What is a Source control

Source control refers to the range of strategies and techniques used to manage rainfall, surface water and pollutants at or near the point where they originate, before they can enter public sewers, watercourses or groundwater systems. In the context of drainage and water management, source control is a key principle of Sustainable Drainage Systems (SuDS) and is designed to reduce runoff volumes, slow flow rates, improve water quality, and promote infiltration and evaporation.

The core idea behind source control is simple but powerful: manage water where it falls. By capturing or slowing runoff close to its origin, the risk of flooding, erosion and water pollution is significantly reduced. This approach not only protects receiving water bodies but also lessens the load on downstream drainage infrastructure.

Objectives of Source Control

The main goals of source control in drainage are:

  • Reducing runoff volumes and flow rates: Preventing sudden surges of water entering the system during storms

  • Improving water quality: Filtering or treating water before it leaves the site

  • Recharging groundwater: Promoting infiltration to support local aquifers

  • Preventing pollution: Keeping oil, sediment, heavy metals and nutrients out of drains and rivers

  • Supporting biodiversity and amenity: Integrating green features that provide ecological and aesthetic benefits

These objectives support both environmental sustainability and urban resilience, especially in the face of climate change and increased urbanisation.

How Source Control Works

Source control measures are typically implemented at the scale of individual buildings, plots, or developments. They intervene at the earliest point in the water cycle, dealing with rainfall as it falls on roofs, roads, driveways, car parks or landscaped areas.

There are three main mechanisms through which source control achieves its objectives:

  1. Interception and storage: Rainwater is collected and held temporarily in green roofs, water butts, swales or permeable paving, reducing immediate runoff.

  2. Infiltration: Water is allowed to percolate into the ground via soakaways, infiltration trenches or rain gardens.

  3. Treatment: Pollutants are removed through filtration, sedimentation or biological processes before water reaches the main drainage network.

These systems are often decentralised and integrated into the landscape or built form, making them cost-effective and multifunctional.

Examples of Source Control Techniques

Source control can take many forms depending on the site layout, land use, soil conditions and local climate. Common examples include:

1. Permeable Surfaces

Permeable paving, porous asphalt and reinforced grass grids allow rainwater to soak through rather than run off. They help reduce the volume and rate of water entering sewers and can filter pollutants in the process.

2. Green Roofs

Vegetated roof systems absorb rainfall, delay runoff and improve water quality. They are especially useful in dense urban environments where ground-level space is limited.

3. Rainwater Harvesting

Water butts and storage tanks collect rainwater from roofs for later reuse in irrigation, flushing toilets or cleaning. This reduces demand on mains water and prevents excess water from entering the drainage system.

4. Soakaways and Infiltration Trenches

These below-ground features allow water to slowly seep into the soil, replenishing groundwater and reducing surface runoff.

5. Filter Strips and Swales

Vegetated strips and shallow channels capture and treat runoff from roads and hard surfaces, removing sediment and pollutants through natural processes.

6. Oil Interceptors and Catchpits

Installed in car parks and industrial areas to trap oils, silt and heavy metals before they enter the public drainage network.

Each of these techniques can be used individually or in combination to address specific drainage and pollution control challenges.

Source Control and Sustainable Drainage Systems (SuDS)

Source control is a cornerstone of the SuDS philosophy. SuDS is a holistic approach to surface water management that seeks to mimic natural hydrological processes in urban areas. The hierarchy of SuDS encourages source control first, followed by site control and regional control.

In practice, this means that every new development should seek to manage water on-site as much as possible before discharging into public sewers or watercourses. Source control not only helps meet planning requirements but also enhances the long-term sustainability of urban drainage systems.

In the UK, SuDS and source control are encouraged by planning policy and Building Regulations, particularly for developments in flood-prone areas or those that will significantly increase impermeable surfaces.

Benefits of Source Control

Implementing source control measures offers a wide range of environmental, economic and social benefits:

  • Flood prevention: By reducing runoff at the source, the risk of surface water flooding is minimised

  • Water quality improvement: Pollutants are filtered out before reaching sensitive water bodies

  • Infrastructure savings: Less demand is placed on sewers and treatment works, reducing capital and maintenance costs

  • Groundwater recharge: Helps to maintain healthy aquifers and support base flows in rivers

  • Urban cooling and greening: Green roofs, swales and trees reduce urban heat and improve visual amenity

  • Compliance with regulations: Supports planning approvals and environmental permits

These benefits make source control an attractive solution for developers, municipalities and property owners alike.

Challenges and Limitations

While source control is widely promoted, there are some challenges and limitations to consider:

  • Space constraints: Some measures require sufficient land or building surface area

  • Soil conditions: Poorly draining soils (e.g. clay) may limit infiltration effectiveness

  • Maintenance requirements: Systems must be maintained to remain effective, e.g. clearing debris from inlets or inspecting rainwater tanks

  • Public awareness: Some systems rely on homeowner or occupant cooperation, which may vary

  • Initial cost: Although often cost-effective over the long term, upfront investment can be a barrier

To address these challenges, careful planning, proper design, public education and robust maintenance regimes are essential.

Planning and Regulatory Context

In the UK, source control is embedded in various policies and guidance documents, including:

  • National Planning Policy Framework (NPPF): Encourages SuDS in all major developments

  • Building Regulations Part H: Promotes the use of infiltration and attenuation techniques before discharging to a sewer

  • CIRIA SuDS Manual (C753): The leading guidance on design, construction and maintenance of SuDS features, including source control

  • Environment Agency Standards: Source control helps meet Water Framework Directive obligations and supports ecological protection

Local planning authorities may also require drainage impact assessments and include conditions for source control features in planning approvals.

Maintenance and Long-Term Performance

Source control systems must be properly maintained to function as intended. Key maintenance activities include:

  • Removing sediment or debris from swales and inlets

  • Inspecting rainwater harvesting tanks for leaks or blockages

  • Maintaining vegetation in green roofs and rain gardens

  • Monitoring infiltration rates in soakaways and adjusting design if necessary

Routine inspection and record-keeping ensure that performance remains consistent over the lifetime of the feature.

Integrating Source Control into Design

Successful implementation of source control depends on integrating drainage thinking early into site design. This includes:

  • Mapping natural flow paths and existing soil conditions

  • Designing buildings and landscapes to work with water, not against it

  • Prioritising infiltration and attenuation over direct discharge

  • Using multifunctional spaces where possible (e.g. car parks with permeable surfacing)

Cross-disciplinary collaboration between engineers, architects, planners and landscape designers is essential to achieve optimal outcomes.

Conclusion

Source control is a vital concept in modern drainage design, focusing on managing water and pollutants as close to their source as possible. By intercepting runoff, promoting infiltration and removing contaminants at the point of origin, source control reduces flood risk, improves water quality and supports sustainable land development.

When properly planned and maintained, source control systems contribute to resilient urban environments and help meet regulatory requirements for surface water management. As urban areas continue to expand and climate change increases rainfall variability, source control will remain a key strategy in protecting infrastructure, property and the natural environment.