Storm surge propagation and crop damage assessment in a coastal polder of Bangladesh

Abstract

The coastal zone of Bangladesh, characterized by a wide network of river and canal systems, faces frequent cyclonic storms as well as associated surges causing devastation of human lives, crops and other properties. Increasing population density, rising sea level, poor socio-economic conditions and low adaptive capacity to natural disasters are likely to increase the coastal people’s vulnerability especially to water related disasters. In the south-west coastal zone, losses and damages due to storm surges in a coastal polder are largely caused by embankment failure which is often the result of human interventions along with natural reasons. This study investigated the storm surge propagation processes into Polder 32 of Dacope upazila of Khulna district and assessed the associated losses and damages, considering Cyclone Aila (2009) as the base event. This study involved the application of a numerical model, called Delft3D, and a mapping tool, called ArcGIS 10.2, to simulate the inundation scenarios associated with polder failure. Primary and secondary information from different sources were analyzed to assess the losses and damages in different sectors including household and agriculture for two modes of embankment failure, namely breaching and overtopping. Primary data were collected using Participatory Rural Appraisal (PRA) tools including social and resource mapping, Focus Group Discussions (FGDs), group discussions, individual interviews and Key Informant Interviews (KIIs). Secondary data were collected from different Government and non-Government organizations and published scientific literatures. The study found that human interventions such as insertion of pipes and holes through the embankment to allow saline water into the shrimp gher aggravated the polder failure during Aila. Results showed that the breaching rate was faster in the earlier period of the event where almost 50% of the total breaching took place within the first 6 hours of the event. Model simulations showed that approximately 25%, 80% and 90% of Polder 32 were inundated due to overtopping, breaching and combined breaching and overtopping, respectively. It was found that 50% of the total impacts were immediate impact, whereas 16% were short term impact. About 58% and 27% of the estimated total impacts in Polder 32 were on agriculture and household sectors, respectively. Crop production dropped down from 4.0-4.5 t/ha to 3.75-4.0 t/ha in the fourth year after Aila though for the first three years the loss was 100%. About 34% of the total impacts in the study area were accounted as long term impact. It was also found that 88% of the long term impacts were only in the agriculture sector. The study concluded that the severity of losses and damages due to Aila was aggravated due to long term inundation inside the polder which was the consequence of breaching of Polder 32, and various human interventions to the polder was a major factor responsible for severe polder breaching.