The coastal zone in southern Bangladesh adjoining the Bay of Bengal is a low lying area. The zone is subjected to tidal flooding due to very high tides and frequent cyclones coming in from the Bay of Bengal. Since 1960s, the construction of polders along the entire coastal belt provided protection to the people and their agriculture land. A polder is a designated area that is enclosed on all sides by embankments and offering protection against tidal floods, salinity intrusion and sedimentation. The ability to drain the land during low tide that occurs twice a day has made it possible to build 139 polders wherein 1.2 million hectares of land are now under permanent agriculture within the coastal embankment system.
The cyclonic storms have caused much destruction in the coastal zone of Bangladesh. At present, there have been unintended consequences of the coastal embankment system due to climate change. The embankment system had also been affected by siltation of the river system triggered by the massive interference with tidal volumes consequent to the prevention of tidal flooding in the coastal zone. The embankment system was designed originally to keep out the highest tides, without any consideration of possible storm surges. Recent cyclonic storm damages and the anticipation of worse future situations on account of climate change, has caused this strategy to be revised. Coastal Embankment Improvement Program will provide protection against anticipated long term impacts of sea level rise and increased cyclone intensity predicted in climate change projections.
The paper describes the use of a cyclone model in the Bay of Bengal and drainage models in individual polders to design a new system that takes into account future storm surge heights, wave run-up and land subsidence in the design of embankment and the redesign of drainage systems to counter sea level rise as well as changes in precipitation. In this study climate change scenario for 2050 has been developed considering sea level rise of 0.5 m and 10% increase in maximum wind speed of cyclone. The maximum surge height was obtained from 106 pre-selected locations in the model domain, for each location of the 38 cyclone simulations. The locations were selected so that it was possible to have sufficient coverage of the storm surges approaching all 139 polders. The 38 values obtained for each location were then analyzed to obtain the 10, 25, 50 and 100 years return period storm surge levels. In addition, the continuing subsidence of all protected lands in the coastal zone and the resulting uncertainty with regard to the existing standard leveling grid and the crest levels of existing embankments required that a new carefully devised strategy is adopted for design of both the new drainage systems and in setting the embankment crest levels. The proposed improvements have been formulated so that the physical and operational conditions in the polders are made satisfactory under the present and the future conditions taking into account climate change projection.