A fresh round of torrential downpours is expected in the lower Central region including Bangkok over the next few days because of a monsoon trough, according to the Thai Meteorological Department (TMD).
For the capital in particular, the TMD forecasts that maximum rainfall over the period could be as high as 60 millimetres, which is the maximum level the capital's water drainage system can accommodate, Thongplew Kongjun, Royal Irrigation Department (RID) deputy director-general, said Monday.
Thailand does not have to worry about Tropical Storm Khanun, he said. However, due to the monsoon trough, some low-lying areas of the city may be inundated, including Vibhavadi-Rangsit Road, Lat Phrao, and Sukhumvit, he added.
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Mr Thongplew said the RID has managed to restrict water discharges from the Chao Phraya barrage to less than 2,800 cubic metres per second, which is the critical level at which flooding would be likely to occur in Bangkok.
The current discharge is occurring in tandem with the diversion of part of the run-off into large fields along two sides of the river, he said.
Prime Minister Prayut Chan-o-cha has meanwhile instructed authorities to ramp up measures to handle the expected floods in Bangkok expected to accompany heavy downpours from Tuesday through Thursday.
Gen Prayut called an urgent meeting with representatives of two of the agencies concerned, the Agriculture and Co-operatives Ministry and Natural Resources and Environment Ministry.
The prime minister has ordered the RID to manage runoff from the North to ease its impact on Bangkok.
Many riverside communities in provinces located behind the Chao Phraya barrage have already borne the brunt of the run-off. More than 2,000 houses in Chai Nat's Sapphaya district have been left under water.
On Friday night in Bangkok, it rained heavily for more than six hours, leaving many areas of the city inundated on Saturday morning. Bangkok governor Aswin Kwanmuang said it was the heaviest rain Bangkok has suffered for 30 years.
Deputy Bangkok governor Chakkaphan Phewngam said Monday that districts vulnerable to flooding have been told to prepare for rain that may hit Bangkok again Tuesday afternoon.
After the severe floods last Saturday, he said the city administration has cleared all canals and drainage paths so they are ready for the upcoming downpours. The city has already ordered water drainage staff to stand by for potential flooding.
Water levels in the Lat Phrao and San Saep canals, which are the key waterways taking rainwater to the Chao Phraya River, have been reduced and will be able to handle up to 90mm of rainfall, said Sansern Rueangrit, head of the city's flood prevention unit.
Pol Gen Aswin said the preparations should be sufficient to cope with this fresh onslaught of downpours.
The governor also unveiled a plan to expand the city's sewers as a more sustainable way to prevent future flooding.
Pol Gen Aswin said he will order a survey of the complete sewage network, especially the older ones which are narrower and more prone to blockages.
"These sewers cannot drain a large amount of water when the city is slammed with torrential rain," he told reporters, adding it is important to start thinking of the sewer system expansion now as such measures will take time to implement.
Small sewers have been blamed for failing to siphon off excess water when rainfall has exceeded 100mm, and calls for a major overhaul of the existing network have grown as the problems in the capital have worsened.
In the early hours of last Saturday, the amount of rainwater that fell exceeded 200mm in many areas. In total, 55 spots were inundated, causing widespread damage to homes and assets and drawing doubts over the city's prevention efforts from critics, including political activist Srisuwan Janya, who also questioned Pol Gen Aswin's capabilities.
"I don't listen to Mr Srisuwan's words," Pol Gen Aswin said, insisting he has taken steps to solve the flooding to the best of his ability.
"A person who acts often keep silent while a person who speaks does nothing."
Pol Gen Aswin also reacted to Pheu Thai key member Khunying Sudarat Keyuraphan's request for the city to check whether its giant tunnels can drain water effectively.
The governor said the eight tunnels, which serve as "expressways" to take rainwater out to the Chao Phraya River, are working well.
Slow drainage is because some water pumps at Bang Sue tunnel are still under construction, he said.