Mitigating Daily Tidal Floods in Bangladesh

The social-ecological system (SES) on the tidal floodplains of the Ganges-Brahmaputra river delta of coastal Bangladesh is one that typically generates a compassion response in people of Western culture, and with good reason. Geographic location and regional conditions historically subjected the area to twice-daily tidal flooding that kept the resident population a small one of poor subsistence farmers who frequently had to deal with insecure and insufficient food resources. Starting in the late 1960s, therefore, the Bangladeshi government instituted the Coastal Embankment Project to improve area conditions. They divided the coastal river delta floodplain into an extensive waffle-like grid of levee-bounded areas called polders that are protected from daily tidal floods. The polders created for flood control permitted the government to introduce commercial shrimp farming opportunities to local residents, and also provided places for other kinds of larger-scale agriculture. Both activities provided people with economic opportunities and increased food security. Population density in the area dramatically increased as a result of these environmental and socio-economic interventions, and the commercial elements of economic activity added external components to the complex social-ecological system (SES) of the region, through global export activities.

In 2017, resilience ecologists Asif Ishtiaque, Nikhi Sangwan, and David Yu analyzed the outcomes of these combined social and ecological interventions, pointing out that the dynamic interplay between social, infrastructural, and hydrological processes in this SES made it a vital case study for understanding the impacts of human intervention on complex social-ecological systems that have been engineered (Ishtiaque, Sangwan, and Yu, 2017; this publication is in the e-journal of resilience ecology, Ecology and Society and the link takes you to a downloadable copy of the paper and its numerous diagrams and maps). They noted that while resilience focuses on a self-organized ecosystem’s capacity to take temporary disturbances in stride, the engineering field’s concept of robustness is a more appropriate concept to apply to an SES that has been even partly engineered. “Robustness focuses on how engineered systems maintain a specific system output when subjected to a much narrower set of known disturbances” than those that would challenge system resilience. Some evaluators have confused robustness and resilience, so this is an important distinction to note: robustness is a property of complex systems that have been engineered for reduced variability in some of the system’s parameters. Robustness is a very specific thing, and it is not a natural property. In this situation, resilience refers to the ability of the coastal deltaic socio-ecological system to persist in the presence of twice-daily flooding from tides. But once the system was engineered into the waffle-like  pattern of levees and polders, flood episodes were reduced and water level variability (flood frequency and amplitude) decreased. So now robustness is the more appropriate characteristic to consider, not resilience. And robustness refers to how well the engineered system persists (continues to function as designed, to “maintain a specific system output”) when its integrity is challenged by the narrower set of known disturbances that remain in the system. For instance, cyclones occur whether polders exist or not. The levees provide at least some storm surge protection in cyclones that the area didn’t have before it was engineered. Robustness looks at (as one example) how well the levee-polder system continues to function when challenged by the far less frequent, and presumably less intense (because of the levees), flooding from a cyclone’s storm surge.

It turns out that suppressing the twice-daily tidal floods by engineering some parts of the complex river delta system created unexpected interactions between the system’s components. The surprising result was emergence of fragility elsewhere in the system. Increased robustness in the economic system resulted in the emergence of unexpected fragility in the region’s economic system too. This pattern of increased fragility in both ecological and economic systems in response to increased robustness has been observed in other engineered SESs as well. It’s a frequent enough occurrence to have earned its own acronym: RFTO, meaning “robustness-fragility trade-off.” Ishtiaque et al (2017) explain: “The notion of robustness-fragility trade-offs (RFTOs) stipulates that by becoming robust or insensitive to a particular set of disturbances, systems necessarily become fragile or sensitive to disturbances outside that set. . . Such RFTOs are dangerous because emergent fragilities are usually hidden and are only revealed through failures.” In the typical robustness-fragility trade-off, managing system variance for short-term increase in system robustness builds up hidden fragilities whose impacts occur over longer time scales.

In the coastal deltaic area of Bangladesh in question, regular tidal daily floods stopped being as much of a threat as they’d once been, but land subsidence caused in part by interruption of processes of sediment-deposition by the rivers, and more consequential impacts of cyclones on the engineered systems — because non-natural structures such as levees are still subject to natural processes of erosion — created new problems that required, for example, unexpected and long-term investments in infrastructure maintenance. But this new system fragility wasn’t apparent until storms had a catastrophic impact on the subsided landscape and its polder levees, damaging the commercial farming and aquaculture activities that now occupy the polders. Because the area’s human population is larger and denser now than it was before this initiative changed it, land subsidence means that when cyclones damage the polder levees and flooding ensues, it impacts far more people and with greater financial consequence and threats to personal safety. The longer the system operates, the more it continues to move in the direction of increasing fragility. It will function very robustly, doing what it was designed to do, as long as it is protected from small outside-the-design perturbations that would cause it to fail in increasingly catastrophic ways. As the land continues to subside because the rivers can’t function normally, the polder levees will have to be built higher and higher to keep out normal tidal floods and periodic storm surges, so maintenance expenses will escalate over time. Of course, this is what John McPhee surmised more than three decades ago.

As Ishtiaque, Sangwan and Yu point out (2017), “many of these fragilities are usually hidden and structural measures designed to suppress similar disturbances result in overestimation of robustness among the coastal community. The structural measures successfully reduce the exposure to disturbances, yet it has been noted by numerous studies that they eventually result in increased vulnerability because the reduced exposure causes the loss in societal memory of floods, thereby encouraging increased development in the risk prone areas . . . and is another example of RFTO between robustness in the short term and fragility in the long run.” This leads to a loss of resilience in area residents that Montz and Tobin (2008) called the “levee effect,” now recognized in communities worldwide (Ishtiaque et al 2017).

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