What is a Impermeable surface

An impermeable surface is any surface that prevents or significantly resists the infiltration of water into the ground. These surfaces do not allow rainfall or other forms of moisture to pass through to the underlying soil, resulting in an increase in surface water runoff. Impermeable surfaces are a major factor in urban hydrology and are central to the design and regulation of drainage systems, flood risk management, and sustainable urban development.

Examples of impermeable surfaces include materials commonly found in the built environment, such as asphalt, concrete, roofing tiles, and certain types of paving slabs or compacted ground. These are contrasted with permeable or porous surfaces, which are designed to allow water to percolate through and recharge the soil.

Characteristics and Behaviour

When rain falls on an impermeable surface, it cannot soak into the ground. Instead, the water quickly accumulates and flows across the surface, often entering road gullies, storm drains, or surface water sewers. This behaviour significantly alters the natural hydrological cycle by:

  • Increasing runoff volumes
  • Reducing infiltration and groundwater recharge
  • Raising peak discharge rates in rivers and drainage systems
  • Decreasing evapotranspiration from soil and vegetation

As a result, urban areas with high proportions of impermeable surfaces are more prone to flash flooding, drainage overload, and polluted runoff entering watercourses.

Common Types of Impermeable Surfaces

Impermeable surfaces are found throughout the built environment. Typical examples include:

  • Roads and motorways constructed from asphalt or concrete
  • Parking lots and driveways with sealed surfaces
  • Building roofs made from tiles, shingles, or metal sheets
  • Footpaths and pavements with tightly compacted or sealed finishes
  • Industrial yards and loading areas with hardstanding materials
  • Tennis courts and playgrounds with impermeable matting or surfacing

Even natural surfaces can become impermeable if heavily compacted, such as in areas with high foot traffic or machinery use.

Implications for Drainage and Water Management

Impermeable surfaces play a critical role in the planning and performance of drainage systems. In areas where the land is predominantly impermeable, traditional infiltration-based drainage may be ineffective or inadequate. This leads to greater reliance on:

  • Conventional piped drainage systems
  • Attenuation tanks or ponds to slow down runoff
  • Flow control devices such as orifice plates or hydrobrakes
  • Sustainable Drainage Systems (SuDS) that attempt to mimic natural processes

Drainage designers must calculate the impermeable area of a development site when assessing required storage volumes, pipe sizes, and discharge rates. Surface water runoff from impermeable areas must often be treated or filtered before discharge to comply with environmental regulations.

Environmental and Urban Planning Considerations

The widespread use of impermeable surfaces in towns and cities has broader environmental impacts, including:

  • Increased risk of urban flooding due to high runoff rates
  • Water pollution from surface contaminants (oil, litter, metals) washed off into drains
  • Reduced groundwater recharge, which can lower water tables and affect ecosystems
  • Heat island effects, as impermeable materials absorb and retain solar radiation
  • Habitat loss, as natural infiltration zones are replaced with artificial coverings

To mitigate these impacts, planning policies increasingly encourage or mandate the use of permeable paving, green roofs, rain gardens, and other SuDS features, especially in new developments.

Regulatory Context

In the UK and other developed countries, impermeable surfaces are a key consideration in planning applications and drainage strategies. For example:

  • Under Schedule 3 of the Flood and Water Management Act (not yet enacted in England), surface water drainage systems may require approval from the Lead Local Flood Authority (LLFA).
  • Many local authorities require SuDS designs that limit runoff from impermeable surfaces to greenfield rates or incorporate on-site attenuation.
  • For domestic properties, paving over front gardens with impermeable materials may require planning permission unless a permeable solution is used or runoff is directed to a suitable infiltration area.

Conclusion

An impermeable surface is a fundamental concept in modern urban water management. By preventing infiltration, these surfaces alter natural drainage patterns and contribute to increased runoff, higher flood risk, and environmental degradation. As cities grow and land becomes more intensively developed, the careful management of impermeable areas — through design, regulation, and sustainable alternatives — is essential to building resilient and environmentally responsible infrastructure. For engineers, planners, and policymakers, understanding the role and impact of impermeable surfaces is critical to achieving long-term hydrological and environmental sustainability.