What is a Percentage runoff

Percentage runoff refers to the proportion of rainfall that becomes surface runoff and enters a stormwater drainage system. It is expressed as a percentage of the total rainfall received over a given area. This value is an essential parameter in drainage and stormwater management, as it helps engineers and planners estimate the volume of water that needs to be conveyed, stored or treated during rainfall events.

Not all rainfall ends up as runoff. Some infiltrates into the soil, evaporates into the atmosphere or is intercepted by vegetation and buildings. The percentage runoff represents what remains and contributes directly to stormwater load.

What determines percentage runoff

Several factors influence the percentage of rainfall that becomes runoff:

• Surface type
  Impermeable surfaces such as concrete, tarmac and rooftops prevent infiltration and lead to high runoff percentages, often exceeding 90%. Natural or permeable surfaces such as grass, soil or gravel allow more water to soak in, resulting in much lower runoff rates.

• Slope and topography
  Steeper slopes increase the speed and volume of runoff by reducing infiltration time. Flat areas tend to absorb more water unless saturated.

• Soil type and condition
  Sandy soils have high infiltration rates, while clay soils absorb less water and generate more runoff. Compacted or saturated soils also contribute to higher runoff percentages.

• Rainfall intensity and duration
  Light rainfall may be fully absorbed, while intense storms can overwhelm infiltration capacity and produce significant runoff.

• Land use and development
  Urbanisation increases the proportion of impermeable surfaces, raising the percentage runoff. Greenfield (undeveloped) sites typically produce 10–30% runoff, while fully developed urban areas can produce 70–100%.

• Vegetation cover
  Trees and plants intercept rainfall and enhance infiltration through root systems, reducing runoff.

Understanding these factors helps engineers make more accurate predictions of system demand and potential flooding.

Typical percentage runoff values

While actual values vary by location and conditions, the following are generalised runoff percentages for different surface types:

These values are often used as design assumptions when planning new drainage infrastructure or retrofitting urban areas for flood resilience.

Role in drainage design

Accurate estimation of percentage runoff is critical in designing stormwater systems, including:

• Sizing pipes, culverts and channels

• Designing attenuation ponds, soakaways and tanks

• Developing Sustainable Drainage Systems (SuDS)

• Preventing local flooding and surcharge events

• Modelling catchment behaviour under different storm scenarios

If the percentage runoff is underestimated, the system may be overwhelmed during rain events. Overestimation, on the other hand, can lead to oversized infrastructure and unnecessary cost.

Calculation methods

Percentage runoff is typically calculated using runoff coefficients or hydrological models. In the Rational Method, for example, the runoff coefficient (C) directly represents the percentage runoff as a decimal:

• A coefficient of 0.9 means 90% of rainfall becomes runoff

• A coefficient of 0.2 means 20% runoff and 80% is absorbed or evaporated

Runoff coefficients are assigned to different land uses or surfaces and weighted based on area coverage in a drainage catchment.

More complex models may incorporate time-varying infiltration rates, soil saturation, and rainfall patterns to provide dynamic estimates of runoff volume and peak flow.

Use in urban planning and flood prevention

Urbanisation leads to a significant increase in percentage runoff, which in turn increases the volume of water entering drainage systems during rainfall events. This has important implications:

• Increased risk of surface water flooding

• Greater stress on existing infrastructure

• Higher likelihood of combined sewer overflows in older networks

• Reduced groundwater recharge and local water table levels

To mitigate these effects, planners often require that new developments limit post-development runoff to match pre-development conditions. This is achieved by incorporating:

• Permeable paving

• Green roofs and rain gardens

• Swales and filter strips

• Detention basins and underground storage tanks

• Flow control devices to delay peak runoff

These features reduce both the percentage and volume of runoff, helping maintain system balance and environmental protection.

Monitoring and validation

In some projects, percentage runoff assumptions are validated using monitoring data. Techniques include:

• Flow meters in stormwater outfalls

• Rain gauges for event-specific comparison

• Soil moisture and infiltration rate testing

• Surface runoff modelling tools calibrated with local data

This validation is particularly important for high-risk sites, sensitive environments or major developments subject to regulatory approval.

Regulatory framework

In the UK, percentage runoff considerations are part of planning conditions under the National Planning Policy Framework (NPPF) and local Lead Local Flood Authority (LLFA) guidance. Developers are often required to submit drainage strategies that show:

• Expected runoff volumes and rates

• Mitigation measures to manage post-development runoff

• Justification for assumed runoff percentages or coefficients

These strategies support sustainable urban development and ensure long-term resilience of drainage infrastructure.

Conclusion

Percentage runoff is a fundamental concept in hydrology and drainage design, representing the fraction of rainfall that enters the stormwater system rather than soaking into the ground or evaporating. It is influenced by land use, surface materials, soil conditions, and rainfall characteristics.

Accurate estimation of percentage runoff is essential for designing effective, resilient and sustainable drainage systems. As climate patterns shift and urban areas expand, managing runoff through informed design and sustainable interventions becomes increasingly vital to reduce flood risk and protect water quality.