バングラデシュ東部ダッカの都市洪水に対する住宅建築の脆弱性と改善行為に関する研究

抄録

 Bangladesh experiences perennial flooding as it is at the confluence and delta area of the Ganges, the Brahmaputra, and the Meghna rivers. In recent years, perennial flooding in urban areas has led to serious situations, which has become a challenge beyond the capacity of local governments. Therefore, this paper aims to investigate the physical vulnerability of the residential buildings and residents’ house modifications during floods in eastern Dhaka. The study area, Sabujbagh was chosen for this research is divided into 10 mahallas (neighborhoods), each consisting of 250-500 households. Two mahallas, Uttor Basabo (M1) and Purbo Basabo (M7) were selected for study based on their previous flood experience, urbanization history, topography, and physical conditions. Therefore, a community survey was undertaken in these two neighborhoods (M1: 55 and M7: 50 households) based on 7 major attributes.

 The results showed that although it is usually assumed that the buildings with impervious surfaces are more likely to be damaged by flooding, in M1, 12 of the 32 surroundings (non-permeable) buildings were not damaged as the plinth level was high. On the other hand, 4 buildings having surroundings with permeable (green) surface were damaged, and 6 buildings having a setback with permeable (green) surface were severely damaged. This was found to be due to the low plinth level and insufficient capacity of the drainage system. A similar trend was evident at M7. In relation to the plinth level, in both M1 and M7, it was found that buildings over 20 years old have a relatively low plinth, and buildings built within 20 years have a relatively high plinth. It has been observed that 2 houses in M1 and 1 house in M7 with higher plinth level experienced flooding. The reason was drainage congestion and blocked sewer lines in M1, and overflow of the pond behind the building in M7.

 In both M1 and M7, more than 60% of buildings were damaged by floods. All buildings built with simple materials (such as mud, thin steel plate, wood, etc.) were damaged in both M1 and M7. However, some buildings built with durable materials (brick, cement, concrete, etc.) were also damaged in both areas. A total of 18 cases were damaged, due to the low plinth level in M1, while in M7, due to overflow of nearby ponds and sewer blockages. The damage is this case was due to drainage infrastructure conditions. The survey also revealed physical improvements by residents to reduce flood damage were 62% and 58% in M1 and M7 respectively. According to the interviews, some residents did not improve their houses because they considered flood damage as an infrastructure problem that should be solved by the local government. Hence, buildings that were not improved were experienced damage. The survey also showed that there is no community-level management system to reduce flooding. The study suggests that measures to address flooding by ensuring water permeability of land surface is not sufficient and should be combined with increasing plinth height and improving drainage infrastructure. Also, land use management and integrated community flood management system are recommended.