The Chao Phraya Dam on Wednesday increased its discharge rate, sending a huge volume of water downstream, where four villages in Bang Ban district of Ayutthaya were flooded.
Extra water is also being released from the Pasak Jolasid Dam, with flood warnings for five central provinces.
The Chao Phraya dam in Chai Nat province increased its discharge rate to 2,766 cubic metres of water per second, the highest rate this year.
Somyos Ketsuwan, chief of Bang Ban district, on Wednesday inspected flooding in areas in front of Wat Kopai in tamon Bang Luang Dod after Khlong Bang Luang canal overflowed and flooded four villages in the tambon.
A total of 162 households were affected, Mr Somyos said as he handed out relief bags to residents.
Meanwhile, five central provinces have placed on full alert for rising water as the Pasak Jolasid Dam will also increase its rate of discharge
The Department of Disaster Prevention and Mitigation on Wednesday sent an urgent letter to governors of Lop Buri, Saraburi, Ayutthaya, Pathum Thani and Nonthaburi provinces and directors of provincial disaster prevention to prepare for discharge from the Pasak Jolasid Dam.
In its letter, the department said the Royal Irrigation Department (RID) had said tropical storm Dianmu and an active low-pressure cell caused widespread flooding
About 515 million cubic metres of runoff was expected to flow into the Pasak Jolasid Dam between Sept 27 and Oct 3. If the discharge rate remained at 8.64 million cubic metres a day, the dam would reach capacity on Sept 30, the RID said.
To make room for more water and prevent the dam overflowing, the RID decided to increase the discharge rate to 3,456 million cu/m a day, about 400 cu/m per second. The rate would be increased gradually. It would flow into Khlong Chai Nat-Pasak canal and then to the Rama VI Dam reservoir at the rate of not more than 700 cu/m per second.
This would see water levels along the Pasak River, downstream of the Rama VI Dam, rise to 2-2.5 metres.