ONWR adjusts water management plan as new inflows hit Bhumibol Dam, satellite images confirm rising floods downstream

The Office of the National Water Resources (ONWR) has urgently revised its water management plan to cope with a new surge of inflows into the Bhumibol Dam. Heavy rainfall north of the Chao Phraya Dam — the second highest since 2022 — has left the reservoir with only 127 million cubic metres of storage capacity. The indirect impact of tropical storm Kalmaegi this November has further intensified the situation.

On November 10, ONWR approved an increase in Bhumibol Dam’s discharge rate from 45 million to 48 million cubic metres per day to manage water levels carefully without opening the dam’s emergency spillway. The agency, together with related departments, has coordinated with the Royal Irrigation Department to divert excess water into designated low-lying fields, especially in Phra Nakhon Si Ayutthaya, where residents have been suffering prolonged flooding.

Additional drainage operations are being accelerated on both sides of the Chao Phraya Dam to maximise water release, with this expected to be the final discharge cycle of the 2025 rainy season. According to forecasts from the Thai Meteorological Department and the Hydro-Informatics Institute, rainfall in the upper regions will decline after 13 November as a high-pressure system moves in, while heavier rain is expected to shift to the South. ONWR anticipates reducing discharge rates to below 1,000 cubic metres per second by mid to late December.

The Geo-Informatics and Space Technology Development Agency (GISTDA) released new satellite images on November 10 confirming flood expansion along the lower Chao Phraya River in Nonthaburi, Pathum Thani, and Bangkok. The images show that previously marked “red and orange” flood-risk zones have now turned “blue,” indicating actual flooding in key areas:

• Pathum Thani: Pathum Thani Municipality, Chiang Rak Yai, Chiang Rak Noi
• Nonthaburi: Bang Kruai, Tha Nam Non, Koh Kret, Bang Srimuang, Bang Khanun
• Bangkok: Northern-western areas, including parts of Bang Phlat district

Authorities warn that water levels in the lower basin remain high and are likely to keep rising. The Chao Phraya Dam may need to increase its discharge rate to 2,900 cubic metres per second to maintain upstream stability.

GISTDA cautioned that if the release reaches 2,900-3,000 cubic metres per second, water levels downstream could rise by another 20-30 centimetres, affecting nearly all riverside zones and potentially pushing floodwaters further into western Bangkok.

Emergency response measures include:

• Local agencies in high-risk zones — including Nonthaburi Municipality, Pathum Thani Municipality, and Bang Phlat District Office — reinforcing sandbag barriers and deploying additional water pumps.
• Riverside communities have been advised to move valuables to higher ground and prepare for rising flood levels beyond earlier forecasts.