A six to seven-hour spell of monsoon rain that dropped 76 mm of rainfall on Dhaka on Sunday was enough to inundate roads across the capital, disrupt traffic, strand commuters and force several schools to postpone examinations.
While the Bangladesh Meteorological Department (BMD) described it as the highest six-hour rainfall recorded in the capital so far this month, the amount was well below Dhaka's historical rainfall records.
Rainfall measured in millimetres indicates the depth of water that would accumulate on a flat, impermeable surface if none of it flowed away or evaporated. A rainfall of 76 mm means that every square metre of land receives 76 litres of water.
Since the rain fell over six hours, the average rainfall intensity was about 12.7 mm per hour. Although rainfall rarely falls at a perfectly uniform rate, this average illustrates the volume of water entering the city's drainage system within a relatively short period.
According to the BMD, Dhaka received 76 mm of rain between midnight and 6:00am and 97 mm during the previous 24 hours. The rainfall was associated with an active monsoon, a low-pressure system over the Bay of Bengal and atmospheric pressure variations that have produced widespread rain across Bangladesh over the past week.
Meteorologically, the rainfall was significant but not exceptional for Dhaka. The capital's highest officially recorded 24-hour rainfall remains 341 mm, recorded on 14 September 2004.
Other major rainfall events include 333 mm in July 2009 and 326 mm in July 1956. Sunday's rainfall therefore represented less than one-third of the city's all-time daily record.
Urban flooding depends not only on the total amount of rain but also on the rate at which it falls. When rainfall reaches drains faster than stormwater can be discharged through canals, pumping stations and rivers, water accumulates on roads and in low-lying areas.
A shorter, more intense downpour can therefore create more severe waterlogging than a larger amount of rain spread evenly over an entire day.
Dhaka's drainage system consists of underground storm drains, open canals, retention areas, regulators and pumping stations that discharge stormwater into surrounding rivers. Responsibility for the network is shared among several agencies.
The two city corporations manage surface drainage and maintenance of many canals, while the Bangladesh Water Development Board operates flood-control structures and pumping facilities in some areas. Coordination among multiple agencies has long been identified as a challenge in managing stormwater.
The capital is built around 43 canals, many of which historically served as natural drainage routes linking the city with surrounding rivers. Government agencies have been working to recover and restore several canals, including 29 canals under Dhaka North City Corporation, while also removing silt and floating waste ahead of the monsoon.
More than 350 personnel have been deployed in Dhaka North this year to improve drainage at flood-prone locations.
Even with these measures, the volume of runoff generated during intense rainfall can exceed the carrying capacity of the network. This is because Dhaka's low-lying urban topography continues to dictate how water accumulates during severe downpours.
Neighborhoods like Mirpur, Mohammadpur, Agargaon, and Dhanmondi are highly vulnerable to flash waterlogging due to their geographical depressions, with water levels reaching knee-to-waist height in areas like Kazipara and Shewrapara within hours of intense rainfall.
This runoff is further bottlenecked during active monsoon cycles when peripheral water levels in the surrounding Buriganga, Turag, Balu, and Shitalakshya rivers rise near or above local danger thresholds. To stop the swelling rivers from flowing back into the internal floodplains, authorities are forced to lock major regulatory sluice gates.
This shifts the city's entire drainage reliance onto heavy mechanical pumping stations like those in Kamalapur TT Para, Dholaikhal, and Hatirjheel.
However, forcing stormwater out against a high river head is incredibly difficult, particularly since these three existing drainage outlets service a massive combined area of 109.24 square kilometers, a capacity the government explicitly acknowledges is insufficient for the city's growing modern needs.
Incidentally, to aggressively tackle these drainage corridors before the current heavy rains, both city corporations rolled out massive mitigation plans.
Dhaka North City Corporation mapped out 108 waterlogging hotspots and deployed 21 Quick Response Teams to clean pits mid-storm, having built 105.89 kilometers of drains last fiscal year with a targeted 120 kilometers more slated for completion.
Meanwhile, Dhaka South City Corporation identified 33 severe flashpoints, executing clean-ups across 22 primary canals and four major box culverts, deploying high-capacity portable pumps, and initiating a World Bank-backed project to build an expansive new drainage outlet from Gulistan to Sadarghat to safely funnel stormwater directly into the Buriganga River.
But because those plans were not implemented in time, Dhaka once again succumbed to its age-old waterlogging problem.
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