"Urban expansion often creates poorer neighbourhoods which lack adequate infrastructure and services, making them more vulnerable to floods. The poor are hit hardest, especially women and children,” said Pamela Cox, the World Bank's vice president for East Asia and the Pacific.
"But rapid urbanisation also means we have the opportunity to do things right the first time, so cities and towns can support sustainable development, saving lives and money,” she said in a statement.
The manual, entitled "Cities and Flooding: A Guide to Integrated Urban Flood Risk Management for the 21st Century", provides practical guidance on how to tackle floods in the face of rapid urbanisation, growing populations and long-term climate change trends.
It notes that urban floods are more costly and difficult to manage than those in rural areas, and can have more serious consequences for societies because towns and cities contain key economic and social assets.
While deaths from floods have dropped dramatically since the 1950s, economic losses have jumped in the past two decades, the guide shows. There are also long-term negative consequences for health and education, which can erode development goals.
Floods affect urban settlements of all kinds - from small villages along Pakistan's Indus river which flooded in both 2010 and 2011, to big cities like Bangkok that were inundated late last year.
The East Asia and Pacific region and South Asia are particularly vulnerable, the World Bank warns. In the past 30 years, the number of floods in Asia amounted to around 40 percent of the total worldwide. More than 90 percent of the global population exposed to floods lives in Asia.
MORE PEOPLE, MORE RISK
Urban flooding is becoming more dangerous and expensive to manage largely because of the growing numbers of people living in cities and towns, the book notes.
In 2008, for the first time, half of the world's population lived in urban areas. That figure is projected to rise to 60 percent in 2030 and 70 percent in 2050, for a total of 6.2 billion people living in urban areas.
As urban settlements swell, unplanned expansion often occurs on floodplains and other at-risk areas, the book says. And many new urban dwellers - especially impoverished rural migrants who come in search of work - end up living in overcrowded, high-risk slums.
"The concentration of the poor within these areas, which typically lack adequate housing, infrastructure and service provision, increases the risk of flooding and ensures that flood impacts are worst for the disadvantaged," it says.
The influx of people into towns and cities, badly planned urban development and natural climate vulnerability are seen as the biggest factors exacerbating the risk of floods now and in the short term.
Climate change impacts such as sea-level rise and more extreme weather - while not regarded as the main driver of worsening flood problems so far - may play a significant role on longer time scales, it adds.
The guidebook recommends that the most effective way to manage flood risk is to take an integrated approach combining structural and non-structural measures.
Structural policies include building drainage channels, floodways, embankments and other defences; incorporating “urban greening” such as wetlands and environmental buffers; restoring floodplains; and making pavements permeable.
Non-structural activities could be creating flood warning systems and public awareness campaigns; land use planning for flood avoidance; insurance schemes; and preparing to evacuate people and keep businesses running when floods do happen.
Mapping flood risks and communities' vulnerability to them can also help direct resources to the places they are most needed, the book says.
It adds that an effective strategy requires cooperation between different levels of government, public-sector agencies, civil society, educational institutions and the private sector, as well as strong leadership from national and local governments.
But as flood risk cannot be completely eliminated, "planning for a speedy recovery is also necessary, using reconstruction as an opportunity to build safer and stronger communities which have the capacity to withstand flooding better in the future", the World Bank said in its statement.
AID AGENCIES ILL-EQUIPPED
The practical approach outlined in the guidebook speaks to a need among those who both respond to and work to reduce the risk of disasters for better strategies for urban areas.
Rapid urbanisation was identified in an AlertNet poll of 41 aid organisations, released in late January, as one of the top factors expected to increase humanitarian need in the coming years. It was ranked the third most important driver, after rising climate-related disasters and displacement caused by climate change, and degraded environments.
“Sprawling unplanned urbanisation means that many more people are at risk in the event of natural disasters hitting cities and towns,” one director of disaster management told the AlertNet survey.
Yet, despite recognising the threat, many respondents said aid agencies remain poorly equipped to deal with urban disasters, as they have traditionally operated mainly in rural areas.
“New response models need to be developed for large-scale urban disasters, such as the earthquake that struck (the Haitian capital) Port-au-Prince in 2010,” said a senior official at one U.S. agency.
“Urban disasters are going to change the dynamics of how we work as an industry,” commented another director of disaster response.
The lead author of the World Bank flood risk guidebook, Abhas Jha, agrees that recent large-scale disasters "emphasise the need for a new approach to disaster risk management and resilience”.
"We need to design systems that recognise the complex and uncertain nature of flood risk management and its impacts," said the World Bank urban disaster specialist.
"Design should be comprehensive, flexible and iterative, being careful to avoid an over-reliance on any one given solution which may not be enough to counter the dynamic nature of risk.”