Polders are low-lying tracts of land surrounded by embankments. The walls, along with pumps and sluice gates, regulate the flow of water through the land and prevent uncontrolled flooding. This system originated in the Netherlands, where 26% of the country is below sea level; therefore managing water in this way is a necessity for the very existence of land.[1] Working from the Netherlands’ example, 139 polders have been built in Bangladesh since the 1960s. Today, the Bangladeshi polder system encompasses 1.2 million hectares of land, (almost half of the 2.8 million hectares defined as coastal), and houses more than 8 million people.[2] As the delta and coastal regions of Bangladesh are comprised of floodplains and traversed by extremely powerful and transient rivers, this system of embankments works to keep the land within them unflooded and safe for houses and crops. The polder system has been extremely effective in controlling flooding in the Netherlands, but Bangladesh’s polder system faces different challenges due to the differences in topography, culture, and political structure.
The embankments surround huge, irregularly shaped tracts of land (Figure 1).[3] Within each polder, sluice gates allow canals to fill with river water, which is then available for irrigation and household use and drains through other sluice gates at the downstream end of the polder (Figures 2 and 3).[4][2] Enclosed by these embankments, the polders are home to crop fields, canals, houses, people, and livestock. Outside the polders, water is everywhere–river water, seawater, and the brackish water in between (Figure 4).[5] The drainage of the polders relies largely on the force of gravity; the openings of the sluice gates must be above the outside water level in order for water to drain passively. If the embankments are overtopped by water or otherwise break, or if the drainage clogs, the land within the polders floods.[6] Houses and possessions are damaged or lost, livestock may drown, and crops are flooded and eventually rot. The floodwater often takes months to recede.[7] In Figure 5, all of the water on the far side of the new embankment is floodwater, covering what used to be dry land and partially submerging many houses.[7] If polder embankments were to fail or be lost entirely, 8 million people would be at the mercy of flooding. While inhabitants of rural Bangladesh managed to live and work in the floodplains before the implementation of the polder system, communities have now adapted to the polders and their loss would likely cause mass population displacement inland, the brunt of which would be absorbed by Bangladeshi cities. These urban centers are already projected to double in population by mid-century with current growth and displacement rates, and therefore likely unable to handle the increased level of rural-to-urban migration that would follow a loss of the polder protection.[8]
Despite its importance to the people of rural Bangladesh and its achievements over the past six decades, the polder system is deteriorating and climate change is likely to hasten its degradation. Even if the embankments were in perfect condition, anticipated rising sea levels would lead to extreme flooding if no mitigating action were taken. Climate change is also causing an increase in severe weather, which is expected to lead to heightened precipitation and river surges that overtop riverside polders and create flash floods.[6] These floods would be unpredictable and highly damaging to crops and infrastructure. Similar flash floods could occur in coastal polders due to oceanic storm surges. If current trends continue, the sea level of coastal Bangladesh is expected to rise 1 meter or more by 2100.[6] This increase would overtop embankment walls and flood coastal polders with saltwater, making large swathes of coastal area uninhabitable. Coastal flooding can be disastrous, as saltwater inundation decreases soil fertility and crop yields.[9] The impact of climate change on severe weather also includes a rise in the intensity of cyclones, which can easily breach or overtop polder walls and devastate the communities within.[6] Even if the polder system worked perfectly today, climate change would soon lead to its failure.
The polder system, however, does not work perfectly, and its current deteriorated condition significantly heightens the threats presented by climate change. In recent decades, embankments have fallen apart, drainage canals have become clogged with sediment, and the buildup of sediment outside the walls has led to lowered embankment heights relative to the ground level.[2] [6] [7] This existing deterioration is only accelerated by climate change, as cyclones, storms, and floods bring greater damage to polders and, in a vicious cycle, leave them more vulnerable to the next round of disasters.
There is no single or simple solution to this issue. We hesitate to provide solutions at all, knowing that we have never been to the affected region and there are doubtless holes in our knowledge. However, based on the information we have gathered and our analysis of the existing structure, we have developed several suggestions regarding adaptations to the current Bangladeshi polder system.
We propose, first, dredging sediment from the land directly outside polder walls. As sediments are deposited outside the embankments, the ground level outside the embankments gradually rises. As this lessens the relative height of the walls as the distance between the water and the top of the embankments shrink, it slowly compromises the polders’ effectiveness. This sediment, once dredged, can be repurposed to strengthen and raise damaged or insufficient embankments, or deposited inside the polders. The soil inside the polders is slowly subsiding as it dehydrates, and agriculture is using up its nutrients. Depositing fresh sediments would serve to reinvigorate the soils and counteract some of the subsidence. This plan uses unwanted sediments to benefit both the embankment structures and crops within, and therefore is advantageous in multiple ways.
Second, we propose sectioning areas of individual polders off into subpolders, some of which would then be designated as flood areas. This would involve construction of additional embankments within polders, completely blocking off water flow so that water entering one subpolder cannot spread to the surrounding land. These ‘sacrifice’ subpolders would line the coast. Therefore, if coastal flooding occurred and seawater entered the polder area, it would be contained within these designated subpolders and the flooding would not be as extensive. However, saltwater intrusion in these subpolders would damage the soil productivity and make these areas undesirable for agriculture; another use for the land is therefore necessary. In recent years, Bangladeshis whose land became too saline have started switching to shrimp farming. While shrimp farming is beneficial to the country’s GDP, (as of 2014-15, shrimp farming comprised approximately 6% of Bangladesh’s fish production), and makes use of land unfit for agriculture, it comes with complex social costs.[10] The poorest rural citizens cannot afford to start shrimp farms, and shrimp farming requires fewer workers than agriculture. Therefore, the spread of shrimp farms leaves many poor Bangladeshis unemployed.[11]
While the social impacts make widespread shrimp farming an undesirable option, coastal subpolders designated for flooding will be incapable of supporting traditional agriculture and therefore would be good places for shrimp farms. If any more shrimp farms are created, they should be limited to these areas, thereby preventing the negative social impacts from occurring on a larger scale.
None of these initiatives will have substantial benefit if the polders are not maintained. Currently, there are no structures for centralized management, maintenance, and repair of polders. The creation of a management structure for each polder, through local governments, could prevent waterlogging by ensuring that polder drainage is frequently cleared of blockages. Such a structure could also ensure that any breaches are repaired more efficiently.
Through a combination of these efforts, an overhaul of the polder system should help the people of Bangladesh to remain in their homes and jobs, providing a measure of protection from the looming threats of climate change.
Figure 1: Map showing polder boundaries throughout coastal Bangladesh
Source: Blue Gold Bangladesh[3]
Figure 2: Polder with sluice gate to allow entrance of river water into canals
Source: Ortega, published in Climatic Change[4]
Figure 3: Water draining from polder sluice gate to river
Source: CGIAR Research Program on Water, Land, and Ecosystems[2]
Figure 4: Polder embankment with water outside and cropland inside
Source: Cornelia Paetz, published in World Food Programme[5]
Figure 5: Bangladeshi citizens work to build new embankment after a flood
Source: Dilshanie Perera, published in University of Michigan Journal of Sustainability[7]
By Caroline Boone
References
- Schiermeier, Quirin (5 July 2010). “Few fishy facts found in climate report”. Nature. 466 (170): 170.
- Pukinskis, Ilse. (2015). The Polder Promise: Unleashing the Productive Potential in Southern Bangladesh. CGIAR Research Program on Water, Land and Ecosystems. Retrieved from https://wle.cgiar.org/polder-promise-unleashing-productive-potential-southern-bangladesh.
- Blue Gold. (2013). Map of Bangladesh Coastal Polders. [Map]. Retrieved from http://www.bluegoldbd.org/more-information/maps-old/.
- Ortega. (2009). Sluice gate in Dacope, Bangladesh. [Photograph]. Retrieved from https://www.researchgate.net/figure/295883223_fig3_Figure-S3-Photograph-of-a-typical-polder-earthen-embankment-above-source-Ortega
- Paetz, Cornelia (2013). Embankment in Bangladesh. https://www.wfp.org/photos/gallery/rising-waters-%E2%80%93-helping-communities-cope-disaster-bangladesh
- Bangladesh Delta Plan 2100 (pp. 1-714, Rep.). (2017). General Economics Division, Bangladesh Planning Commission.
- Perera, Dilshanie. (2016). Uncertain Waters: Deltaic Interventions in Bangladesh amid Precarious Conditions. Michigan Journal of Sustainability, Vol. 4. http://dx.doi.org/10.3998/mjs.12333712.0004.003
- Streatfield, P. K., & Karar, Z. A. (2008). Population Challenges for Bangladesh in the Coming Decades. Journal of Health, Population, and Nutrition, 26(3), 261–272.
- Salinity in Agriculture. (n.d.). United States Department of Agriculture, National Resources Conservation Service. Retrieved from https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/quality/tr/?cid=nrcs143_010914.
- Hussain, Monawar. (2016, February). Fisheries Statistics in Bangladesh: Issues, Challenges and Plans. Retrieved from http://www.fao.org/fileadmin/templates/ess/documents/apcas26/presentations/APCAS-16-6.3.2_-_Bangladesh_-_Fisheries_Statistics_in_Bangladesh.pdf.
- Paul, B. G., & Vogl, C. R. (2011). Impacts of shrimp farming in Bangladesh: Challenges and alternatives. Ocean & Coastal Management 54, no. 3, 201–211.