New research suggests that the probability of flooding in cities and megacities built on river deltas is on the increase and over time, the Mississippi and the Rhine may become up to eight times more at hazard from rising tides, storm surges or catastrophic downstream floods.

The study, published in the journal Science, calculates the challenges ahead for 48 major coastal deltas in the Americas, Europe and Asia, right now home to populations of more than 340 million people.

Deltas are natural sites for cities: they offer direct access to the sea and upriver to the hinterland; their wetlands provided good hunting and, when drained, became fertile farmland, ever renewed by fresh deposits of silt from frequent flooding. So civilisation got a head start in the Nile delta, the Indus, the Ganges and the Brahmaputra, the Yangtze, and elsewhere.

But dams upriver to conserve water or generate electricity stopped the downstream flow of silt. The draining of the wetlands allowed the soils to compact. The abstraction of water for industry and for huge numbers of new citizens meant that soils compacted even more. The reclamation of wetlands meant that the highest tides had nowhere to go but on to the streets and into city basements. And the sea began to encroach, which meant more investment in sea defences.

In the Mississippi delta, according to another study in the same journal, the citizens of Louisiana and New Orleans have bade goodbye to up to 100 sq km of land washed away every year since 1900, and in the Netherlands, after centuries of soil drainage and subsidence, 9 million people live below sea level, behind costly sea dikes.

By 2100, according to recent research, as sea levels rise in line with global warming, coastal flooding could be costing nations $100,000bn a year.

Zachary Tessler of City University in New York – a city taken calamitously by surprise by Hurricane Sandy in 2012 – and colleagues report in Science that they looked at relative risks and how these might be altered over time, not just by sea level rise and climate change and a greater frequency of extreme events, but by the things humans continue to do to lower the level of the land at the same time as sea levels continue to rise.

They found that while 340 million were directly at risk in their 48 major coastal deltas, there were also an estimated 140 million people living within a 25km radius of the vulnerable regions.

And they found that although the poorer nations were inevitably more vulnerable, relative risk continued to grow – from four to eightfold in some cases – for the wealthiest delta regions as well. These included not just the Mississippi, the Rhine, the Han, Chao Praya and Yangtze, but also the Parana, Rhône and Pearl deltas. Not surprisingly, they conclude that deltas are “highly sensitive” to the balance between sediment supply and wave energy.

“Our study demonstrates that economic ability and decisions to deploy engineering solutions will be key factors in determining how sustainable deltas become in the long term,” they warn.

In the Ganges Brahmaputra delta in Bangladesh and India, home to 170 million people, the surviving wetlands are up to 1.5 metres above the embanked and reclaimed land. When the wetlands are disconnected from the rivers the land no longer naturally builds up with sea level rise.

So Stijn Temmerman of the University of Antwerp and Matthew Kirwan of the College of William and Mary at Gloucester Point in Virginia, in the US, spell it out: the answer may lie in “eco-based” engineering systems to put sediment-laden water back on to the delta plains. In the Mississippi, that would prevent the loss of 500,000 hectares of wetlands and reduce annual flood damage to New Orleans and the Louisiana coast by between $5.3bn (£3.4bn) and $18bn in 50 years.

And by judiciously letting the river do the job, instead of transporting settlement by barge, or by pipeline, the costs could be reduced. 

Tim Radford

This article first appeared in the Guardian

edie is part of the Guardian Environment Network

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