Geospatial analysis of the depletion of surface water body and floodplains in Dhaka City (1967 to 2008) and its implications for earthquake vulnerability

Abstract

Dhaka city the capital of Bangladesh experienced unprecedented urban growth since the country’s independence in 1971. A significant portion of the urbanization took place in the encroached surface water body and floodplain (SWB and FP) areas. We thus aimed to capture the depletion of SWB and FP in Dhaka city from 1967 to 2008 using geospatial analysis and evaluated its implications for earthquake vulnerability. We used CORONA (1967) and QuickBird (2008) satellite images to compare the pre- and post-urbanization status of SWB and FP. We identified a striking change occurred during this period with a total loss of 33% of SWB and FP areas. The channelized water body faced a depletion of 15%, while the open water body which mainly includes the floodplains experienced a loss of 38%. The urban structures built on the depleted SWB and FP are highly vulnerable to ground motion and liquefaction of earthquake. We found that 90% of the depleted areas have a shear wave velocity in the top 30 m of soil thickness (Vs30) of 180–360 m/s, while the rest has a Vs30 value of 15. We produced an earthquake vulnerability map based on the information of surficial geology, Vs30, LPI and found that substantial areas near the southern, eastern, and western boundaries of the city scored highly in the vulnerability map. The results of the study can support formulation of rational earthquake preparedness strategy for Dhaka.