Tidal River Management (TRM) Schemes in Southwest Bangladesh

Written By Roots and Wings

Authors: Cai Ladd & Emilie Cremin, University of Glasgow

Cyclone Amphan struck the Bay of Bengal on 21st of May 2020 causing extensive flooding of cultivated land across the Ganges and Brahmaputra delta. We are still discovering the extent of the damage and misery caused by the cyclone, but we already know that Amphan is another devastating example of coastal flooding which has become more extensive and prolonged since the mid 20th century. In the Satkhira district of Bangladesh, areas of perpetually waterlogged land have increased 40-fold since 1973 (Islam et al. 2018). In some cases, land flooded by Cyclone Aila in 2009 remained underwater for nearly three years afterwards (Auerbach et al. 2015, and Fig. 1). To understand why flooding has become so much more severe, we need to understand how wider land management has changed across the region.

Figure 1 (A): Polder 32 in southwest Bangladesh which remained underwater for nearly three years after Cyclone Aila struck in 2009; (B): approximate location of Polder 32; (C): diagram from Cornwall (2018) showing dominant directions of sediment movement in southwest Bangladesh.

For generations, local communities practiced “overflow irrigation” to manage cultivation in the delta. Doser badh (community embankments) or osthomasi badh (embankments lasting eight months of the year) were erected along riverbanks, protecting arable land from tidal flooding during the dry season. During the rainy season, when delta waters were less saline, embankments were opened to allow the sweet water in, bringing with it sediment and nutrients which rejuvenated the land for the following year’s harvest. Overflow irrigation was a sustainable practice, perfected over generations (Kibria, 2011). The mid 1960s saw the birth of the Green Revolution in Bangladesh, where a drive for increased food production aimed to boost national GDP. Rice yields in southwest Bangladesh were increased from a single harvest to two to three cuttings each year largely through the construction of earthen embankments in Dutch-inspired polder systems and funded under structural schemes by bodies including the World Bank. However, the polders effectively disconnected tidal rivers from the catchment and set in motion longer-term degradation of the delta.

Because rivers could no longer flood the catchment, sediment instead accumulated in riverbeds (Fig. 2). The drained land also subsided and compacted without the regular supply of water and sediment. In extreme cases, this process led to land behind embankments actually being topographically lower than adjacent riverbeds. Shallower river channels also meant embankments were more easily overtopped during river floods and storm surges, and in addition the now-sunken land behind embankments took longer to drain. By the mid 1980s, flooding was often widespread and the land could be waterlogged for up to six months a year, making rice cultivation impossible (Hussain et al. 2018a).

Figure 2: How rivers silt up and polders subside once the catchment is cut off from the wider river system.

Community-led activism in the 1990s saw the deliberate breaching of some embankments, ostensibly in order to reinstate traditional and sustainable land management practices. Once breached however, the rate of change in the landscape was staggering. At Beel Dakatia, the flooded land accreted at rates of 20 to 50 cm a year. Two years later, more than 1,000 ha of land could once more be cultivated (Al Masud et al. 2018a; 2018b). Restoration of catchment practices was formally adopted by the government under the Tidal River Management (TRM) scheme in 1993.

TRM essentially involves reconnecting catchments with the tidal river without the need for major infrastructure projects. A new dyke is constructed around a portion of the catchment, and sections of the earthen embankments enclosed by the dyke are opened to the river, creating a Beel (Tidal Basin). The Beel is temporarily dammed during the dry season, but when water levels rise during the rainy season, the dam is opened and strong currents, especially during high tides, transport sediment between the riverbed and the Beel (Sterrett, 2011).

Figure 3: Flow of water and sediment through Beel Pakhimara Tidal River Management scheme. Drawing by Emilie Crémin.

Water flowing into the Beel during the flood tide is fast, meaning that sediment is lifted from the riverbed and carried into the Beel. As the water disperses over the surface of the Beel, friction slows the water down and sediment begins to settle out of the water onto the Beel surface (Fig. 3). This deposition process is fastest at high tide, when the water is relatively motionless. After high tide, the ebb flow gradually increases and starts to lift some of that deposited sediment back into suspension. All the water running from the Beel through the river channel lifts more sediment from the riverbed and disperses it further downstream. Some sediment is carried all the way to the ocean. With every tide, there is a twice-daily migration of sediment between the Beel and the riverbed. However, the flood tide is slightly faster than the ebb tide in the Beel due to the fact that, as the same amount of water takes longer to leave the Beel during the ebb tide, the ebb tide is slower than the flood tide. This is known as an asymmetric tide, where the slower ebb current doesn’t pick up quite as much sediment from the Beel compared with the faster flood current. Due to this asymmetry, over time there is a net gain in sediment within the Beel, and a net loss of sediment from the riverbed. This means the Beel surface gets higher as the riverbed incises (Fig. 4). The difference in height gradually reaches an equilibrium as the flood and ebb currents begin to equal (with the asymmetrical tide becoming symmetrical).

Figure 3: Flow of water and sediment through Beel Pakhimara Tidal River Management scheme. Drawing by Emilie Crémin.

Because the Beel surface is higher than the riverbed, the land drains effectively and doesn’t waterlog. TRM is thus an elegant way of restoring the productivity of the land in a sustainable way. Restoring the natural sediment transport pathways helps the delta remain resilient to sea level rise, because sedimentation in the Beel allows the surface to keep pace with sea level rise. Accretion in the Beel can only occur if there’s enough sediment in the delta, however, and this is a future topic for discussion in this blog series.

Whilst TRM offers a way to restore the land and promote sustainable land use in the long run, it does require that some land be ‘sacrified’ to the regular flooding of the tide in the short-term. Sacrificed land is, inevitably, a sacrifice of someone’s livelihood (Gain et al. 2019). Without proper compensation, some communities have been forced to take individual and sometimes drastic action to raise their livelihoods by other means. For example, drainage channels in the Beel Bhayna TRM scheme are being closed in order to farm shrimp, preventing the deposition of sediment and potentially putting the entire TRM scheme at risk (Fig. 5).

Figure 5: An illegal embankment being constructed at a channel mouth in the Beel Pakhimara Tidal River Management scheme.

The success of TRM is dependent not only on a careful understanding of hydrodynamics and sediment flux within deltas, but also on meaningful engagement with communities who depend on the delta for their livelihoods. More information about our scoping trip to Beel Pakhimara in Satkhira District can be found here. We thank Andy Large for his valuable comments on this post.

References:

Auerbach L, Goodbred Jr S, Modal D et al. (2015) Flood risk of natural and embanked landscapes on the Ganges–Brahmaputra tidal delta plain Nat Clim Chang 5:153-157. https://doi.org/10.1038/nclimate2472

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Bangladesh Scoping Visit, March 2020