Expert: Recent floods in Southern Central Coast is unusual

Several major rivers simultaneously surged past historical flood levels, a situation virtually unseen in over 50 years of observation and beyond normal calculated flood thresholds.

Record-shattering rainfall and flood levels

Reporting on the recent historic flooding, Mr. Mai Van Khiem, Director of the National Center for Hydro-Meteorological Forecasting, stated that the flood event in the Southern Central Coast from November 16–22 was an extreme phenomenon that broke all previous records. Rainfall totals at many stations, such as Son Hoa (Dak Lak) with 601.2 mm and Quy Nhon (Gia Lai) with 380.6 mm, exceeded all past historical records. Other stations, including Son Thanh Tay, Son Thanh Dong, Hoa My Tay, and Song Hinh, recorded between 1,000 to 1,200 mm in just a few days. According to the World Meteorological Organization (WMO) classification, these are rare events whose precise rainfall quantities are nearly impossible to forecast accurately.

Compounding this, accumulated rainfall from October to mid-November 2025 in the region had already been extremely high, 120–200% above the multi-year average. This saturated the soil and raised baseline river water levels, meaning that an additional 300–500 mm of rain during the main event was enough to trigger massive flooding.

This scenario led to major, especially large, and historic floods across numerous rivers from Quang Tri to Khanh Hoa and Lam Dong that have not been seen in decades. Severe, large-scale inundation occurred across the provinces/cities from Quang Tri to Khanh Hoa.

The flood peaks on several rivers exceeded Alarm Level 3, including: Thach Han (Quang Tri); Bo River (Hue City); Vu Gia-Thu Bon River (Da Nang City); Ve, Dak Bla Rivers (Quang Ngai); Ky Lo, Ba, Kon, Krong Ana Rivers (Dak Lak); Dinh Ninh Hoa, Cai Phan Rang, Cai Nha Trang Rivers (Khanh Hoa).

Notably, the flood on the Ba River exceeded its 1993 historic peak.

The peak at the Cung Son station reached 40.99m, exceeding the historic peak by 1.09m.

The peak at the Phu Lam station reached 5.40m, exceeding the historic peak by 0.19m. Other rivers also set new records: the Ky Lo River (Dak Lak) peak at Ha Bang station was 13.90m, 0.43m higher than the 1993 historic flood; the Da Nhim River peak at Dai Ninh station was 836.17m, 2.72m higher than the 1993 historic flood; and the Dinh Ninh Hoa River (Khanh Hoa) peak at Ninh Hoa station was 6.77m, 0.19m higher than the 1986 historic flood.

As of November 23, the Srepok River (Dak Lak) was fluctuating near its peak of 178.17m, 4.17m above Alarm Level 3.

Mr. Khiem stressed that the simultaneous breaking of records across 3–5 river basins is extremely rare, almost never observed over 50 years of hydro-meteorological observation. This magnitude of event falls outside the “normal calculated flood level,” which is typically determined based on recurrence frequency and statistical analysis of long-term rainfall and flood data for planning and forecasting purposes.

According to Mr. Khiem, the rainfall event resulted from the combination of multiple extreme weather patterns. A strong disturbance within the Easterly wind belt at altitudes of 1,500–5,000m was active, combining with a firm cold air mass, especially after the night of November 17, 2025. This created a convergence zone for moisture from low to high altitudes. Moisture was continuously transported from the East Sea into the Central Coast and Central Highlands, while the Truong Son range acted as a "windbreak," intensifying convection and sustaining prolonged heavy rain. Rainfall totals in many areas exceeded the natural drainage capacity.

In addition to the weather factors, the Central Coast’s topography and hydrology also exacerbated the flood risk. Rivers in the region are short with steep gradients, causing rainwater to concentrate rapidly downstream, leading to flash floods and quick-rising river floods within hours. High tides also slowed water drainage. Furthermore, climate change has made extreme rainfall patterns stronger and harder to forecast, while increasing the frequency of large floods over the past 10-15 years.

Timely and continuous forecasting

The National Center for Hydro-Meteorological Forecasting demonstrated advanced preparation:

The Center issued an early warning four days before the heavy rain began (November 12) to the National Steering Committee for Civil Defense, predicting a strong cold air mass combined with widespread heavy rain from November 16–22. The Hydro-Meteorological Agency followed up with an official heavy rain warning bulletin on November 13, and based on these forecasts, the National Steering Committee for Civil Defense issued a directive on November 14 urging urgent preparation for the large flood event. The directive specifically forecast very heavy rain from November 15-18, with common accumulations of 300-600 mm, along with a risk of floods reaching Alarm Levels 2-3.

The Agency continually issued flood warning bulletins, increasing frequency to twice daily from November 15 and setting the disaster risk level for floods at Level 3-the highest defined warning level. From November 17, these were upgraded to special large flood and emergency flood bulletins for the relevant rivers and provinces. The disaster warning level for eastern Dak Lak was raised to Level 4 on November 20, signaling a very high risk from a multi-hazard combination (flood, flash flood, and landslide).

In total, the Center issued 55 bulletins for the heavy rain event and 31 flood bulletins for rivers from Ha Tinh to Khanh Hoa and Dong Nai. Of the flood bulletins, 24 were categorized as emergency or special large emergency floods for key river systems. The timely and comprehensive information was provided to the National Steering Committee for Civil Defense, local Civil Defense Steering Committees, and media outlets to minimize damage to life and property.

Lessons learned for disaster response

Mr. Khiem noted that while forecasting accuracy has improved, even accurate forecasts are insufficient to prevent damage from historic-level natural disasters. Given the accelerating and severe nature of such events, enhancing forecasting capabilities, risk management, and prevention organization is crucial.

A key success story of coordination was seen in Thua Thien - Hue province. The local Hydro-Meteorological Station issued early warnings 48–72 hours in advance and continuously released bulletins. Warnings were rapidly transmitted through a multi-channel system, including the HUES - IOC Hue application for leaders, the Smart City Center sending alerts via Zalo, SMS, and electronic boards, and the City Media Center providing constant updates. At the grassroots level, warnings were relayed via loudspeakers.

This multi-platform warning system ensured that weather alerts reached the community very quickly, enabling proactive prevention and damage reduction. Furthermore, the Hydro-Meteorological Agency provided direct consultation to the City Civil Defense Command. Based on these forecasts, the Command was able to timely order the regulation of major reservoirs (Ta Trach, Binh Dien, Huong Dien) according to standard operating procedures. The "Consultation-Command-Real-Time Update" mechanism significantly improved flood forecasting accuracy, helping local authorities close roads, warn residents, and carry out evacuations at the right time, thereby markedly reducing the severity of inundation and the extent of damage.

The data over the past 30 years indicates that major floods in the Southern Central Coast typically occur before November 15. The 2025 flood event, however, arrived later, reflecting an increasing trend of extreme, unpredictable weather due to climate change and highlighting Vietnam's vulnerability to such events.