Bangladesh is one of the countries at most risk from climate change, as it is low-lying and could be swamped by rising seas – particularly if they rise by several metres (see “Ice sheets on course for collapse”). Now it seems the very embankments built to protect its people could be making them more vulnerable to floods.
Bangladesh is about to upgrade 600 kilometres of coastal embankments in the Sundarbans delta region in its south-west. It is using $400 million from the World Bank. The bank says the upgrade will “build resilience to climate change”.
But British geomorphologist John Pethick says the upgraded embankments will put millions at risk. He has angered Bangladeshi scientists by arguing that coastal defences in the Sundarbans delta are doing more harm than good.
In the past 50 years, Bangladesh has built 4000 kilometres of embankments along the coast. About 30 million people live on polders – land enclosed by embankments – or in areas earmarked for poldering. Most of them are fishers or farmers. Many don’t own their land, and must move around as shifting tides move the sand bars on which they live.
The area’s low-lying river deltas are disappearing beneath the waves faster than can be explained by global sea level rise. Conventional thinking points the finger at land subsidence, but Pethick has evidence that the embankments are the problem.
“Embankments create floods if you don’t do them right. That has certainly been the case in Bangladesh,” says Colin Thorne of the University of Nottingham in the UK, who wasn’t involved in Pethick’s research.
With Julian Orford of Queen’s University Belfast in the UK, Pethick found that on the Pussur estuary, high tides are rising 16 millimetres a year, five times faster than mean sea level.
That is partly because the delta is subsiding. But the pair say the embankments must take much of the blame. They constrict the width of the delta’s estuaries. As the same volume of water now has to pass through a narrower channel, high tides rise higher, causing deeper and more widespread floods (Global and Planetary Change, doi.org/s8h).
Pethick says this funnelling effect causes many of the rising tides. He calls flooding in the Sundarbans “mainly a self-inflicted wound”, and advocates both abandoning some of the polders and setting embankments further back from estuaries to reduce the funnelling effect.
The findings are controversial. Bangladeshis don’t want to give up land to rising tides, and local scientists complain that Pethick’s analysis relies on only three tidal gauges.
“I do think that the data show an increase in the tidal range, and the impact of poldering is a reasonable explanation,” says Michael Steckler of Columbia University in New York. He is making the first GPS measurements of sea levels in the area.
Tidal amplification is common in estuaries and river deltas around the world, agrees coastal engineer Marcel Stive of Delft University of Technology in the Netherlands.
Despite Pethick’s work, Bangladesh is going ahead with upgrading the embankments. The World Bank’s Bangladesh director, Johannes Zutt, says Pethick’s findings will be taken into account.