Under natural conditions, rainwater sinks into the ground and refreshes and sustains the aquifers. However, in urban areas, where surfaces are sealed with asphalt and dense construction materials, precipitation may not reach the subsurface. Instead, the water in urban areas is often led into inadequate drainage systems which may cause numerous problems.
When there is heavy precipitation over a short period of time, the drainage system may become overloaded, flooding roads and damaging buildings. It may seem paradoxical, but often when the surface is overflowing with water, the subsurface may be drying out. Too little water in the underground is an invisible problem that over time can cause as much damage as flooding. Stable, high groundwater levels are often critical to the stability of surface structures (link to subsidence damage) and for the preservation of archeologically valuable material.
Decentralizing management of urban runoff is necessary to mitigate damage caused by flood and ground settlement. At a local level, rainwater must be led to the subsurface by replacing impervious surfaces with permeable solutions, and where the soil conditions are suitable, establish local infiltration. These interventions will minimize runoff to the streets, relieve the drainage system, remove pressure on water treatment plants and replenish groundwater in the urban environment.
Capturing urban storm runoff in is possible by using rain gardens, swales or green roofs. A rain garden could look similar to an ordinary garden, but hidden beneath the plants is filter medium and drainage that is designed to collect, absorb, clean and possibly filter water from surrounding, impervious surfaces. Such systems can hold a lot of water and thereby lower the peak flood level. Predictable flood ways are developed with drainage to the stormwater management facilities.