Permeable and impermeable surfaces

In urban areas, there are significantly fewer permeable surfaces (surfaces that absorb water), such as forests or hedgerows. Instead, there are more impermeable surfaces (surfaces that do not absorb water), such as  roads, pavements and driveways.

This means more of the water has no choice but to build up and flow along the surface (surface water run-off). If too much water is forced into over-capacity drainage systems, or rivers and lakes, they can overflow or burst their banks, resulting in flooding.

An increase in green space could dramatically reduce run-off in built-up areas.