The National Water Command Centre has warned flooded provinces along the Chao Phraya River of rising water levels as it slows the downriver surge from the North to protect Bangkok.
The centre issued the warning on its Facebook page on Tuesday.
It said a monsoon trough would cover the lower Central Plains, the East and the upper South from Monday to Sunday and cause heavy rain in parts of the lower North and the Central Plains, including Greater Bangkok.
During this period the centre expected faster flows into the Pa Sak Jolasid dam in Lop Buri province and said the Royal Irrigation Department would have to increase the discharge rate at the dam from 600 to 800 cubic metres per section to protect its structure.
That would raise downstream levels of the Pa Sak River by 0.40-1.00 metres from Phatthana Nikhom district of Lop Buri to the Rama VI dam in Tha Rua district of Ayutthaya.
The level of the river downstream from the Rama VI dam in Tha Rua district would rise by 40-60 centimetres and be higher by 25-50cm at its confluence with the Chao Phraya River in Phra Nakhon Si Ayutthaya district of Ayutthaya.
"The flow (of the Chao Phraya River) through Bang Sai district of Ayutthaya province will be at 3,300-3,500 cubic metres per second and water will surge upstream into the Noi River, the level of which will rise," the National Water Command Centre said.
The centre also said that the flood deluge from the Yom and Ping rivers was flowing into the Chao Phraya River. Officials had tried to detain water in the low-lying Bang Rakam field in Phitsanulok province, but the field was already full.
"To reduce flooding in low-lying areas below the Chao Phraya barrage from Chai Nat province to Samut Prakan province, it is necessary to regulate the discharge rate at the Chao Phraya barrage at 2,700-2,800 cubic metres per second."
"Consequently, the level of water above the Chao Phraya barrage in Muang and Manorom districts of Chai Nat province and Muang district of Uthai Thani province will rise by about 1.30 metres," the National Water Command Centre warned.
The centre ordered irrigation officials to maximally divert water from the Chao Phraya River to adjoining canals on both sides of the river. People in low-lying areas above the Chao Phraya barrage in Chai Nat and Uthai Thani provinces should brace for overflow from the Chao Phraya, the centre said.