The water retention area, proposed in Dhaka's Detailed Area Plan (DAP), has declined alarmingly over the last seven years, said an official of the Rajdhani Unnayan Kartripakkha (RAJUK).
In its wake, the city's waterlogging problem aggravated, he added.
There is an area of 1,590 sq km under the DAP gazetted in 2010.
Of the area, 5,540 acres or 22.42 sq km were proposed to be protected as a water retention area including ponds, canals, lakes and other water bodies.
But a Rajuk study in 2016-17 found that only 1,744 acres or 7.06 sq km existed as the water retention area.
The city development authority conducted the study for preparing a revised DAP for the period of 2016-35.
On the condition of anonymity, an official said the Rajuk cannot shirk its liability for the city's awful situation.
Rajuk prepared the master plan that other agencies like city corporations, Dhaka WASA, Water Development Board, Rajuk and Cantonment Board were supposed to implement, he added.
"Rajuk has failed to monitor and coordinate the master plan as the custodian of it," he said.
"It should have strictly monitored what the other agencies were doing," he further said.
He said the Dhaka WASA could not save the 26 canals under its jurisdiction.
Some of them have been grabbed while others have become moribund. Some of them are being used for making concrete walkways while the others are silting up.
The natural flow of the canals must not be obstructed, the official observed.
Bangladesh Institute of Planners General Secretary Adilur Rahman Khan gave even more disturbing information on the extinction of flood flow zones under the DAP area.
He said 2,000-3,000 acres of flood flow zones are lost every year in the DMDP area.
Rajuk as the city's development authority and regulator has no comprehensive data on it due to a weak monitoring system.
The Rajuk source said a high-powered cabinet committee made 158 amendments to the DAP in eight years.
It paved the way for influential quarters to fill the water bodies and flood flow zones, he added.
A seven-member ministerial committee was assigned to approve the DAP in 2010. But it turned into a committee allowing the conversion of land use, he said.
Dhaka city had 43 canals. Of them, 17 have been grabbed fully. Twelve are on the verge of extinction.
Only eight canals are maintained by the deputy commissioner of Dhaka and 26 by the Dhaka WASA as part of a storm drainage system.
The canals under the WASA, each 75 kilometres long, remain mostly clogged with solid waste.
Most of these canals need regular dredging and cleaning, but WASA has failed to do so over the years.
Ten kilometres of the canals have been turned into concrete box culverts to make way for roads on them, significantly reducing their capacity to carry water.
According to experts, the drainage system depends on the topography of any particular city.
After the devastating flood in 1988, Japan International Cooperation Agency (JICA) prepared a Flood Action Plan.
It recommended that a minimum of 12 per cent area be kept as water retention reservoir in the forms of pond, canal, lake, agricultural land and flood plain to save Dhaka city from damaging floods.
Besides, there should be a system to pump out extra water. If the pump system is not kept, the water retention reservoir or flood plains should be increased, it said.
Experts say rainfall above 40 millimetres at a time in Dhaka floods the city with water which takes at least three hours to recede.
The available pumping facility can drain out up to 20mm rain. Rainwater is supposed to be carried away instantly to low-lying retention areas by a well-networked storm drainage system.
They said the city's temperature increased from 28-32 degrees Celsius a few years ago to 38-42 degrees Celsius in recent years.
The experts find a link of the weather conditions with the climate change.
Waterlogging is a big problem nowadays due to the erratic rainfall.
Experts say the situation may aggravate with the monsoon approaching fast. The rainy season usually begins in June.
Talking to the FE, DAP project director Ashraful Islam said the present roadside drainage network does not have the capacity to drain out excess rainwater as downpour's intensity has increased.
"Areas like Dhanmondi, Mirpur and Karwan Bazar are inundated even after a brief spell of rain," he commented.
He suggested that the capacity of the roadside drainage network be increased along with regular cleaning.
Rainwater harvesting can also be a major way out for conserving excess rainwater for some time, said Mr Ashraf.
The city has some 0.15-0.2 million houses with rooftops where tanks can be installed for rainwater harvesting, he said.
This should be made mandatory and enforced by taking community initiatives and motivating the people, he added.
Aggressive tree plantation or mandatory green space has to be ensured by city corporations and other authorities to reduce the city's temperature, he mentioned.
"The residents may be given incentive to create green space in their houses."
