Evaluation of Dhaka city flood protection embankment under 1998 flood condition

Abstract

Embankment is the most favoured option for flood control due to its simple construction method and use of local labour and manpower. In 1987 and 1988 Bangladesh experienced two of the most severe floods on records. Vast areas of the country including the capital city Dhaka were flooded to an unprecedented degree with flood level 1.50 m higher than normal for periods up to four weeks. In the wake of these floods, in October 1988 the government of Bangladesh (GOB) established a committee for flood control and drainage of greater Dhaka with the primary objectives of preparing a flood control plan for Dhaka metropolitan area, based primarily on the 1987 JICA study on storm drainage system improvement for Dhaka city and the 1988 " Jansen Report" on cause of 1988 flood and recommended solutions. In January 1989 the committee submitted a detailed scheme for phased investment in flood protection. In view of the high priority, the govenunent immediately initiated phase 1 of the recommended works on a crash programme basis using their own resources. These works included construction of about 30 km of embankment and 7 km of protection wall along the westerly periphery of the city to provide protection to about 136.50 sq. km. in the higWy urbanised westerly part of the city. The recent flood in 1998 is one of the catastrophic flood in the history of Bangladesh. The 1998 flood is specially characterised by its prolonged duration. In 1954 the water level above danger level continued for 19 days and that in 1988 flood continued for 25 days while in 1998 the water level above danger level exceeded even more then 6 weeks at many places. 1998 flood is the fIrst severe flood that Bangladesh experienced after the construction of Dhaka city flood protection embankment. The objective of the study is to evaluate the effectiveness of the existing flood protection embankment for Dhaka city with respect to the 1998 flood conditions. The evaluation is done based on fIeld visit and collection of data from the Flood forecasting and Warning centre of BWDB. The fIeld visit started from Tongi Railway Bridge and covered the whole peripheral embankment along Turag river up to Kellar mohr and the DND project area. During fIeld visit few locations were identifIed where the embankment suffered failures. The evaluation is carried out with respect to various causes leading to failure of earthen embankment. The present physical condition as observed are also mentioned in this report. From the analysis it is concluded that the existing embankment section in most of the locations are adequate against hydraulic failure with respect to over toping but the free board is inadequate at few locations. The existing embankment section is mostly inadequate against seepage through embankment body. The seepage line cut the down stream face of the embankment section but remains well within the down stream face with respect to design section. In this respect the embankment needs to be re-sectioned at several locations as per design. However the embankment section is adequate against sliding of foundation against horizontal pressure of water under 1998 flood condition. The sections of the existing drainage channels of the city are mostly inadequate to discharge stonn water to the rivers. In this respect the storm drainage system needs to be maintained in proper sections needed for the individual catchments. Seepage occurs through the embankment body at many locations of DND embankment. The existing flood wall has no sufficient free board and in many locations there is no free board with respect to 1998 flood conditions.