Urban Flooding

Concretisation is the replacement of soil and vegetation by buildings, roads, pavements and other hard surfaces. These impervious surfaces prevent rainwater from infiltrating into the ground, thereby increasing both the volume and speed of surface runoff reaching urban drains and low-lying areas.

Hydrological mechanism

In a natural catchment, rainfall is divided among infiltration, surface storage, evapotranspiration and runoff. Concrete and asphalt have very low permeability, so a larger proportion becomes surface runoff instead of entering the soil.

• Rapid runoff shortens the time of concentration, causing water from different parts of the catchment to reach drains and low-lying areas almost simultaneously.
• The resulting higher peak discharge can exceed the carrying capacity of stormwater drains, producing waterlogging and flash flooding.
• Reduced infiltration also diminishes groundwater recharge, weakening the city's capacity to absorb later rainfall.

Why the urban effect is severe

Roofs, roads and paved plots form connected impervious surfaces that quickly channel water towards drains. Concretisation becomes especially damaging when natural storage areas have also been lost.

• Building over wetlands, ponds and floodplains removes spaces that could temporarily detain stormwater.
• Paving open soil and removing vegetation reduce interception, infiltration and evapotranspiration.
• Inadequate, silted or waste-blocked stormwater drains further compound the runoff generated by impervious surfaces.
• Rainfall intensity, local topography and drainage capacity therefore determine the final severity of flooding.

Restoring urban absorption and storage

Urban flood management should reduce runoff at its source rather than depend only on larger drains. A sponge-city approach combines permeable surfaces, storage and protected natural drainage.

• Permeable pavements, rain gardens, bioswales and vegetated open spaces allow infiltration and slow runoff.
• Rainwater-harvesting structures and detention or retention ponds temporarily store peak flows.
• Protecting wetlands, floodplains and natural channels preserves drainage and flood-storage functions.
• Catchment-based planning should limit excessive site coverage and connect land-use planning with stormwater management.