How Do You Size a Rising Main?

Rising main sizing is an important part of designing an adoptable pumping station, but it is not simply a case of selecting a pipe diameter that will carry the required flow.

The rising main affects the hydraulic conditions the pumps must overcome, the velocity of wastewater through the system and how long it remains in the pipeline.

A rising main that is too small can increase friction losses and pumping costs. One that is too large may result in low velocities and excessive wastewater retention, potentially creating septicity and maintenance problems.

For developers, consultants and groundworkers, the key is making sure the rising main is designed alongside the pumping equipment and wider drainage infrastructure, rather than treating it as a separate element.

How is rising main size calculated?

Rising main sizing begins with the required pumping flow and the velocity that wastewater needs to achieve.

For a circular pipe flowing full, the internal diameter can be estimated using:

D = √(4Q / πv)

Where:

• D = internal pipe diameter in metres
• Q = pumping flow in cubic metres per second
• v = wastewater velocity in metres per second

For example, if a pumping station needs to deliver 10 litres per second at a target velocity of 1.0 m/s:

D = √(4 × 0.010 / (π × 1.0))

This gives a preliminary internal diameter of approximately 113 mm.

The next step is to assess suitable pipe sizes and check how their actual internal diameters affect the hydraulic performance.

The calculation provides a starting point, not a final rising main specification.

What rising main velocity is required?

For relevant adoptable pumping stations in England, Water UK’s Design and Construction Guidance (DCG) specifies an operating velocity range of 0.75 to 1.8 metres per second.

However, the intended adopting Water Authority or NAV may have additional requirements or accepted variations.

Velocity matters because:

• Low velocities can contribute to sediment deposition and wastewater retention problems.
• High velocities increase friction losses and the head the pumps must overcome.
• Changes in pipe diameter can affect pump duty, operating efficiency and the overall hydraulic design.

The aim is to select a rising main that achieves the required performance without creating unnecessary operating or maintenance problems.

Why rising main sizing affects pump selection

The rising main is directly connected to the pump duty.

Its diameter, length, route, levels and fittings all influence the hydraulic resistance within the system.

A smaller pipe may achieve the required velocity but increase friction losses. A larger pipe may reduce friction losses but create problems with wastewater retention.

This means the designer needs to consider:

• Required pumping discharge rate.
• Static head and friction losses.
• Rising main diameter and internal roughness.
• Valves, bends and other fittings.
• Minimum and maximum operating conditions.
• Wastewater retention and potential septicity.
• Applicable adoption requirements.

If the rising main route or diameter changes after the pumps have been selected, the calculated duty may change as well.

That can become a problem if equipment has already been ordered or the surrounding civil design has progressed.

At Advantage, we consider the pumping equipment and rising main as one hydraulic system. Our manufacturer-independent approach gives us flexibility to select suitable pumps around the actual site requirements rather than designing the development around a predetermined equipment range.

Wastewater retention is another important consideration

Rising main design is not only about achieving the required velocity while the pumps are operating.

The designer also needs to consider how long wastewater remains within the pipeline between pumping cycles.

Long retention periods can increase the risk of:

• Septicity and associated odours.
• Corrosion.
• Sediment accumulation.
• Additional maintenance requirements.

The DCG includes a combined wet well and rising main retention criterion of no more than six hours for relevant adoptable pumping station designs.

Longer rising mains or developments with relatively low flows can require particular attention.

In some cases, additional assessments may also be necessary for hydraulic surge, air accumulation and pipeline pressure conditions.

The right solution balances pipe diameter, pump performance, wastewater retention and long-term operation.

Adoption requirements should be considered early

For an adoptable pumping station, the rising main must satisfy the requirements of the organisation that will eventually own and operate the infrastructure.

Water Authorities can apply approved Local Practices alongside the national DCG, while NAVs may have their own technical and operational requirements.

These can affect:

• Minimum pipe diameter.
• Operating velocity.
• Pipe material and pressure rating.
• Air valves and discharge arrangements.
• Maintenance access.
• Hydraulic surge assessment.
• Technical submission requirements.

Where the intended adopter has not yet been confirmed, it is important to understand which organisations are being considered before the rising main design is fixed.

A change in adoption requirements later in the project could affect both the pipework and the pump selection, potentially resulting in additional design work or changes on site.

Explore Water Authority & NAV Pumping Station Guidance

See how this works in practice

At Allison Homes’ Witheridge Phase 2 development in Devon, Advantage provided the adoptable pumping station design for 56 homes under Independent Water Networks Limited (IWNL) requirements.

One of the key challenges was achieving the required 8,960 litres of emergency storage while maintaining acceptable wastewater retention within the rising main.

Our engineers developed a solution that kept rising main purge intervals below the applicable six-hour threshold, avoiding the need for additional septicity control equipment.

As the drainage layout evolved, the hydraulic calculations and pumping station design were updated to reflect the changing requirements.

The project demonstrates why rising main design needs to be coordinated with pump selection, storage and the wider drainage layout from an early stage.

Read the Witheridge Phase 2 Case Study

Working on a rising main design?

If the rising main route, diameter or discharge requirements are still being established, this is a good point to involve Advantage.

Our engineers can work alongside your consultant and groundworker to review the hydraulic requirements, assess suitable pumping equipment and help coordinate the design around the intended adopting organisation.

Discuss Your Pumping Station Design