The nuts and bolts of what we need to do
The severity of the 1987 and 1988 floods that inundated more than 60% of the total area of greater Dhaka, to depths ranging between 0.3 to over 4.5 metres, created global awareness and drew attention of both national and international organizations about management of flood problems in the country.
As a follow up, the Greater Dhaka Flood Protection Project (GDFPP) was started. The planning approach of the GDFPP was to protect Dhaka city and surrounding floodplains and agricultural land from the intrusion of flood water, and drain out the internal stormwater run-off due to local rainfall.
The implementation of the GDFPP was divided into two phases. Construction work of phase-I started for the western part of Dhaka in 1989 and continued until 1992, while construction of phase-II for the eastern perimeter of Dhaka has not started yet.
This eastern part of Dhaka roughly occupies between latitude 23045’ to 23057’ and longitude 90023’ to 90030’. It is composed of alluvial terraces and low-lying areas and surrounded by tributaries and distributaries of three famous international rivers: The Ganges, the Brahmaputra, and the Meghna. The river system is composed of the Dhaleswari, Bansi, Turag, Balu, Buriganga, and Lakhya rivers and their tributaries.
The water levels in those rivers are likely to be affected not only by discharge from the Brahmaputra-Jamuna River and local rainfall, but at times also backwater from the Meghna River, if there is heavy rainfall in the north-eastern part of Bangladesh and high tide in the Bay of Bengal at the same time.
The existing built-up areas are located mostly at the higher land above 5.0 m (GTS). The higher land is usually flood free. However, most other areas, under current conditions, are low-lying and have been underwater for over half a year, of which perhaps half is under 2.0 m (GTS).
The prime objectives of flood protection schemes were to ensure flood-free life for about 10 million people (1989 estimates of population) and protect surrounding agricultural lands from flood water intrusion. But, as observed, soon after the construction of phase-I (Dhaka-West) a significant portion of agricultural land was changed for residential use.
In case of phase-II (Dhaka-East), construction of which has not started until June 2021, similar trends were observed. This area was initially a low-lying agricultural land and normally remained underwater for over half of the year. This is the area where, among others, the upscale Bashundhara residential community started to develop in 1980.
Over the past four decades, the contribution of this residential area to the development of urban housing and civic amenity creation has been amazing. In keeping with the rapid population growth in the metropolitan Dhaka megacity, Bashundhara has stretched itself to cover new boundaries and responded effectively to the long-term needs of the residential facilities of the ever-growing urban population. This is now a peaceful residential area with groceries, planned houses, hospital, mosques, banks, and all other necessary social and urban services.
However, despite all these upscale residential developments in this area, this area still remains as a floodplain as phase-II embankment hasn’t started yet, and the embankment construction process is uncertain now. A floodplain by definition is an area of low-lying ground adjacent to a river, formed mainly of river sediment and subject to flooding.
This area was flooded by one to three metres depth of water in 1998, and the flooding lasted for more than one to two months. If we try to identify the causes of 1988 or 1998 flooding, the La Niña events, among others, of those years are to be blamed. The year of 1998 faced the historically strongest La Niña event. The 1988 La Niña was moderately strong.
The latest Global Climate Model-based study provides a strong message about the increasing risk of extreme La Niña conditions for future generations. It demonstrates that both the frequency and intensity of the strong events will increase significantly.
Those studies revealed that the frequency of extreme events doubles under the 1.5C Paris target and could occur more frequently than before. Therefore, the probability of La Niña-related flooding like that of 1988 and 1998 is on the rise for the near future.
As the phase-II project has not been completed yet, the area is not protected from flooding. As a result, a flood of 1998-type will bring a great disaster for the eastern perimeter of Dhaka, including the Bashundhara residential area. Fortunately, there was no 1998-type severe flooding in the last 22 years, but this can occur any time in the near future.
Therefore, in order to save money, property, and lives, it is necessary to fully implement the construction work of GDFPP (phase II) embankment project immediately so that the newly constructed upscale residential areas are safe from flood water intrusion.
Md Rashed Chowdhury is a research faculty in the University of Hawaii at Manoa, USA. Email: [email protected]