Nature knows best: How working with wetlands can reduce flood risks in cities

This week, the rising waters of the Seine caused a partial closure of the Louvre in Paris and flooded social media with yet more photographs of boats sailing along inundated city streets. Last year, massive floods in cities in Bangladesh, India and Nepal had a far more tragic impact, killing 1,300 people and affecting 45 million, while a “1,000-year flood” soaked Houston in Texas.

These events highlight a challenge that is becoming more obvious with each urban inundation: cities around the world are at risk from flooding and, with climate change and continued development, that risk is growing.

Floods are among the most costly natural disasters worldwide. In recent years, annual global damages have ranged between US$30–60 billion (2017 will be far higher when the final numbers are tallied), while floods displaced more than 100 million people between 2008 and 2014 alone. There is strong
evidence that, in a warming world, destructive floods will become more common and intense, and affect the lives of even more people. Furthermore, nearly half of all urban development between today and 2030 will occur in areas with elevated risks of flooding. That’s an additional 500,000 square kilometres of urban development — more than 300 Houstons — that will be at risk of generating more of the catastrophic scenes we have witnessed in the past six months.

Without significant investments to reduce risk, flood damage to coastal cities alone will increase to an unacceptably high one trillion dollars per year by 2050, according to a recent study led by a World Bank economist.

To respond to this challenge, governments must invest in a broad range of responses to reduce risks to people and property. As they do, the theme of this year’s World Wetlands Day — “Wetlands for a Sustainable Urban Future” — points them in an unexpected direction.

Wetlands may not be as obvious a solution to managing flood risk as engineered investments such as dams, levees, floodwalls and drainage systems. But experiences from around the world — recently, but also dating back a century — show that wetlands should be part of the mix of solutions as cities confront this challenge. Within cities, urban wetlands are part of a mix of investments — including rooftop gardens and permeable pavements — that can intercept and absorb rainfall, so that more water sinks slowly into soils and groundwater and less becomes rapid runoff that submerges streets and
overwhelms storm drainage systems.

In China, a new initiative is encouraging cities to pursue this mix of investments to address rising urban flood risk. The initiative’s name, “sponge cities,” quite cleverly evokes its goal — urban areas designed to soak up rainfall: the sponge part gets wet so the city part can stay dry. Thirty Chinese cities are now part of the sponge city initiative. The initiative’s ultimate goal is that, by 2030, 80 per cent of each city will feature sufficient wetlands and other sponge-y features to capture and absorb 70 per cent of storm water runoff.

But what about cities, such as New Orleans, that have big rivers flowing through them? For them, flood risk arises not only from rain that falls within their borders but also from river systems that gather rainfall and runoff from vast areas of a country or continent. To paraphrase Chief Martin Brody from
: you’re gonna need a bigger sponge.

Luckily, sponges — wetlands — do come in bigger sizes.

Floodplains, the low-lying lands that flank rivers, are seasonal wetlands. When a river floods, its floodplain functions like a release valve for floodwaters, reducing flood levels elsewhere, such as in cities. River managers have strategically reconnected rivers — including the Mississippi, Sacramento and Rhine — to portions of their historic floodplains, allowing floods to spread out in some places and reduce risk to cities and farms. The 2011 flood on the Mississippi carried a greater volume of water than the historic 1927 flood, which caused hundreds of levee failures and a terrible human toll, but did a tiny fraction of the damage thanks to several massive floodways that carried floodwaters away from cities.

In addition to contributing to flood-risk management, floodplains and urban wetlands provide a diverse range of other benefits to cities. The 24,000-hectare Yolo Bypass in California provides open space for a growing region and the best remaining lowland floodplain habitat in the Central Valley, supporting vast flocks of birds and endangered fish species, such as salmon. In cities, urban wetlands and other “green infrastructure” projects double as parks and open spaces, improving air quality and making neighbourhoods more verdant and livable.

To realize these diverse benefits, cities must first protect and manage the natural defenses they already have. We can’t keep repeating the mistakes of the past as unplanned growth obliterates wetlands and floodplains, erasing their protective functions, and sending higher volumes of floodwater toward homes and businesses.

To catalyze restoration, investments in wetlands can be coupled with reconstruction funds, as every dollar spent on risk mitigation is worth at least four dollars of future reconstruction. Storm water credit trading programs can create markets for green infrastructure to help deliver the needed investments.

Wetlands have an important role to play, but are just one part of the solution for cities to reduce flood risk. There also needs to be a greater emphasis on improved zoning, better building codes and realistic insurance programmes that are intended to keep people out of harm’s way in the first place. But sound policies reducing exposure won’t be enough, in part because so much at-risk development has already been built. Cities need to prepare for the worst by resorting to the best mix of solutions.

In a warming world, we risk seeing nature’s power as only a threat. By investing in wetlands, we also get nature’s power on our side.

Jeff Opperman is WWF’s global lead scientist for freshwater and author of the book Floodplains: processes and management for ecosystem services.