The urban water cycle

Cities break the natural cycle. Paving stops infiltration, so rain that would have soaked away becomes runoff that arrives all at once.

Impermeability: A natural grassland catchment might turn 10% of rainfall into runoff. A dense urban catchment can turn 70% or more, and deliver it in a fraction of the time.
Why paving front gardens matters: Small individual decisions aggregate. Widespread front garden paving measurably increases peak runoff across a whole neighbourhood.
Sustainable drainage: Permeable paving, swales, rain gardens and green roofs restore infiltration and storage, reducing both flooding and sewer overflow.

Text description

In a natural catchment, most rainfall infiltrates into the ground and only a small share becomes runoff. In an urban catchment, impermeable paving and roofs prevent infiltration, so a large share becomes rapid runoff, arriving at the drainage system as a sharp peak. This causes surface water flooding and triggers combined sewer overflows. Sustainable drainage restores infiltration and storage.