Climate Change: Meeting sea level rise by raising the land
Community groups in coastal Bangladesh have shown that the land can be raised to match sea level rise. Their success has been hard fought, initially contested by aid agencies, engineers and the police. But they have won over policy makers and scientists who realise this will be important in the struggle to adapt to climate change.
Most of Bangladesh is a huge delta built by major rivers flowing down from the Himalayas. The land is rich and this densely populated country is self-sufficient in rice. But it is also flat and subject to devastating floods and cyclones, which will be made much worse by climate change. Since independence in 1971, Bangladesh has made substantial efforts to reduce the damage done by the dramatic disasters. And since the disasters will be made worse by climate change, for three decades Bangladeshi scientists, politicians and communities have been working to adapt to climate change and to press industrialised countries to curb greenhouse gas emissions. We tell this story in our new book “Bangladesh confronts climate change“. And Bangladeshis will again play a leading role in the annual climate change negotiations, this year in Marrakech from 7th-18th November.
One quarter of the population lives in the coastal southwest, which is less than 3 metres above sea level, most at risk to floods and cyclones, and most vulnerable to climate change. Soon after independence, donors pushed a disastrous scheme to build 6,000 km of dykes and embankments to create Dutch style polders. They failed to realise the heavy monsoon rains would flood the polders behind the embankments. More than 130,000 ha are so waterlogged that crops cannot be grown. Tens of thousands of farmers abandoned their land and moved to the cities. But some communities began to cut holes in the dykes to allow the polders to alternately drain and flood, adopting and updating a 18th century system that used the natural actions of the delta.
The delta has been built up because for millennia more than one billion tons of sediment flows down each year in the rivers from the Himalayas. During the monsoon, huge water volumes in the rivers and natural flooding of the land prevent tidal intrusion and push the sediment out to sea in the Bay of Bengal. But in the dry season, tidal water reaches hundreds of kilometres into the country, bringing some of the sediment back. As each high tide covers the low lying land, sediment is dropped on the land, naturally raising the level.
Recent studies have shown that the sediment naturally compacts, lowering the land level. Other research has confirmed that sea level is rising. But studies also show that in protected coastal areas without embankments, that low lying land which was just above sea level 300 years ago still is, because natural processes slowly raise the level of the land.
But the interaction of tides and monsoon has an unusual effect in the southwest, where there are two different kinds of floods at two different times of year. In the monsoon, rainfall floods the land and allows rice to be planted. Then in the dry season high tides pass over this land and drop sediment.
The Mughal rulers in the 18th century developed a system to use both floods effectively. What were called “eight month embankments” were constructed during the monsoon, trapping rain to grow rice in flooded fields and keeping out any salt water. After the harvest, the dykes were then cut to allow any remaining rainwater to drain to prevent waterlogging, and to allow tidal seawater and sediment flooding in the dry season. The seawater is saline but the next monsoon rains wash out the salt, allowing a rice crop.
This system partially continued into the 20th century and there was a local understanding of the flood patterns – and of the way aid-funded dykes ignored this pattern. Two decades ago, communities began cutting the dykes, leading to bitter fights with government and the police. At Beel Dakatia the whole community turned out, not just with spades and buckets but with drums, pipes and other musical instruments. Cutting the dykes raised the land level inside the polders by one metre or more over three years, ending the waterlogging. The success in raising land levels and ending waterlogging won over first local councillors and then the government’s respected Centre for Environmental and Geographic Information Services (CEGIS), which also does much of the modelling for climate change planning. CEGIS and communities developed what is now called Tidal River Management (TRM) in which polders are opened one at a time for three years and allowed to flood in the dry season, raising the land level. This will need to be done every 30 years, but can keep up with climate change induced sea level rise, and has been adopted as government policy.
It is not a story known in industrialised countries, because it was done by Bangladeshis and not foreign consultants (who always seem to assume Bangladesh’s coast and rivers can be tamed in the same way as those in Europe or the United States). But Bangladesh will need help from the industrialised countries. First they need climate change to be curbed, because TRM can cope with some sea level rise but not the massive rise that will be caused by unchecked greenhouse gas emissions. Second, Bangladesh is still a very poor country and the farmers and landless workers who lose some income during the three years of TRM floodings must be compensated. The Bangladesh government budget already pays three quarters of all its spending on climate change, with only one quarter coming from donors and lenders – must Bangladesh pay to raise the land in response to the sea level rise caused by the industrialised countries?
Bangladesh confronts climate change: Keeping our heads above water is being launched on