The legacy of polders : diagnosing complex flooding processes and adaptation options in the coastal region of Bangladesh

Polders (enclosed coastal embankments) are often constructed to protect river and coastal floodplains in deltas. In Bangladesh, a total of 139 polders have been constructed between the 1960s and 1980s to safeguard agricultural lands from potential damage due to tidal floods, storm surge floods, and salinity intrusion. Whilst such flood control measures reduce the probability of flooding, they have had complex impacts on geomorphological and hydrological processes with consequence for human wellbeing. This legacy of interventions in the coastal system and subsequent human adaptations, now makes decision making even more challenging at a time of increasing environmental pressures. This thesis analyses historical floods in the coastal zone in Bangladesh, diagnosing whether the floods were attributable to monsoonal precipitation (pluvial flooding), high upstream river discharge into the tidal delta (fluvio-tidal flooding), or cyclone-induced storm surges. It quantifies polders’ effectiveness by modelling different forms of inundation to estimate what flooding might have been had the polders not been constructed. Considering the societal impacts of flooding, the effects of changing land-use and flood risk on poverty are quantified. The results answer the questions of (i) how the pattern of land use/land cover (LULC) change influences flood risk at present and continue to do so in the future; (ii) how LULC change and flood risk impact poverty spatially. The last section of the thesis focuses on estimating the impacts of potential Tidal River Management (TRM) on elevating the floodplains and reducing the annual probability of flooding. It identifies suitable TRM sites to model sediment deposition in those areas. The potential impact of TRM on flooding is investigated by comparing flood susceptibility ‘before and after’ the implementation of TRM. By analyzing complex coastal flood processes and adaptation options spatially and quantitatively, this thesis provides new evidence to the effectiveness of the past (polders), and potential future (TRM) adaptation options. It also presents a new approach to flood risk estimation, considering the societal impacts of flooding. The results of this thesis could be of importance both in research and practice for making scientifically informed decisions on coastal flood risk management, particularly in Bangladesh.