What is a Outfall

An outfall is the location where wastewater, stormwater or treated effluent is discharged from a pipe, drain, channel or other conduit into a receiving environment. This may include rivers, lakes, coastal waters, soakaways or man-made water bodies. Outfalls mark the final stage of a drainage or sewerage system, where collected flows are released into the wider environment following conveyance or treatment.

Outfalls are found in both natural and engineered systems and are essential for managing water movement, preventing flooding, and completing the water cycle. Their design, location and management are critical to ensuring safe, compliant and environmentally sound discharges.

Types of outfalls

Outfalls vary depending on the type of system they serve and the nature of the discharged water. Common types include:

  1. Surface water outfall
    Discharges rainwater or surface runoff from roads, roofs and paved areas. These are usually connected to separate surface water systems and often release water directly into rivers or ditches with little or no treatment.

  2. Foul water outfall
    Releases treated wastewater from sewage treatment works into rivers, estuaries or coastal waters. These outfalls must meet strict quality standards under environmental permits.

  3. Combined sewer outfall (CSO)
    Allows overflow from a combined sewer system to be discharged during heavy rainfall. Used to prevent sewer surcharges and property flooding, though they release untreated water during overflow events.

  4. Industrial outfall
    Used by industrial facilities to discharge process water, cooling water or effluent. These require close monitoring and regulation due to potential contamination.

  5. Emergency or storm overflow outfall
    Used in critical situations where system capacity is exceeded. These are designed to minimise damage but can be a source of pollution if activated frequently.

Purpose and function

Outfalls serve several important functions in water and drainage systems:

  • Completing the transport of water from its source to a discharge point

  • Allowing treated or collected flows to return to the environment

  • Relieving pressure on sewer or drainage networks during peak flows

  • Providing controlled pathways for overflow and emergency discharge

  • Supporting natural water cycles in sustainable urban drainage systems

While outfalls are essential to system performance, their environmental impact must be carefully managed.

Design considerations

Proper design of outfalls is crucial to ensure they function safely and comply with regulatory standards. Key design factors include:

  • Location
    Outfalls must be positioned to minimise environmental harm. For example, discharges into rivers should occur downstream of water abstraction points and away from recreational areas.

  • Flow rate and volume
    The expected volume of discharge must be considered to prevent local flooding or erosion at the outfall site.

  • Outlet structure
    May include concrete headwalls, flap valves, trash screens, stilling basins or energy dissipation features to protect the receiving environment.

  • Access
    Outfalls must be accessible for inspection, cleaning and maintenance.

  • Backflow prevention
    In tidal or flood-prone areas, non-return valves or tide gates are used to prevent water from flowing back into the system.

  • Erosion control
    Measures such as rock armouring or vegetation planting may be used to stabilise banks and prevent scouring.

Regulation and environmental control

In the UK, all outfalls that discharge to controlled waters are subject to regulation by the Environment Agency or equivalent bodies in devolved regions. Operators must obtain a permit under the Environmental Permitting Regulations and adhere to specific discharge conditions.

Requirements may include:

  • Limits on flow rate and discharge volume

  • Water quality standards for pH, temperature, nutrients, or pollutants

  • Monitoring and reporting of discharge frequency and composition

  • Installation of monitoring equipment such as event duration monitors or flow meters

Unauthorised discharges from outfalls are considered pollution incidents and may lead to prosecution, fines or enforcement notices.

Environmental impact

Outfalls can have both positive and negative effects on the environment, depending on how they are designed and maintained.

Potential negative impacts include:

  • Pollution from untreated or poorly treated effluent

  • Harm to aquatic life from temperature changes, chemicals or nutrients

  • Erosion and physical damage to riverbanks or seabeds

  • Public health risks where outfalls are near bathing waters

To mitigate these impacts, modern outfall systems are often integrated with treatment facilities or sustainable drainage systems. In some cases, discharge is routed through reed beds, wetlands or filter systems to improve water quality before it reaches the environment.

Maintenance and inspection

Routine maintenance of outfalls is essential to ensure effective performance and environmental compliance. This may include:

  • Removing blockages, debris or sediment

  • Inspecting structural integrity of the outlet headwall or pipe

  • Checking for signs of erosion or scouring

  • Testing water quality and flow levels

  • Ensuring access points are safe and usable

Neglected outfalls can lead to pollution, flooding or system failure and are a key focus in long term drainage asset management plans.

Outfalls in sustainable drainage

In sustainable urban drainage systems (SuDS), outfalls play a more natural and integrated role. Instead of high-volume point discharges, SuDS may release water gradually into soils, watercourses or green areas through features like:

  • Filter drains

  • Swales

  • Retention ponds

  • Infiltration trenches

These systems aim to replicate natural hydrology and improve water quality before it reaches a formal outfall location.

Conclusion

An outfall is the essential endpoint of any drainage or sewer system, where water is finally released into the environment. Whether carrying treated effluent, surface runoff or overflow, an outfall must be properly designed, regulated and maintained to ensure safety, environmental protection and compliance with legal requirements.

As drainage systems evolve to address climate change, urbanisation and ecological pressures, outfall management will remain a critical element of sustainable and responsible water infrastructure.