What is a Surface water/rainwater

The term surface water/rainwater refers to precipitation that flows from roofs, driveways, patios, gutters, and other hardstanding surfaces, typically in domestic and urban environments. It is generally considered uncontaminated and can often be discharged directly into the environment or surface water drainage systems without treatment.

For professionals working in drainage design, plumbing, construction, and environmental management, understanding how rainwater behaves, how it should be handled, and what regulations govern its discharge is essential to ensure both compliance and environmental protection.

What is Surface Water / Rainwater?

Surface water/rainwater is defined as water resulting from precipitation that collects on exposed surfaces such as rooftops, pavements, roads, and garden areas. In this context, the water is assumed to be free from contamination by foul sewage, hazardous substances, or industrial pollutants.

Unlike foul or grey water — which includes waste from toilets, sinks, showers, and appliances — surface water/rainwater is typically clean enough to be discharged untreated, provided it does not mix with contaminants. In the UK, such water may be directed to:

  • Surface water drainage systems

  • Sustainable Drainage Systems (SuDS)

  • Watercourses such as rivers and streams

  • Soakaways or infiltration systems

  • Rainwater harvesting systems for reuse

Common Sources of Surface Water / Rainwater

Surface water/rainwater can originate from a variety of surfaces and features within residential, commercial, and public settings. Typical sources include:

  • Roof areas – Water collected via gutters and downpipes

  • Driveways and paths – Runoff from sloped hardstanding

  • Paved courtyards and patios – Particularly when impermeable materials are used

  • Car parks and footways – In urban environments, these can shed large volumes of rainwater

  • Decking and balconies – Often drained to gutter systems or downpipes

In well-managed systems, these sources are kept separate from foul drainage to prevent overloading treatment works and to maintain clean runoff suitable for direct discharge or infiltration.

Conditions for Safe Discharge Without Treatment

Not all surface water is suitable for untreated discharge. To qualify as safely dischargeable, the rainwater must meet the following conditions:

  • Uncontaminated – It must not be mixed with foul water, grey water, chemicals, or oil

  • Free from industrial waste – No pollutants from vehicle washing, commercial kitchens, or chemical handling should be present

  • Physically unobstructed – Leaves, debris, and sediments should be filtered where possible

  • Discharged to an appropriate location – Such as a soakaway, SuDS feature, or watercourse with legal consent

When these criteria are met, surface water/rainwater can usually be discharged directly into the environment or public drainage systems without requiring treatment.

Drainage Options for Surface Water / Rainwater

There are several accepted methods of managing clean rainwater, depending on the site conditions and regulatory requirements:

1. Soakaways

A soakaway is a subsurface structure that allows rainwater to percolate into the surrounding soil. It is a common solution for individual houses and small developments. Soakaways require permeable ground and must be sized correctly to accommodate local rainfall.

2. Watercourses

Where available, water from roofs and paved areas may be discharged directly into streams, rivers, or ditches. Discharge to a watercourse often requires consent from the Environment Agency or relevant authority.

3. Sustainable Drainage Systems (SuDS)

SuDS aim to replicate natural water movement and treatment processes. Rainwater is managed through features such as:

  • Swales

  • Permeable paving

  • Retention ponds

  • Rain gardens

  • Green roofs

SuDS not only allow for safe discharge but also improve water quality, reduce peak flows, and support biodiversity.

4. Rainwater Harvesting

Rainwater from roofs can be collected, stored, and reused for non-potable applications such as:

  • Garden irrigation

  • Toilet flushing

  • Car washing

Harvesting systems can reduce demand on mains water and lower utility bills, particularly in commercial or public buildings.

Key Benefits of Direct Surface Water Discharge

When safely managed, the direct discharge of clean rainwater brings a range of benefits:

  • Reduces pressure on foul sewers and treatment works

  • Decreases flood risk by managing water at source

  • Minimises energy use and costs associated with unnecessary treatment

  • Supports regulatory compliance with drainage and flood risk policies

  • Contributes to sustainable building certifications such as BREEAM or Code for Sustainable Homes

Regulatory and Planning Considerations

In the United Kingdom, various regulations govern the design and discharge of surface water systems. Relevant documents include:

  • Building Regulations Part H: Requires provision for the disposal of rainwater from roofs and paved areas

  • Sewers for Adoption / Design and Construction Guidance (DCG): Standards for adoptable drainage systems

  • Environment Agency permits and guidance: Necessary for discharge to watercourses or groundwater

  • Lead Local Flood Authority (LLFA) requirements: Planning authorities often require a drainage strategy or flood risk assessment for new developments

The drainage hierarchy, promoted in planning policy and guidance, stipulates that rainwater should be dealt with in the following order of preference:

  1. Discharged to the ground (e.g. soakaways)

  2. Discharged to a surface water body

  3. Discharged to a surface water sewer

  4. Discharged to a combined sewer (only as a last resort)

Planners and engineers must demonstrate that higher-priority options are unfeasible before resorting to less sustainable alternatives.

Misconnections and Environmental Risk

Although rainwater is generally clean, it must not be discharged through or mixed with foul drainage systems. Misconnections — where rainwater pipes are plumbed into foul drains — can result in:

  • Overloaded sewers and increased flood risk

  • Reduced performance of wastewater treatment plants

  • Unauthorised discharges from combined sewer overflows (CSOs)

  • Potential legal penalties for non-compliance

Proper installation and inspection are necessary to prevent such issues, particularly in refurbishment or mixed-use developments.

Surface Water Charging and Financial Implications

Some water companies in the UK apply surface water drainage charges as part of business and domestic sewerage bills. These charges fund the maintenance of public surface water sewers.

Customers may be entitled to a reduction or exemption if:

  • Their property drains surface water to a soakaway or watercourse

  • No part of the property contributes rainwater to the public sewer

Verifying this often involves submitting drainage layout evidence or photographs to the relevant water company.

Future Considerations: Climate and Urban Growth

As climate change drives more frequent and intense rainfall events, and urban development increases impermeable surface area, the volume of surface water and the importance of managing it effectively continue to grow. Key future challenges include:

  • Preventing surface water flooding in dense urban areas

  • Improving resilience through the wider adoption of SuDS

  • Encouraging rainwater reuse to reduce potable water demand

  • Adapting existing infrastructure to accommodate greater runoff volumes

Drainage professionals must remain up to date with best practice guidance and technological advancements to meet these evolving demands.

Conclusion

Surface water/rainwater — when clean and properly managed — is a relatively benign part of the water cycle. Its effective discharge through sustainable and compliant drainage systems is a core responsibility for professionals in plumbing, drainage engineering, and environmental design.

Whether directed into a soakaway, reused on-site, or conveyed to a public sewer, the management of rainwater must prioritise safety, sustainability, and regulatory alignment. With increasing climate pressures and urban growth, the importance of surface water management will only continue to rise across the built environment.