バングラデシュ北西部の地形・洪水特性からみた水害対策計画の問題点

抄録

The World Bank has made “Bangladesh Action Plan for Flood Control” which consists of 26 components after severe floods of 1987 and 1988. Japanese and UK governments donate the Component No.2, for Northwest Region. The author attended the preliminary study in 1990 as a geomorphologist and found some problems on flood mitigation projects related with characteristics of landform and flood feature.
The northwest region is divided into the following four geomorphological units: 1) Barind Tract, 2) the Himalayan piedmont plains, 3) the Alluvial lowland along the Brahmaputra-Jamuna and 4) the Alluvial lowland along the Ganges. The embankments are frequently broken by floods because river beds are unstable in the Himalayan piedmont plains. In the alluvial lowland along the B.-Jamuna, the river channels are unstable and change their courses frequently during annual floods with breaching embankments, since the B.-Jamuna changed its course in the 18th century. In the alluvial lowland along the Ganges, distinct large natural levees have developed along the Ganges and a broad back-marsh area lies along the lower Atrai Basin. The “Polder Project” for flood protection is progressing in the lower Atrai Basin. However, each polder is too broad to protect the whole area. In order to protect their houses, people often cut embankments for draining inundation water not only from inside of the polder to river, but also from outside of the polder into the protected area.
In the Flood Action Plan, the World Bank proposed two major projects in this area, namely, an interceptor drain from the Upper Atrai into the B.-Jamuna, and a diversion channel from the Lower Atrai into the Ganges. The main technical problem on both the Interceptor and the Diversion channel is the water level at the outfalls. The Interceptor is proposed along the Karatoya-Atrai, a heavy free-meandered river. The Interceptor needs high embankments whole from the Atrai to the B.-Jamuna. The Diversion channel is less feasible ; since the river bed of the Atrai is lower than that of the Ganges.
Apart from these major projects, the author suggests smaller but reasonable projects for this area. In the southern part, the Polder Project is to be improved: smaller units of polders with consideration of geomorphological setting and local land-use conditions. A comprehensive project which contains both structural and non-structural methods for flood control and rural development programs as waterside district will support the development of this area.