Sage Vals comments on the Nature news piece ‘Holding Back the Tide’ by Quirin Schiermeier (1)
This is an interesting article that contains a few questions for both flood and sediment management, and raises concerns regarding the nature of funding for international development projects
6000 km² of the Ganges-Brahmaputra delta is less than 2 m above sea level. It is therefore highly susceptible to flooding, especially when tropical cyclones sweep through the Bay of Bengal, leading to much damage and loss of life.
Quirin Schiermeier notes that global warming is raising sea levels by 2 to 3 mm per annum, but “relative sea-level may be rising by up to 2 cm each year” in this delta. It is clear then that climate change is not the main problem facing the residents of the delta area of Bangladesh. The problem is that the delta itself is sinking over time at a much faster rate than any sea-level rise that can reasonably be associated with global warming.
More than 1bn tonnes of sediment every year is deposited into the Bay of Bengal. This is derived from erosion of the Himalaya and carried down the rivers into the Ganges-Brahmaputra delta. This sediment is easily compacted and so the delta is slowly sinking under the pressure of its own weight, therefore there is a relative sea-level rise. In a natural situation, it would be expected that additional sediment settling on top of the delta would largely off-set this sinkage leading to an equilibrium position, at least on a centennial scale. “But”, says Schiermeier, “agriculture, industry and hydroelectric dams have diverted water and choked off the flow of sediments, so the land is no longer being rebuilt”.
The delta’s behaviour differs from one place to another, and there are varying subsistence rates in different places. “Geological forces and erosion have shifted the lower stretch the Ganges steadily to the east, leaving the western parts of the delta especially starved of sediment”. This means that the south-west part of the delta is particularly vulnerable to flooding.
Some parts the delta are sinking more slowly. Schiermeier reports evidence for this derived from the burial of the bases of ancient mosques and temples. This apparently suggests that the areas in the west of the delta are sinking at a rate of only 1 to 2.5 mm per year.
The effects of earthquakes can exacerbate the rate of delta sinkage. Schiermeier cites the earthquake in 1762 which was of magnitude of 8.8. At this time the land around Chittagong sank by several metres. Another such quake could, and this is highly likely given the unconsolidated nature of the delta material, produce a further significant sink in land levels.
Failures of Mitigation
Attempts at flood control seem to be making things worse. Schiermeier describes places where embankments along the tidal channels have reduced the area of land which can be covered by water at high tides. Thus, in places where the tide water can spread out, the water goes farther inland. This increased tidal range in the less protected areas of the Delta means that some areas have experienced average annual high-water-level increases of 15 to 20 mm. This process is called ‘tidal amplification’.
Schiermeier also gives the example of Polder 32, an area so-called because of the help of the Netherlands in creating it during the 1960s. This area sank by 1m over a period of 50 years (i.e. 2cm y-1) relative to the land outside it. This was due to its embankment preventing the influx of water so that there was no new supply of silt to replenish it. But the breach of dykes due to Cyclone Aila, meant that for the two following years there was no full protection from flooding in the area. As a result tens of centimetres of extra sediment from daily tides entered the area, thus reducing the amount of overall sinkage in Polder 32.
The problems of subsidence and associated flooding led has to a joint Netherlands and Bangladeshi programme, the “Bangladesh Delta plan“. The plan looks at the effects of this sea-level rise in the delta and will make recommendations for, and implement, improvements in delta management.
One suggestion is to return to the style of embankments that were used by local people before the 1960s. These temporary embankments were raised in the dry season to keep out the salty water, but partly dismantled in the wet season to allow sediment in. Homes in the areas so managed are usually set on higher ground which allows flooding to take place without disturbance to the residents.
Schiermeier reports that “in its 2011 to 2015 economic plan, Bangladesh earmarked more than… $1.15 billion… for climate adaptation and disaster management. Furthermore it is currently channelling a $117 million multi-donor climate-change-resilience fund, set up in 2010 into projects including flood protection. More money could come from the multi-billion-dollar international Green Climate fund”. Given the scale of this and other suitable aid funding, Schiermeier notes that there is some scope for optimism for resolving the issues of flooding in the delta region in the foreseeable future.
It is to be welcomed that the problems of delta subsidence in Bangladesh is being properly addressed by both the scientific community and by governments.
The importance of the natural cycle of erosion, transport and deposition of sediment to deltas, and the concomitant subsidence of the delta cannot be ignored. In the longer term, failure to recognise and hence accommodate these processes will always lead to longer-term problems where human interventions are concerned. This has certainly happened in the delta as described in this article. Indeed, it was a contributing factor to the flooding in south-west of England this winter. It should also be noted that, in many ways, traditional or traditionally-inspired methods are better.
It is clear that, so far at least, flooding problems in the Ganges-Brahmaputra delta have little or nothing to do with climate change, despite a good deal of comment to the contrary. However, poor management of the delta itself and of the silt-flow from the Himalaya is causing problems. Of course, one could argue that an increase in cyclone activity might be due to climate change, although at time of writing at least, this is very debateable. The IPCC suggests a 50% chance of increased hurricane activity, but this prediction is limited to the Western North Pacific and North Atlantic (2) (and, based on my own numbers, there seems little evidence even in these areas so far). Increased migration away from the delta cannot therefore be construed as the movement of ‘climate refugees’.
It is understandable that a relatively underdeveloped country such as Bangladesh requires some international funding to help with such major projects. However, it does seem quite incredible that so much funding is given on the basis of climate change where there is no suggestion that this is actually causing the problem. It does raise the question of the political nature of ‘climate change’, in that, as here, it would seem to be simply a way of justifying government aid. In this case, I would suggest that a straightforward humanitarian justification is all that is actually needed. It would be a cause for regret if the only way that funding could be found was under a pretence.
1. Schiermeier, Quirin. Holding Back the Tide. Nature. April 10, 2014, Vol. 508, 7495. (For the avoidance of doubt, this is a ‘News Feature’ piece, not a peer-reviewed article or letter).
2. IPCC. Summary for Policy Makers. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change . 2013. Fig SPM.1