In a world of melting ice caps, storm surges, and tropical cyclones, the most resilient cities aren’t the ones that fight the water back—but the ones that absorb it.
By Fred Pearce
The ramshackle river port of Khulna in southwest Bangladesh is one of the most flood-prone urban areas on Earth. The third-largest city in one of the world’s poorest and most populous nations is at constant risk of inundation. It lies 125 kilometers inland from the shores of the Indian Ocean. And yet a tenth of this city of 2 million people is flooded at least ten times a year on average.
When Cyclone Aila swept inland in May 2009, unleashing a tidal surge up the Pussur estuary, nearly 2,000 kilometers of embankments in Khulna and surrounding districts were damaged. Many have never been repaired. The city is almost literally disappearing beneath the water.
The conventional explanation is that rising sea levels due to climate change are leaving such places at ever-greater risk from storm surges. But that is only part of the story. Sea levels in the open ocean are rising by three millimeters a year, but high tides in Khulna and along the estuaries of southwest Bangladesh are rising six times faster.
The destruction by farmers of mangrove forests along the coast in the Sundarbans swamp is partly to blame for the ocean’s greater ferocity. The mangroves once provided a wide buffer to soak up the winds and cyclonic tidal surges. But a new theory is gaining ground—a theory with important implications for many coastal regions around the world as they face up to the risks of climate change.
It seems that across the lowlands of southern Bangladesh, embankments erected to protect people from the rising tides are often making the problem much worse. They are constricting and funneling tide flows, pushing them further inland, and amplifying the tidal range. Low tides may often be lower, but the high tides in places such as Khulna—and in other delta regions around the world, from the mouth of the Rhine in the Netherlands to the Mississippi in Louisiana—are higher.
The physics isn’t difficult to grasp. When a surge of water runs into a hard barrier in one place, it doesn’t simply dissipate; it often rushes with increased ferocity somewhere else. And, with the water more tightly straitjacketed, the funneling effects can be lethal. What’s more, hard barriers such as concrete seawalls, steel floodgates, and engineered embankments lack the one trait most needed in a world of melting ice caps and expanding oceans—flexibility.
And therein lie the beginnings of a sea change in the world of coastal engineering: hard is giving way to soft. Instead of fighting water with concrete and mud, new teams of geomorphologists, urban designers, and ecologists are experimenting with softer infrastructure—mangrove forests, salt marshes, and sand dunes—that moves with the water and absorbs it like a giant sponge. It turns out that the form of coastal defense most likely to prove resilient against those forces of nature that we have unleashed is—nature itself.
Traveling by boat on the great rivers that cross southern Bangladesh is an overwhelming experience. The forces of nature often dwarf the puny efforts of engineers to hold them back. Almost half the country is built on wide and constantly shifting delta flats where two of the greatest and most ferocious rivers on Earth, the Ganges and the Brahmaputra, reach the Indian Ocean.
Here, 30 million people are in harm’s way. Floods that would be front-page news in other countries barely register here. Thirty villages disappeared under water in July this year, after 20 kilometers of embankments collapsed. That made three paragraphs. “Several thousand marooned for the seventh time in the last four months,” ran another headline.
In the past century, successive governments have allowed hard-pressed farmers to chop down the natural flood defenses provided by mangroves and have sought to replace natural defenses with thousands of kilometers of earth embankments. During the 1960s and 1970s, USAID paid for 1,600 kilometers of embankments in the Khulna district alone. The banks surround some half-million hectares in 40 protected zones known as polders.
But there is mounting evidence that the polders may not be equal to the new realities of sea level rise. In fact, the embankments that seal off the polders from the wider delta may make things worse. Shahjahan Mondal of the Bangladesh University of Engineering and Technology reported last year in the American Journal of Climate Change that in southwest Bangladesh, the “maximum tidal high water level is increasing at a rate of 7–18 mm a year.” (1) The highest figure was recorded on the tidal gauge at Khulna. Counterintuitively, “Extremes in tidal water levels are more prominent in inland areas compared to those near the sea,” Mondal said.
In a more detailed study for the World Bank, British geomorphologist John Pethick found a similar surge in high tides coursing inland through the delta, arguing that the funneling effect of embankments was also critical. (2) He warned that, combined with the effects of the global rise in sea levels, the result could be a 2.5-meter rise in high tides by the end of the century. Yet remarkably, his findings have been largely set aside. The Bangladesh government and the World Bank, seemingly caught in their own inertia, are set on a $400 million upgrade of some 600 kilometers of embankments south of Khulna, regardless.
Down the coast from Khulna, the people of India’s Mahanadi Delta in the state of Odisha are trying a different approach. The delta of the Mahanadi River, a smaller version of the Bangladesh delta region, is also prone to cyclone disasters. In 1999, “super cyclone” Kalina claimed more than 10,000 lives there; the majority of people drowned as a storm surge rushed across the low-lying delta.
Once again, the loss of mangroves is part of the story. Saudamini Das of the University of Delhi looked at 400 coastal villages in Odisha that half a century ago had, on average, a 5.1-kilometer-wide belt of mangroves protecting them. (3) Today that figure has fallen to an average of 1.2 kilometers, replaced in some places by rice paddies and elsewhere by ports, power plants, and other industrial infrastructure. During the super cyclone, no deaths occurred in villages that still had four or more kilometers of mangroves. Furthermore, in areas where protection got below three kilometers, death rates rose steeply. As in Bangladesh, efforts to create barricades against floods have proved counterproductive.
The Netherlands-based NGO, Wetlands International, along with the Indian government and Odisha’s new Integrated Coastal Zone Management Project, is trying to reverse decades of mangrove destruction. They are helping villages to cultivate and plant mangroves along the coastline and on the banks of all tidal rivers along Odisha’s coast.
When I visited the Mahanadi Delta earlier this year, communities proudly showed me stands of mangroves they had planted along the shoreline. In Tandahar, a coastal village along the coast, the secretary of the local disaster committee, Pramod Swain, told me when we met in the village’s cyclone shelter: “The sea is coming ever closer. In 50 years, the coast has come in 1.5 kilometers. Our fathers’ land is now under water.” But they, too, were planting mangroves for protection.
The decline of coastal mangroves is one of the world’s least reported environmental tragedies. Mangroves grow in partially flooded sediments along thousands of kilometers of tropical coastlines—nurturing fisheries, absorbing storm surges, storing carbon, and cleaning up pollution. Yet we are destroying an estimated one per cent of them each year, a rate several times faster than the rate of landward deforestation.
Daniel Alongi of the Australian Institute of Marine Science in Townsville, Queensland, says modeling studies show that a mangrove belt as little as 100 meters wide can significantly reduce deaths from tsunamis. (4) And left to themselves, mangroves can keep pace with rising sea levels. In fact, they thrive on it. He found that the faster sea levels rise, the faster healthy mangroves accumulate sediment in their roots. They can keep up with a rise of up to 25 millimeters a year—eight times the current global rate. No seawall can do that.
What goes for mangroves in the tropics also goes for other forms of natural coastal protection in other climes. Salt marshes, sand dunes, lagoons, mud flats, and other coastal wetlands have been diked, drained, and destroyed for generations in Europe and North America. Deemed wastelands, they were appropriated for grazing land and crops, heedless of the coastal protection they provided. But as the folly of that destruction becomes increasingly apparent with rising sea levels, coastal engineers are beginning to change course.
There is, says Keryn Gedan of Brown University in Providence, Rhode Island, “overwhelming evidence even small wetlands afford substantial protection from waves.” A meta-analysis of 75 studies by Christine Shepard of the University of California at Santa Cruz concluded that a barrier of salt marshes as little as ten meters wide could reduce wave heights by 50 percent. By 500 meters, the reduction was typically 90 per cent. (5)
That revelation has caught the attention of planners and architects in space-starved New York City. When Superstorm Sandy inundated low-lying areas of Manhattan and Staten Island in 2012, it humbled the city’s man-made flood defenses. But in the years since, there’s been a flood of ideas to make the Big Apple “greener”—and more absorbent.
In one proposal, New York architect Stephen Cassell envisions a fringe of tidal marshes and wetlands around the lower edges of Manhattan, spreading out from Battery Park. In some places, the marshes would be built on open ground—such as Battery Park. Elsewhere, Cassell proposes extending the land onto the foreshore for a couple of blocks, using landfill. But in contrast to the usual practice of building more high-rises, the newly created land would be infused with marshes.
Behind the shoreline wetlands, Cassel says, streets and other hard surfaces should be reconstructed so they can absorb water and let it seep underground. Porous concrete could drain excess water into the coastal marshes. “Man pushed nature out of most of Manhattan,” says Cassell. “Maybe it is time to bring some of nature back, for our own good.”
The same philosophy underlies a project on the estuary of the Delaware River (the second-largest estuary on the eastern coast of the U.S.), where scientists and managers are dumping bundles of coconut fiber on the mud flats beyond the cord grass. The bundles dissipate wave energy and capture sediment, helping to raise the level of the shoreline and keep pace with rising sea levels. The idea is to regenerate marshes (which, until recently, were being lost at a rate of an acre a day) and to build a “living shoreline” on the estuary.
It doesn’t take long to do. In the Heislerville Wildlife Management Area, where the Maurice River enters the estuary, healthy salt marsh established itself behind the coconut fiber “biologs” within a year. As the salt marshes form, mussels attach themselves to the fibers and filter the polluted water. This system of “soft armor” could become a model for protecting low-lying areas from New Orleans to San Francisco.
As the shape of our coastlines changes, so too can the shape of our seawalls. In the iconic English seaside resort of Blackpool, built on a stretch of sand dunes along the coast in northwest England, engineers have designed a seawall that behaves like a sand dune. Those dunes, topped with deep-rooted marram grass that catches and traps loose sand, absorb the energy of Atlantic storms and resist erosion. But over the years, the town removed the dunes to expand its famous “golden mile” of seaside stalls and attractions; the dunes were replaced with a sea wall.
But in recent times, the tides advanced, pushing the town’s beach back against the wall and exposing it to ever higher tides and the pounding of waves. Soon, engineers said, there would be no beach left—nowhere for English holidaymakers to indulge their love of erecting deckchairs, building sandcastles, and soaking up the sun. Meanwhile, the sea wall itself proved increasingly ineffective. The golden mile started to flood.
Engineers responded by mimicking the old dunes. They noticed that the surviving dunes along the coast absorb waves far better than does the vertical sea wall that protects Blackpool. The sloping dunes spread and dissipate the force, whereas the sea wall confronts it—and eventually crumbles as a result. Their solution has been to create a new, sloping sea wall—essentially a flight of shallow steps down to the beach. The new wall, says landscape architect James Haig Streeter, who helped design it, is lower than the old one but provides more protection by neutralizing the waves rather than fighting them. Sand, meanwhile, migrates up and down the steps with the seasons.
Then again, maybe we should stop worrying about rising tides and float free of the perils they create. Only floating structures can provide true resilience against sea level rise. So says Dutch architect Koen Olthuis, founder of the firm Waterstudio and a member of UNESCO’s Flood Resilience Group. He thinks we need a grander vision: we should, he says, embrace the rising waters by disconnecting ourselves from the land and building floating cities.
Not surprisingly, the Dutch are out in front on this one. In the Amsterdam suburb of Steigereiland, Marlies Rohmer has designed an estate of 43 floating houses. They are proper houses, not just converted boats. They can sit on the land or rise up and float to keep pace with the waters.
Cities are destined to suffer more than elsewhere from rising sea levels because many of them are sinking even faster than the waters are rising. When cities pump underground water to fill taps, pores in rocks collapse and the ground subsides. Coastal Jakarta, the megacity capital of Indonesia, is sinking by as much as 15 centimeters a year. Olthuis believes the solution lies in building floating structures right in the heart of the city to house up to a million of Jakarta’s 10 million inhabitants.
This idea is not so far-fetched. Dutch engineers have teamed up with the Indonesian Ministry of Public Works and Jakarta city authorities to turn Jakarta Bay, around which the city is constructed, into a huge, wet polder. Covering 100 square kilometers, an area twice the size of Manhattan, this man-made lagoon (estimated to cost $US25 billion) would have the capacity to take on much more water during storm surges or other flood events. Olthuis suggests that this huge lagoon might be the ideal place to put a floating city—with amphibious houses, office blocks, schools, hotels, sewage works, and even fields. The buildings could rise and fall with the watery buffer that—paradoxically—protects Jakarta from flooding.
In Jakarta, many of the poor live in houses built on stilts over creeks and riverbanks. Olthuis’s idea is just to go upmarket—and lose the stilts. In a city where 40,000 people were evacuated during floods last year, this makes a lot of sense. And soon it could be attractive in many other places as well. New York, Singapore, Hong Kong, and other cities were built on land “reclaimed” from the sea. Now, as land goes down and real estate prices go up, letting the sea back into the city—in essence shifting into reverse—looks more and more like a smart line of defense.
Sea level rise will continue, no matter what happens now. We need to get used to it. And plan for it. A few millimeters a year of rising tides doesn’t sound like much—until you comprehend that, according to the IPCC, the next century is likely to see waters rising by a meter or more. The current melting of ice caps may prove unstoppable, meaning that the seas will continue to rise even if we manage to halt atmospheric warming. So think two, three, or even four meters of rising over the coming centuries.
Back in Bangladesh, what should be done? The government still opposes re-creating mangroves and other natural flood defenses, stating that this would mean “abandoning” some of the delta lands currently occupied by 30 million people eking out a living on the dangerous mud banks. The government still wants man-made defenses.
The World Bank has decided to back its client, despite the evidence of its own consultants that man-made defenses will prove worse than useless. The bank told me that Pethick’s findings will not halt its efforts to upgrade embankments near Khulna, though they will be “taken into account” in drawing up the detailed plans. Maybe so, but it makes you wonder whether an embankment plan of any kind is really the best place to sink all that money. The people of Bangladesh, crammed onto their shrinking sand spits, may be doomed by such engineering hubris.
1. Mondal, M.S. et al. 2013. American Journal of Climate Change, doi:10.4236/ajcc.2013.21007.
2. Pethick, J. and J.D. Orford. 2013. Global and Planetary Change, doi:10.1016/j.gloplacha.2013.09.019.
3. Das, S. and J.R. Vincent. 2009. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.0810440106.
4. Alongi, D.M. 2008. Estuarine, Coastal and Shelf Science, doi:10.1016/j.ecss.2007.08.024.
5. Shepard, C.C., C.M. Crain and M.W. Beck. 2011. PLoS One, doi:10.1371/journal.pone.0027374.
Fred Pearce is an author and journalist based in London. He has reported on environment, popular science, and development issues from 64 countries over the past 20 years. Currently the environment consultant of New Scientist magazine and a regular contributor to British newspapers, he is the author of more than 15 books including The Climate Files, Peoplequake, and Confessions of an Eco-Sinner. His latest book, The New Wild: Why Invasive Species Will Be Nature’s Salvation, is due out in April 2015.