The Bangkok Metropolitan Administration (BMA) is preparing to reinforce its flood defences in nine riverside districts.
The Thai Meteorological Department (TMD) has forecast heavy rain in 60% of areas in the northern, eastern and central regions.
Meanwhile, the National Water Command Centre has estimated the volume of rain set to accumulate in the upper river and Pa Sak stream may gradually increase water levels in the Chao Phraya Dam during Oct 1-5, which may affect those who live outside of the river's floodwalls.
Bangkok governor Aswin Kwanmuang has ordered the reinforcement of of 80 kilometres of floodwalls along the Chao Phraya River running from Rama VII Bridge to Bang Na.
Piles of sandbags have also been laid as a precaution along low-lying areas in nine districts -- Dusit, Phra Nakhon, Yannawa, Bang Kholaem, Sathon, Bang Phlat, Thon Buri, Klong San and Rat Burana.
The water station in Ayutthaya's Bang Sai district measured the water flow at 3,052 cubic metres per second (m³/s); the amount of water around Bangkok's Pak Khlong Talat flower market remains lower than the 1.15-metre floodwall at present.
Meanwhile, the Chao Phraya River overflowed into Soi Samsen 23 in Dusit district in the capital last night.
District chief Thanin Niamhom said the river's water levels suffered from inflows of sea water and was aggravated by the increase of headwaters last night.
The district had managed to drain much of the excess and was able to protect households in adjacent areas, Mr Thanin said.
The Disaster Prevention and Mitigation Department said Tropical Depression Dianmu has prevailed across the northeast and central regions, resulting in heavy showers that began last month and continue to cause flash floods and forest runoff in 31 provinces.
The situation needs close monitoring in 81 districts of 17 provinces -- Sukhothai, Phitsanulok, Phetchabun, Phichit, Khon Kaen, Chaiyaphum, Nkhon Ratchasima, Ubon Ratchathani, Nakhon Sawan, Uthai Thani, Chai Nat, Lop Buri, Saraburi, Suphan Buri, Sing Buri, Ang Thong and Ayutthaya, according to the agency.
In Ayutthaya, a two-metre high dam located behind Wat Chulamanee in tambon Ban Khum of Bang Ban district broke at 6am yesterday morning, resulting in water from the Chao Phraya River submerging the area surrounding the temple.
San Soisaeng, chief of the Ayutthaya Disaster Prevention and Mitigation Office, said water from Chao Phraya Dam has been discharged at 2,779 m³/s, resulting in excessive water from the Noi River, Phong Pheng and Bang Ban canals flooding 18,812 households in eight districts.
A floodwall is also being constructed around Chedi Sri Suriyothai in Phra Nakhon Sri Ayutthaya district in an attempt to protect the site.
Defence Ministry spokesman Gen Kongcheep Tantravanich said the armed forces have been instructed to assist local residents near the Chao Phraya River and Pa Sak River.