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Using Green Roofs To Solve Flooding Problem In The Cities
Many cities are facing flooding problem.  The world weather has changed. Global warming allows more water to be retained in the atmosphere.  So much water that when it rains, it really pours.  This is causing serious flooding problem in urban areas from Spain to Singapore and all over the world.  Many city planners soon realised that they have reached the limited of their canals' capacity.  During torrential rain, the drainage canals quickly overflow and causes serious floods.  Now experts are recommending the use of holding reservoirs and green roofs to alleviate the problem.  A holding reservoir or water tank essentially stores water that the canals cannot divert to the sea fast enough.  In a highly developed urban area, it can be a problem to build holding reservoirs. The alternative is to build green roofs.  It is in effect, using the roofs of buildings to hold the water and release it at a controlled rate.
What Is A Green Roof?
A green roof is basically a rooftop garden.  It is also known as a rain garden. The roof is completely or partially covered with plants and is equipped with a drainage and irrigation system.  A large amount of water can stored in the green roof system.  The water retention system consists of soil-like material, porous mebranes and water storage trays.  The water retention system reduces the rate of water flowing to the canals.  Some of the water are recycled for irrigation purpose.  The water can also be recycled for various usages such as cooling and washing.  A green roof system can easily retofitted to existing buildings and provides many benefits to the building owners as well as improves the surrounding environment.
A variation of the green roof is the green wall, whereby plants are grown vertically on a wall.  A green wall offers the same benefits as a green roof. Both are great ideas for greening a building or home.

What Are The Benefits Of A Green Roof?
Besides improving the building's aesthetics, a green roof can save money for a building owner by conserving energy use. It keeps temperature low on hot days, so that less power is required for air-conditioning. The water collected can be used for washing and other uses, further saving the building owner money.
A green roof can improve the quality of air by filtering off dust particles, pollutants and carbon dioxide.  A green roof also helps to insulate the building from noise as well as reduce noise pollution in the immediate surrounding.
Most importantly, a green roof can prevent flooding by storing rainwater, reducing the rate and the amount which water will flow to the ground. A typical green roof can store between 6 to 20 litres of rainwater per square metre.  Building in a water reservoir feature can further increase the storage capacity.