Over the last few months I have been comparing the way in which flood risk is managed in major cities around the world. In particular I have sought to distinguish how communities, and their governments, respond to the two ends of the flood spectrum, that is, the frequent but less severe flood events, and the rare but extreme events. Most countries invest in managing flood risk across the spectrum. Traditional solutions to frequent flooding often involve improved drainage conveyance or local channel realignment works, while extreme events are dealt with by building flood defence structures. Risk in both categories is growing however, despite this investment. This is mainly due to the effects of increasing urbanisation and increased rainfall intensity as a result of climate change. In addition many urban areas suffer from the process of urban creep, that is, the infill of open space with new development and the extension of existing building footprints.
Urban creep is typically increasing paved areas at a rate of between 0.25% and 2.5% per annum. So over the last 30 years some paved areas will have doubled in size, even though population may have remained static. Climate change could increase rainfall intensities by up to 40% in some areas. Existing drainage infrastructure cannot cope with such changes, and investment in increasing infrastructure capacity has lagged well behind the growth in need. This is leading to a significant increase in the frequency of more modest flood events and substantial increases in devastation from extreme events.
The challenge therefore is how to reverse this trend. Planning restrictions and building regulations can help to limit further urban creep, but many countries now accept that more sustainable approaches to draining urban development need to be found. There has been significant progress in implementing sustainable drainage measures (sometimes referred to as best management practices) to limit run-off from new development. But there is little evidence of progress on retrofitting such measures, and in particular creating safe flood pathways for managing extreme events. The more notable exceptions to this are in Holland and parts of Sweden and the United States where innovative approaches to retrofitting have resulted in the disconnection of existing roof areas to be drained locally, and the creation of designated flood pathways using roads and sacrificial flood areas in parks and open space. Such innovation can do much to reduce flood risk without the need for investment in heavy infrastructure.
These new methods differ substantially from traditional infrastructure. They are more closely related to natural drainage, using vegetated ponds and channels and mimicking natural systems. They need to be more carefully integrated into the urban landscape but they can deliver benefits beyond those of flood risk management (improved water quality, amenity and bio-diversity). The skills needed to progress such innovation extend beyond those of engineers to include architects, planners and social scientists. It is yet another example of the great benefits that can be realised from interdisciplinary approaches to flood risk management.