Colombo’s drainage system, built to carry about 30mm of rain an hour, was overwhelmed when nearly 100mm fell in two hours on Friday 6 November, an official of the Colombo Municipal Council (CMC) said.
Deepika Wijethilake, Deputy Director of Drainage and Water Supply at the CMC, told the Daily Mirror that the unusually intense rainfall over a short period was the main reason for the flooding. She said the city had not seen rain of that intensity in such a short time in recent years.
The downpour flooded several roads and damaged shops across the city. A section of the National Hospital was also inundated, affecting hospital services.
Wijethilake said the situation could have been different had the same amount of rain fallen over a longer period, because the drains would have had time to carry the water away. She said the existing capacity could not handle such heavy downpours and that steps were being taken to expand the system and improve existing drains.
Responding to criticism over poor maintenance, she said the CMC cleans the drains every day despite a shortage of workers. She said more workers were being recruited and that the support of the Tri-Forces was expected to be sought to help clean the drainage network.
Heavy rain flooded roads in the city again on Wednesday. Police said roads in Maligawatta, Sangharaja Mawatha and Khettarama were inundated, and that traffic was disrupted around Armour Street, Nawam Mawatha and Borella. Police urged motorists to take care in the affected areas.
Background
Colombo is a low-lying coastal city, and its drains, canals and lakes are designed around expected rainfall rates. Drainage systems of this kind are generally sized for a given amount of rain per hour, so rainfall far above that rate can cause flooding even when the drains are clean and working as intended.
The figures given by the CMC show the scale of the gap. Nearly 100mm in two hours works out at roughly 50mm an hour, well above the 30mm an hour the system is designed to handle. The official did not say how many roads or shops were affected on 6 November.
The CMC has faced criticism over the maintenance of drains after the flooding. The official’s response was that daily cleaning continues, although the council is short of workers.
What it means
For residents and commuters, short and intense downpours are likely to bring waterlogged roads and delays in the affected areas, as happened on Wednesday. Police advice is to drive carefully through flooded sections.
The CMC says work to expand the drainage network and improve existing drains is under way, along with recruitment of more workers and a possible request for Tri-Forces help with cleaning. It has not given a timeline for these steps.
