Flooding is one of the most damaging natural hazards, affecting millions every year. There have been many notable flood events around the world in recent years, in countries such as Pakistan, Thailand, Australia, the USA, and in early 2014, parts of England experienced extensive floods. These events have caused widespread damage to infrastructure and buildings, as well as detrimental effects on human well-being.
Flooding in urban areas is a particularly pressing problem. Cities are highly concentrated centres of human and economic activity. They exhibit many complex relationships, which can be disrupted by flooding. The world is witnessing a number of trends which are expected to increase the risks of urban flooding.
By 2050, for example, the UN estimates that an additional 2.6 billion people will live in urban areas.1 Increased urbanisation means that more people will live in cities and will be exposed to flooding. Urban development also typically increases the area of impervious surfaces and subsequently the volume of surface runoff. Further, economic growth will increase the value of exposed assets and it has been projected that by 2025, 600 cities will accommodate 20% of the world’s population and generate 60% of global Gross Domestic Product.2 When combined with the possibility that climate change could lead to more frequent intense rainfall, there is a pressing need for cities to improve the way that floods are managed.