Impervious Surfaces and Urban Runoff

Impervious surfaces, such as roofs, roads and paved areas, prevent rainfall from soaking into the soil. They convert a larger share of intense rainfall into rapid surface runoff, causing water levels in drains and rivers to rise faster and reach higher peaks.

Changes in rainfall partitioning

Urban paving disrupts the natural processes that temporarily store or absorb rainwater.

• Reduced infiltration and soil-water storage increase the volume of direct runoff generated by a storm.
• Loss of vegetation reduces interception and evapotranspiration, leaving more rainfall available for runoff.
• The urban catchment consequently has a higher runoff coefficient, meaning that a larger proportion of rainfall becomes surface flow.

Effect on the flood hydrograph

Impervious surfaces and artificial drainage rapidly connect different parts of a city to channels and low-lying areas.

• Smooth surfaces, gutters and storm sewers shorten the lag time between peak rainfall and peak discharge.
• Rapid concentration of runoff produces a steeper rising limb and a higher flood peak in the drainage channel or river.
• Runoff from many paved areas can arrive almost simultaneously, overwhelming the carrying capacity of drains, culverts and channels.

Conditions that worsen urban flooding

The effect becomes severe when natural drainage and temporary storage have also been reduced.

• Construction over wetlands, ponds, floodplains and natural channels removes spaces that store and slowly release stormwater.
• Encroached, undersized or silted drains restrict discharge, causing pluvial flooding even before nearby rivers overflow.
• High antecedent soil moisture and intense rainfall further reduce available storage, while rapid urban runoff can also raise downstream river levels.
• Permeable pavements, urban wetlands, rain gardens and detention basins reduce peaks by restoring infiltration and storage.

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