Historic flood erases many survival experiences in South Central Vietnam

(Dan Tri) - 4 billion cubic meters of water poured into the Ba River basin in just 7 days, far exceeding the historic flood of 1993, not only pushing the water level to unprecedented levels, but also erasing safety habits and experiences that have been considered "standard" for decades.

Ms. Le Thi Ngoc Phuong, residing in Tuy Hoa ward, Dak Lak province, one of the areas directly affected by the flood discharge on Ba river, said that she has lived here for more than 20 years, and in previous years, the highest flood peak only inundated her house about 0.5m. Therefore, every time she heard the forecast of heavy rain, she often relied on the old flood levels to prepare: put all her children's belongings and books up on the attic about 2m high, and the whole family took shelter there, safe as in every familiar flood season.

But this time was different. Completely different. “My husband was working far away, and even though I was a woman, I still tried to carry all the things up high to avoid damage,” she said. However, the water rose too quickly, and the waves continuously crashed into the house. In the emergency, she only had time to hold her daughter and run to a neighbor’s house.

From then on, she stood and watched as the house that the couple had worked so hard to build for many years… completely collapsed in the whirlpool.

In Tay Hoa commune, Ms. Nguyen Thi Tri also experienced a similar moment. “After this historic flood, I realized that the old experience of prevention based on flood peaks is no longer suitable,” she said.

Previously, the water only reached the yard of her house, but this time it was more than 2 meters deep; all the furniture was damaged, and her herd of sows worth more than 150 million VND also died. “The damage was too great,” she choked.

According to Mr. Nguyen Thanh Hoai, Vice Chairman of the People's Committee of Duc Binh Commune (Dak Lak), the recent historic flood shows that people's experience of "flood escape" in the old direction of watching for flood peaks is no longer suitable with the changing reality of natural disasters.

“Hearing the forecast of heavy rain and floods, the local authorities warned and urged people to evacuate their properties and take shelter. But many people were still complacent, thinking that the flood peak for many years was only about 1 meter, so they refused to leave. Unexpectedly, the water rose rapidly, and people only had time to run away, but their properties were swept away and severely damaged,” said Mr. Hoai.

He advised: in the coming time, when there is information about heavy rain or natural disasters, people need to closely follow the media, and especially must strictly follow the evacuation instructions of the authorities - because subjectivity only needs to make one mistake to pay the price of a lifetime of savings.

The floods from November 16 to 22 in the South Central and Central Highlands provinces were assessed by the meteorological agency as an extreme phenomenon, surpassing all historical records ever recorded.

Rainfall at many stations such as Son Hoa in Dak Lak region reached more than 600mm, some places recorded from 1,000 to 1,200mm in just a few days, causing many rivers in the Ba, Dinh, Cai, and Da Nhim river basins to simultaneously reach new flood peaks.

As of the morning of November 24, floods have left 102 people dead and missing, with estimated economic damage of more than VND13,000 billion, more than 186,000 houses flooded, more than 80,000 hectares of rice and crops damaged, and over 3.2 million livestock and poultry swept away.

The most severely affected provinces were Dak Lak, Khanh Hoa, Gia Lai , and Lam Dong, with many residential areas submerged in water, traffic paralyzed, and power and communication systems disrupted.

Associate Professor, Dr. Nguyen Tien Giang, Head of the Department of Meteorology and Hydrology, University of Science (Photo: Minh Nhat).

From a professional perspective, Associate Professor Dr. Nguyen Tien Giang, Head of the Department of Hydrometeorology, University of Science (Vietnam National University, Hanoi) commented that this was a rare "combined flood" where many unfavorable factors appeared and resonated together.

Complex Floods: When All Factors Are Unfavorable

Associate Professor Dr. Nguyen Tien Giang said that in hydrology, each flood must be viewed in the entire hydrological cycle: from the moisture source at sea, the air mass systems moving in the atmosphere to the buffer conditions below the ground.

Only when we put all three of these “layers” together can we fully understand why a flood could be as devastating as it was.

Record floods cause great damage to the Central provinces (Photo: PV).

“This flood is a complex flood, or in other words, a flood that combines many unfavorable factors appearing at the same time. From the sea, the atmosphere to the buffer surface, all simultaneously fall into the most unfavorable state for flood drainage,” Associate Professor Giang emphasized.

At the marine level, Vietnam is being affected by the La Nina phase of the ENSO system. When La Nina is active, the southern East Sea and the coastal strip of the Central and South Central regions are in a “water-rich” area, meaning the probability of heavy rain, strong storms, tropical depressions and rain-causing disturbances increases significantly.

Climate forecasts have shown that La Nina is likely to last until the end of 2025, with the potential to increase rainfall in the Central and South Central regions compared to the average of many years.

According to Associate Professor Giang, many unfavorable factors combined to create a complex flood (Photo: Minh Nhat).

On the background of an already "wet" climate, the South Central region during the flood was simultaneously affected by the strengthening cold air from the north and strong easterly wind disturbances.

The cold air flowing down acts as a barrier, guiding the hotter, moist air mass from the sea, which is blocked by the Truong Son mountain range and rises, creating a “rainy area” on the eastern slope of the mountain range.

“In the Eastern Truong Son region, the rain poured down heavily, with high intensity and lasting for a long time, causing the actual rainfall to far exceed the historical floods recorded before,” Associate Professor Giang analyzed.

Record-breaking rain and billions of cubic meters of floodwater

According to Associate Professor Dr. Nguyen Tien Giang, when talking about the level of destruction of a flood, just looking at the flood peak is not enough. The flood peak indicates the highest water level at a point, at a time, but the actual damage also depends on the total flood volume, the time of flood rise, the time of flood recede and the intensity of the flood.

“With the flood from November 16 to 22, we estimate the total flood volume in 7 days to be about 4.0 billion cubic meters up to Cung Son station on the Ba River basin. Meanwhile, the historic flood in 1993 was only about 2.6 billion cubic meters. That means the total volume of water this time is about 1.4 billion cubic meters larger than a flood that was already considered historic,” said Associate Professor Giang.

According to Associate Professor Dr. Nguyen Tien Giang, when talking about the level of destruction of a flood, just looking at the flood peak is not enough (Photo: Minh Nhat).

That is why he affirmed that the most direct and obvious cause of the recent devastation was record-breaking rainfall in the context of the land being saturated with water after many consecutive weeks of rain.

Data from the Department of Hydrometeorology also shows that from October to mid-November, total rainfall in the South Central region was 120 to 200% higher than the average of many years. When entering the main rainy season, only 300 to 500mm more is enough to form a large flood, even a historic flood.

In other words, the basin entered the flood with the “cup already full”, so only a small amount of additional water was enough to cause the river system to “overflow”.

The buffer surface is transformed by double floods.

The third element in the hydrological cycle that Associate Professor Dr. Nguyen Tien Giang emphasized is the "buffer surface", which means everything located below the ground, from the terrain of the channel, the reservoir system, the dike, to the traffic infrastructure, urban areas, and industrial zones.

According to him, to determine exactly which factors play a key role in buffering, detailed studies for each basin are needed. However, some general trends can be seen.

The first is the change in the topography of the riverbed after large floods. Previous floods erode and deposit, changing the shape of the riverbed, resetting the hydraulic “level” for the next flood.

Studies in Europe have demonstrated the phenomenon of “flood after flood”: after each large flood, the channel is changed, hydraulic resistance increases, causing the water level in the next flood to be significantly higher than the previous one with the same flow rate.

“In my opinion, one of the main causes related to the buffer surface is the terrain of the river basin. In particular, two days after the historic flood, the rivers received a major flood. The change in the terrain of the river bed after previous floods increased hydraulic resistance and caused the water level to rise,” Associate Professor Giang analyzed.

Big waves, high tides and infrastructure bottlenecks block flood drainage

Not only rain in the basin, the process of water flowing from upstream to the sea also encounters bottlenecks.

Firstly, big waves due to the Northeast monsoon, the high tide regime usually peaks in the period from October to January of the following year in the South Central region.

Associate Professor Dr. Nguyen Tien Giang recalled the field research that he and his colleagues conducted at two estuaries in this area, Da Nong and Da Dien.

According to Associate Professor Giang, not only rain in the basin, the process of water flowing from upstream to the sea also encounters bottlenecks (Photo: Minh Nhat).

“During the peak months of floods, offshore wave heights can reach 2.5 to 2.7 meters, when combined with high tides. When floods from the river flow out at the same time as big waves and high tides, it is like encountering a wall, significantly slowing down the process of flood discharge to the sea,” Associate Professor Giang analyzed.

The ability of floodwaters to escape to the sea being “held back” has contributed to this year’s exceptionally wide and deep flooding. River water cannot flow out to the sea in time, but rises back into the tributary river system and low-lying areas, causing many residential areas that were considered “relatively safe” to still be deeply flooded.

Second is the system of constructions built by humans. For large floods, we often use water levels to assess the alarm level. But water levels do not only depend on the amount of water flowing in but are also influenced by the drainage capacity of the downstream.

Infrastructure systems can become "bottlenecks" for draining floodwaters into the sea when faced with record-breaking floods like this year.

According to experts, it is necessary to take into account infrastructure factors that obstruct flood drainage (Illustration: Nam Anh).

“Upstream, water flows very quickly. If the downstream drainage is poor due to narrowing by infrastructure and construction, the water level will rise much higher. This is something we must seriously consider when assessing the role of infrastructure in urban and riverside flooding,” Associate Professor Giang noted.

The Great Flood Redraws the Map of Natural Disasters

Associate Professor Dr. Nguyen Tien Giang said that this year's flood cannot be considered a "once in a hundred years" event, but is a clear warning about the trend of extreme weather under the impact of climate change and pressure from socio-economic development.

“We are witnessing increasingly complex combinations of unfavorable conditions: high accumulated rainfall, a period of extremely heavy rain on top of it, La Nina activity, strong cold air, large waves and tides, altered channel channels, rapidly developing infrastructure that has not kept up with new extreme scenarios. All of these combine to create an unimaginable flood,” said Associate Professor Giang.

As floodwaters receded, many items in Tay Nha Trang ward (Khanh Hoa province) were covered in mud and severely damaged. People were forced to leave them on the streets, causing a sudden increase in garbage (Photo: Trung Thi).

According to him, the biggest lesson from the 2025 “great flood” in the South Central region is that we cannot just look at each piece separately. Forecasting needs to be more closely linked to risk assessment at the basin scale, taking into account climate change and changes in the buffer surface.

Planning for infrastructure, urban and industrial park development needs to be "re-examined" from the perspective of flood drainage, not just as a simple economic, traffic or landscape problem.

“In the future, floods may not occur as often, but when they do occur, the severity will increase. If we do not promptly adjust our perspective, planning and operation of projects, the damage will not stop at 13,000 billion, but could be much higher,” Associate Professor Dr. Nguyen Tien Giang warned.

He believes that each flood provides a realistic "map" of the most vulnerable locations, "bottlenecks" in the flood drainage system, and areas that need priority for infrastructure reinvestment.

The question is whether we have enough determination to turn the very hot lessons learned after natural disasters into concrete actions, before another historic flood strikes.

Photo: Reporter Group

Source: https://dantri.com.vn/khoa-hoc/tran-lu-lich-su-xoa-nhoa-nhieu-kinh-nghiem-sinh-ton-tai-nam-trung-bo-20251125065615219.htm