What is a Drains/drainage

The term drains or drainage refers to a comprehensive system of infrastructure designed to collect, transport, and safely discharge wastewater and stormwater from buildings, roads, and public spaces. This system is essential for maintaining hygiene, preventing flooding, and preserving the structural integrity of built environments.

A fully developed drainage network typically encompasses two main categories: foul water drainage and surface water drainage. Both are fundamental to urban planning, construction, and public health management, though they serve distinctly different purposes.

Foul Water Drainage

Foul water drainage deals with wastewater that originates from household, commercial, or industrial sources. This includes water from:

  • Toilets (black water)
  • Showers, baths, and sinks (grey water)
  • Dishwashers and washing machines
  • Commercial food preparation or industrial processes

Foul drainage systems are designed to transport this contaminated water away from properties through sealed pipework to either a public sewer, a septic tank, or an on-site treatment plant. The key objective is to prevent exposure to harmful bacteria, pathogens, and pollutants.

Foul drains are typically constructed using uPVC, vitrified clay, or concrete pipes, and are connected via inspection chambers or manholes for maintenance. The pipes are watertight to prevent leakage and laid with a gradient that ensures a self-cleansing flow.

Surface Water Drainage

Surface water drainage, on the other hand, is responsible for removing rainwater, snowmelt, and other forms of natural runoff from impervious surfaces such as:

  • Roofs
  • Roads and driveways
  • Pavements and car parks

Unlike foul water, surface water is not considered contaminated under normal conditions, although it may carry debris, hydrocarbons, or sediment. The drainage system channels this water away from developed areas to soakaways, attenuation tanks, watercourses, or dedicated storm sewers.

In newer developments, surface water management often includes Sustainable Drainage Systems (SuDS), which are designed to mimic natural water processes. These systems aim to slow runoff, encourage infiltration, and reduce the risk of flash flooding.

Combined vs. Separate Drainage Systems

Historically, many older towns and cities in the UK were built with combined drainage systems, where both foul and surface water were carried through the same set of pipes to wastewater treatment facilities. While economical at the time, these systems are now recognised as problematic, especially during periods of heavy rainfall, when treatment plants become overwhelmed and overflow events occur.

Modern developments typically use separate drainage systems:

  • Foul drains carry only wastewater to treatment facilities.
  • Surface water drains direct clean runoff to rivers, soakaways, or retention basins.

This separation improves environmental outcomes and reduces treatment costs.

Key Components of a Drainage System

An effective drainage network will include a wide range of components tailored to the specific needs of a site or locality. These include:

  1. Gullies and grates – For collecting water from roads and paved surfaces.
  2. Inspection chambers and manholes – For access, maintenance, and inspection.
  3. Backflow prevention devices – Especially important in low-lying areas prone to flooding.
  4. Pipes and culverts – For underground conveyance of water.
  5. Interceptors and filters – To trap silt, oil, and debris.
  6. Outfalls and soakaways – For safe discharge into the environment.

The design of these components must comply with local regulations such as Building Regulations Part H in the UK, and guidance from bodies such as the Environment Agency or Water UK.

Planning and Design Considerations

Effective drainage design requires a multidisciplinary approach that considers:

  • Topography – Water must flow by gravity wherever possible.
  • Soil permeability – Determines the suitability of infiltration systems.
  • Rainfall intensity and volume – Essential for sizing pipes and storage units.
  • Urban density and surface type – Hard surfaces generate more runoff.
  • Legal constraints – Connections to public sewers require permission, and discharge to watercourses may require permits.

Hydraulic modelling software is often used in the design phase to simulate system performance under different weather conditions and ensure long-term reliability.

Maintenance and Common Issues

Poorly maintained drainage systems can lead to:

  • Localised flooding
  • Backflow and sewer surcharges
  • Water ingress into buildings
  • Pollution incidents

Common causes include:

  • Blockages from grease, wet wipes, or silt
  • Tree root intrusion
  • Collapsed or misaligned pipes
  • Overloaded systems during heavy rainfall

Routine inspection using CCTV surveys and cleaning via jetting or rodding are standard practices to ensure long-term functionality.

Conclusion

Drains and drainage systems are fundamental to modern infrastructure, managing both foul and surface water to ensure public health and environmental protection. While the two systems serve distinct purposes, their integration into coherent, well-maintained networks is essential for the sustainable development of any built environment. As urban areas grow and climate change leads to more intense rainfall, the role of intelligently designed and resilient drainage systems will become ever more critical in shaping safe and liveable cities.