Deputy city clerk Jumpol Sampaopon said on Thursday that work crews took a long time to drain off the water because subsidence in the city means the canals now have only a gradual drop and the water flows out much more slowly.
Land in the city was subsiding at a rate of one to 10 centimetres a year, he said.
Mr Jumpol denied that the flooding was caused by undredged canals or clogged drains, as many people claimed.
Responding to former Bangkok governor Bhichit Rattakul's suggestion that all the drains in the capital city should be replaced with new ones, the deputy city clerk said the task would be difficult and would require a large budget.
However, he said, Bangkok authorities might lay new drains in some flood-risk areas.
Officials will ensure that the drains are not clogged with debris such as leaves, sticks and garbage so that water can flow unobstructed, he added.
Sujarit Koontanakulvong, a water engineer at Chulalongkorn University, said rainfall is an important variable for flooding in Bangkok.
"The drainage system has been in use for more than 20 years and it is designed to handle 60 millimetres of rain per hour," Assoc Prof Sujarit said. "If there is more than 130mm of rain in an hour, it will take more than two hours to drain off."
He also suggested that the Bangkok Metropolitan Administration install more drains and speed up the draining process on main routes in the city.
"The BMA should consider setting up an electrical drainage system. This system is a central public utility system which each agency can operate together," he said.
Meanwhile, Rak Thailand Party leader and MP Chuvit Kamolvisit posted a question in his Facebook page on Thursday asking whether the BMA's giant tunnel can really be used to drain water out to the sea.
His question was directed to Bangkok Governor MR Sukhumbhand Paribatra.
"I want to know whether the giant tunnel, in which a huge sum of money was invested for sustainable flood prevention, is really effective.
"An official working at the BMA office told me that the city's water pumps and water pumping system were full of defects, making the BMA's water pumping capacity ineffective," Mr Chuvit posted on his Facebook page.
The Rak Thailand leader suggested it was time that people started looking into various projects of the BMA to see how much it had actually done to prevent Bangkok from being flooded.
Mr Chuvit also posted a picture taken last year of a flooded spot in Bang Khae area.
The picture shows how sad and disappointing it was that when Bangkok was hit by the great flood, the giant tunnel could not be of any help, he said.