BANGLADESH is one of the countries at most risk from climate change, as it is low-lying and could be swamped by rising seas 鈥� particularly if they rise by several metres (see 鈥�Ice sheets on course for collapse鈥�). Now it seems the very embankments built to protect its people could be making them more vulnerable to floods.
Bangladesh is about to of coastal embankments in the delta region in its south-west. It is using . The bank says the upgrade will 鈥渂uild resilience to climate change鈥�.
But British geomorphologist 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.
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In the past 50 years, Bangladesh has built 4000 kilometres of embankments along the coast. About 30 million people live on polders 鈥� land enclosed by embankments 鈥� or in areas earmarked for poldering. Most of them are fishers or farmers. Many don鈥檛 own their land, and must move around as shifting tides move the sand bars on which they live.
The area鈥檚 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.
鈥淓mbankments create floods if you don鈥檛 do them right. That has certainly been the case in Bangladesh,鈥� says of the University of Nottingham in the UK, who wasn鈥檛 involved in Pethick鈥檚 research.
聯Embankments create floods if you don鈥檛 do them right. That has been the case in Bangladesh聰
With of Queen鈥檚 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鈥檚 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 ().
Pethick says this funnelling effect causes many of the rising tides. He calls flooding in the Sundarbans 鈥渕ainly a self-inflicted wound鈥�, 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鈥檛 want to give up land to rising tides, and local scientists complain that Pethick鈥檚 analysis relies on only three tidal gauges.
鈥淚 do think that the data show an increase in the tidal range, and the impact of poldering is a reasonable explanation,鈥� says 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 of Delft University of Technology in the Netherlands.
Despite Pethick鈥檚 work, Bangladesh is going ahead with upgrading the embankments. The World Bank鈥檚 Bangladesh director, , says Pethick鈥檚 findings will be taken into account.
This article appeared in print under the headline 鈥淏angladesh sea walls may make floods worse鈥�
