocities closer to 1.0 m/s or more may be necessary.

  • Combined sewers: Require balancing both sanitary and runoff conditions, often using a minimum velocity of 0.9 m/s under dry weather flow.

These values are supported by numerous studies and practical experience in the UK and internationally.

It is also important to consider peak, average, and minimum daily flows when designing pipe gradients and diameters to meet these targets under real-world conditions.

Factors Affecting Self-Cleansing Velocity

Several variables influence whether self-cleansing flow is achieved within a drainage or sewer system:

  • Pipe gradient (slope): Steeper gradients increase flow velocity, supporting better cleansing action.

  • Pipe diameter: Larger pipes may have lower velocities under low-flow conditions, even with steeper slopes.

  • Flow rate: Systems must be sized to ensure sufficient velocity during periods of minimal use.

  • Surface roughness: Smooth materials like uPVC generate less resistance, supporting higher velocity at the same flow rate.

  • Solid characteristics: Heavier or coarser solids require greater velocities to remain in suspension.

A balance must be achieved between hydraulic performance and construction feasibility, as excessively steep gradients may be costly or impractical.

Application in Foul Water Systems

In foul water systems, where the primary flow consists of wastewater from toilets, sinks, showers, and appliances, solid content includes organic matter, paper, and small debris. To prevent these materials from settling in the pipe, flow must be maintained above the self-cleansing threshold.

For example:

  • WC discharge: Provides intermittent high-flow bursts, aiding in scouring.

  • Greywater (from sinks and baths): Has lower solids and flow rates, making pipe gradient more critical.

  • Low-use fixtures: May require minimum diameter or steeper slopes to maintain cleansing velocity.

The Building Regulations Part H and British Standards recommend pipe gradients and diameters to achieve these flow conditions. In general:

  • A 100 mm pipe should have a minimum fall of 1:40 (2.5%) for optimal self-cleansing.

  • For lower gradients (e.g. 1:80), the risk of solids build-up increases unless flow volume is high.

Application in Surface Water and Combined Systems

Stormwater systems and combined sewers face different challenges. These systems receive runoff from roofs, roads, and open areas, bringing with it silt, sand, leaves, and litter. These solids are denser and more abrasive than foul sewage, requiring higher flow velocities to remain mobile.

In such systems, designers aim for:

  • Pipe self-cleansing velocities above 1.0 m/s during rainfall events

  • Use of flushing systems or flow regulators in flat or low-lying networks

  • Incorporation of sediment sumps or catch pits at manholes and gullies to trap materials before they enter the main network

Hydraulic modelling is often used to verify whether self-cleansing conditions will be met during design storms.

Preventive Measures in Low-Flow Scenarios

There are many instances where achieving self-cleansing velocity through gravity flow alone is not possible — for example, in long, flat sections or buildings with low discharge volumes. In these cases, supplementary measures may be required:

  1. In-system solutions:

    • Flow control chambers that store and then flush water in batches

    • Backfall traps or branch designs to encourage surge cleaning

    • Narrower pipes to maintain velocity in low-flow sections

  2. Maintenance strategies:

    • Scheduled jetting or rodding

    • Use of inspection chambers for monitoring sediment build-up

    • CCTV inspections to identify low-velocity or problem zones

Good system design aims to minimise the need for mechanical maintenance by achieving reliable self-cleansing through proper sizing and gradient.

Hydraulic Calculations and Modelling

To determine whether a system meets the required self-cleansing velocity, engineers use hydraulic design tools and formulae. The Manning equation is the most common:

V = (1/n) × R^(2/3) × S^(1/2)

Where:

  • V is the velocity of flow (m/s)

  • n is the Manning roughness coefficient

  • R is the hydraulic radius (m)

  • S is the slope or gradient of the pipe

By inputting flow depth, pipe material, and slope, designers can predict whether the flow will exceed the self-cleansing threshold. If not, the pipe diameter or gradient must be adjusted.

Software packages also allow simulation of flow conditions across entire networks, assessing risk areas and identifying where maintenance access is most important.

UK Standards and Guidelines

Design recommendations for self-cleansing velocity are embedded in several UK regulatory documents:

  • BS EN 752 (Drain and Sewer Systems Outside Buildings): Specifies minimum design velocities for different flow types.

  • Building Regulations Part H: Gives practical advice on pipe falls and sizes for gravity drainage.

  • CIRIA SuDS manuals and WSUD guidelines: Address runoff velocity and sediment transport in sustainable drainage systems.

Adherence to these standards ensures the long-term reliability and serviceability of drainage infrastructure.

Implications for Sustainability and Maintenance

Achieving self-cleansing velocity supports sustainable operation by:

  • Reducing the need for energy-intensive maintenance

  • Minimising use of water for manual flushing

  • Preventing overflow events caused by blockages

  • Extending the life of pipes and pumping equipment

From a whole-life cost perspective, initial investment in proper gradient design and flow control pays off in reduced operating expenses and environmental impact.

Conclusion

Self-cleansing velocity is a critical design parameter that ensures solids within drainage and sewer systems are effectively transported, rather than settling and causing operational problems. By maintaining minimum flow velocities through appropriate pipe sizing, gradient selection, and flow control, engineers can deliver systems that are robust, low-maintenance, and sustainable. Whether in domestic foul drainage, urban stormwater networks, or combined systems, achieving and maintaining self-cleansing flow is key to reliable, long-term performance.