What is a Contributing Area

The term contributing area refers to the portion of a catchment or drainage basin from which stormwater runoff is collected and directed into a sewerage system. In the context of urban drainage and sewer design, a contributing area represents all surfaces—natural or constructed—that feed water into surface water drains, combined sewers, or stormwater infrastructure during rainfall events.

Understanding the characteristics, extent, and hydrological behaviour of contributing areas is fundamental for designing efficient, compliant, and resilient drainage systems. It influences everything from pipe sizing and network layout to flood risk assessments, pollution control, and asset management strategies.

Definition and Scope

A contributing area is not limited to a geographic zone outlined on a map. Instead, it is defined by hydraulic connectivity—i.e., the way water physically travels across surfaces, infiltrates soils, or enters collection points such as gullies, inlets, or overland flow paths. The term applies across all types of sewerage systems, including:

• Surface water drainage networks 
• Combined sewer systems 
• Separate foul and stormwater systems 
• Sustainable Drainage Systems (SuDS) 
• Open channel and culvert systems 

Importantly, the term may also refer to the contributing catchment to individual assets, such as a manhole chamber, storm tank, or treatment facility. In each case, defining the contributing area accurately is vital to ensuring proper performance, longevity, and environmental compliance.

Types of Contributing Areas

Contributing areas are generally classified according to their surface characteristics and their connection to the drainage system. The two principal types are:

1. Impervious areas: 
   • Surfaces that do not allow water to infiltrate into the ground. 
   • Examples: asphalt roads, rooftops, concrete driveways, car parks. 
   • These areas generate high volumes of runoff, often with minimal delay. 
2. Pervious areas: 
   • Surfaces that absorb some or all incoming rainfall. 
   • Examples: grass verges, soil plots, permeable paving. 
   • These contribute to runoff indirectly, often through subsurface flow or when saturated. 

The runoff coefficient of each surface determines how much of the rainfall becomes surface runoff, with impervious surfaces typically having coefficients between 0.8 and 1.0, and pervious surfaces ranging between 0.1 and 0.4.

Some drainage systems also account for partial contributing areas—zones that only contribute under certain conditions (e.g., extreme storms) or through overflow paths from basins, ponds, or overland flow systems.

Hydrological and Hydraulic Significance

The size and characteristics of a contributing area directly influence flow rates, hydraulic loads, and peak discharge volumes entering a sewer or drainage asset. Key factors include:

• Rainfall intensity and duration 
• Surface slope and roughness 
• Land use and surface type 
• Soil permeability and antecedent moisture 
• Infiltration, evaporation, and interception 

Accurate estimation of the contributing area is essential for:

• Designing stormwater systems (pipe sizing, detention volumes, inlet spacing) 
• Modelling runoff volumes for flood risk mapping and asset planning 
• Predicting pollution loads entering combined sewer systems or watercourses 
• Planning sewer upgrades or retrofits 
• Complying with discharge permits and runoff attenuation standards 

Modern design tools such as MicroDrainage, InfoWorks ICM, or SWMM allow engineers to model contributing areas in detail, applying spatially variable rainfall and land use data for precise simulation.

Delineation and Calculation

Defining a contributing area begins with topographic analysis and field surveys to identify which surfaces drain toward a specific point or section of the sewer network. This includes:

• Mapping roof drainage connections (direct or via gutters and downpipes) 
• Surveying gully positions and kerb channels 
• Identifying flow paths across hard and soft landscaping 
• Confirming property boundaries and levels 

In modern developments, architects and drainage engineers often specify contributing areas at the design stage using CAD tools or GIS mapping. In existing systems, CCTV surveys, flow monitors, and infiltration testing may be required to verify assumptions.

The calculation of contributing area involves:

1. Summing the effective impervious area (EIA): 
   • Includes all impermeable surfaces directly connected to the drainage system. 
2. Adjusting for infiltration and disconnection: 
   • Some impervious surfaces may drain to pervious areas or SuDS components before entering the sewer. 
3. Incorporating runoff coefficients: 
   • Weighted averages are applied to mixed-surface catchments. 
4. Applying temporal and spatial rainfall data: 
   • Determines peak flow and total volume for system design. 

A common simplification used in design is the Rational Method: Q=CiAQ = CiAQ=CiA Where:

• QQQ = peak flow (m³/s) 
• CCC = runoff coefficient (dimensionless) 
• iii = rainfall intensity (mm/hr) 
• AAA = contributing area (hectares) 

However, for larger or more complex systems, dynamic modelling is required.

Design Implications

Underestimating or miscalculating the contributing area can lead to:

• Undersized pipework 
• Surcharging and flooding during storms 
• Overloaded SuDS or detention systems 
• Premature wear on pumping stations and mechanical assets 
• Non-compliance with discharge consents or environmental standards 

Conversely, overestimating may result in unnecessarily high construction costs and over-designed infrastructure.

Designers must also consider future changes to contributing areas, such as:

• Urban expansion or densification 
• Additional impermeable surfaces (e.g., extensions, patios, driveways) 
• Changes in land use or landscaping 
• Climate change and increased storm intensity 

It is often good practice to apply climate uplift factors to account for future rainfall variability and urban creep, ensuring that systems remain functional throughout their design life.

Role in Sustainable Drainage

Contributing area analysis is central to the design of Sustainable Drainage Systems (SuDS). These systems aim to mimic natural hydrology by controlling the volume, rate, and quality of runoff at source.

Key SuDS strategies influenced by contributing area data include:

• Design of attenuation basins and detention ponds 
• Sizing of infiltration trenches and soakaways 
• Permeable paving layout and base design 
• Rainwater harvesting and greywater reuse systems 

Many SuDS schemes are designed to treat only a portion of the contributing area—for instance, high-risk pollutant zones such as car parks—while allowing clean roof water to drain separately or be reused.

National planning policy, including the Non-Statutory Technical Standards for SuDS and guidance from CIRIA (e.g. C753: The SuDS Manual), places emphasis on accurate contributing area assessment as part of best practice drainage design.

Regulation and Compliance

Contributing area data is often required for:

• Drainage Strategy Reports submitted with planning applications 
• Flood Risk Assessments (FRAs) 
• Build Over Agreements with water companies 
• Environmental Permits and Discharge Consent Applications 
• Adoption processes under the Sewerage Sector Guidance (SSG) 

For developments where sewers are to be adopted by a statutory undertaker, the designing party must submit accurate information about the contributing area, supported by hydraulic calculations or simulation results. Water companies may require verification through surveys, as-built drawings, and post-construction monitoring.

In addition, regulators increasingly expect developments to include monitoring provisions, especially for large-scale or sensitive sites, to ensure that actual performance matches design assumptions.

Conclusion

The concept of a contributing area is fundamental to effective, sustainable, and compliant drainage design. It defines the physical and hydrological space from which stormwater enters a sewer system, influencing everything from pipe sizing and flow modelling to SuDS implementation and pollution control.

As urban environments evolve, and rainfall patterns become more intense and unpredictable, accurate assessment of contributing areas will be essential for building climate-resilient drainage networks. For engineers, planners, developers, and asset managers, mastering this concept ensures the delivery of drainage systems that are not only functional and cost-effective, but also environmentally responsible and future-ready.