Expect more images of flood evacuees scrambling into rescue boats with their wet pets and possessions unless cities seize the chance to build both high-tech and simple "green" flood prevention measures into future infrastructure, experts say.

But as sky-high real estate prices in already packed megacities lead to wetlands and mangroves being concreted over for apartment blocks and offices, convincing cities to find space for water-absorbing parks and other ways to reduce flood risk could prove a hard sell.

“The cheapest way of dealing with it is forward planning,” said the World Bank’s Niels Holm-Nielsen, noting that 60 per cent of the urban areas that will exist by 2030 have yet to be built.

“It has a huge financial cost, but what the city is really doing is making their space and their people more productive,” said the bank’s global lead for disaster risk management and resilience. “Of course, it’s a political winner, as you don’t get a lot of votes from people that are drowning.”

Powerful Hurricane Irma has been smashing its way through the Caribbean, hard on the heels of Hurricane Harvey dousing Texas and Louisiana, while more than 1,400 people have been killed in South Asia’s latest monsoon floods.

This extreme weather flags the need for cities to step up preparations for floods and storms, which generate 70 per cent of natural disasters and are predicted to become more intense as the global climate heats up.

“Urgency makes it easy,” said Patricio Zambrano-Barragan, a housing and urban specialist at the Inter-American Development Bank.

“When you have a big flood Houston-style, or the one Panama is seeing right now, there seems to be much more agreement that something needs to be done – which opens windows of opportunity.”

But despite the pressing need to protect booming populations, cash-strapped cities struggling to provide even basic drainage systems in sprawling slums may only dream of replicating the massive pipes and cisterns under Tokyo’s streets, or the vast barriers protecting Rotterdam from the sea.

Taking the lead from China, which is investing in lush green “sponge cites” fitted with permeable pavements, experts say governments should aim to put cheap, low-tech ways to absorb water, such as rain gardens and parks, into expanding urban areas.

Harriet Festing, co-founder of Chicago-based environmental non-profit Anthropocene Alliance, said much infrastructure still falls into the “grey” category, including concrete flood barriers and embankments. “With the green infrastructure… you know quickly whether it works,” she added.

Enforcing regulations to stop settlements sprouting up near river beds and flood plains can limit losses, experts said, while retaining trees and wetlands allows water to be absorbed, and conserving mangroves along coastlines curbs the effect of storm surges.

“We simply can’t go on with haphazard urbanisation – it hits the poor very badly,” said Rohinton Emmanuel, professor of sustainable design and construction at Glasgow Caledonian University.

“Saying that we can’t afford to have space for greenery and parks is very short-sighted… no one has looked at the whole life cost of these things: how much does the flood event cost?”

Many anti-flood measures have multiple benefits, he added, with trees and parks helping curb urban pollution and heat, for example.

Hard infrastructure such as car parks can also act as temporary storm-water reservoirs, while drainage systems built under highways or football pitches can provide water for households or irrigation.

Yet persuading cities to turn down development proposals that create employment or ease a housing crisis, so as to preserve green areas, is a tough task, said experts.

“In Chennai, we have lost more than 90 per cent of wetlands in the past 40 years,” said Arivudai Nambi Appadurai, head of climate adaptation strategy for the World Resources Institute in India.

The city’s IT corridor was built on wetlands, he noted, and in Mumbai, an airport is being constructed in a mangrove forest.

“The damage has been done,” he said. “We cannot really bring these buildings down as now the entire city’s economy revolves around it.”

Urban planners should consider how best to build near watersheds on city fringes which will be the “next frontier” for development, experts said.

The limited revenue opportunities from flood prevention projects mean cities must also be more agile in finding ways to pull in private-sector investment, they added.

In areas where regular flooding slashes the value of land, some cities are negotiating with investors to pay for flood defences, permitting them to build in the newly protected zones on the condition they provide safer housing, the World Bank’s Holm-Nielsen said.

Slum communities in Dar es Salaam and Nairobi are also looking at ideas to fix local flooding problems and increase the value of land, he added.

While green infrastructure might take longer to establish and its worth is hard to quantify ahead of a disaster, mayors are realising the need to invest in longer-term schemes beyond their own terms in office, experts said.

“It’s always cheaper to sort out adaptation (to climate change) now, rather than sort out the problems that come with not being prepared,” said Alfredo Redondo, manager for climate change risk assessment at the C40 Cities network.


By Sophie Hares