Experts explain the information about "flood frequency once every 5,000 years" in Nghe An

According to experts, the information 'floods that only happen once every 5,000 years' must be understood correctly as a flood with a magnitude (scale) corresponding to a flood with a recurrence period of 5,000 years, not occurring once every 5,000 years.

Ban Ve Hydroelectric Reservoir

Regarding the information about the water flow upstream of Ban Ve hydropower reservoir reaching 9,543m3/s, nearly reaching the peak flood flow of 10,500m3/s (equivalent to "flood frequency of 5,000 years"), Dr. Luong Huu Dung - Director of the Center for Hydrological and Oceanographic Research (Institute of Meteorology, Hydrology and Climate Change, Ministry of Agriculture and Environment ) said that this is a probability value, not related to the time of occurrence.

Previously, on the evening of July 22, the Chairman of the People's Committee of Nghe An province issued urgent notice No. 604/TB-UBND, informing: At 9:00 p.m. on July 22, the flow upstream of Ban Ve hydropower reservoir reached 9,543m3/s, nearly reaching the peak flood flow of 10,500m3/s (frequency 0.02%, which means it only happens once every 5,000 years).

The "invisible" information has raised many questions - is it true that a flood similar to the one that just happened in Nghe An only happens once every 5,000 years?

Sharing the above information with the press, Dr. Luong Huu Dung said that the above information is the value that occurred around the night of July 22, when the water flow into the hydroelectric reservoir reached 9,543 m3/s; then at 2:00 a.m. on July 23, 2025, the peak flood flow reached 12,800 m3/s, exceeding 2,300 m3/s compared to the peak flood test with a frequency of p=0.02 (corresponding to a 5000-year recurrence cycle) of 10,500 m³/s.

"Saying this shows that the level of abnormality, extremely rare flooding has occurred on the upper reaches of the Ca River, posing potential danger to the reservoir project, residents and infrastructure," Mr. Dung emphasized.

Further analysis, Mr. Dung said that the information "floods only happen once every 5,000 years" must be understood correctly as a flood with a magnitude (scale) corresponding to a flood with a recurrence period of 5,000 years, not once every 5,000 years, and this only refers to the possibility of occurrence and is not related to time. Because floods occur randomly, they can happen at any time.

According to Mr. Dung, with a flood with a return period of 5,000 years corresponding to a probability of 0.02%, it means that the possibility of a flood greater than or equal to the flood at Ban Ve Lake (specifically 10,500 m3/s) is very rare. If a flood has a return period of 5,000 years, that means the probability of it occurring is 1/5,000, or 0.02%. This is a probability value (likelihood of occurrence), not tied to the time of occurrence.

“We should understand that saying ‘5,000-year flood’ is how experts statistically interpolate based on a series of rainfall and flood data over many years. It is similar to saying ‘historical flood’ but here the probability is clearly calculated,” said Mr. Dung.

Many agencies and houses in Muong Xen commune, Nghe An were severely flooded.

Mr. Dung also noted that if the actual measured data on rainfall and flood flow are still within the same statistical threshold as the previous event, it can still be assessed as a frequency of 0.02% corresponding to a 5,000-year recurrence cycle. This is not contradictory, because it only reflects statistical probability.

"Therefore, people and the media should not misunderstand that if there has been a flood, the next one will not happen for another 5,000 years or that Vietnam's history is only 4,000 years old, so how can we know about a 5,000-year flood? It should be noted that the lower the frequency of the flood, the greater the rarity and extremity of the phenomenon, but that does not mean it will not happen again," Mr. Dung shared.

According to Mr. Dung, it is important that the media and professional agencies clearly state that this is the flood frequency (or the possibility of a flood), not a specific, fixed time when a flood occurs.

The assessment of “100-year flood” and “5,000-year flood” is to calculate the typical magnitude of rain, water level, and flow in the design of dikes, reservoirs, and disaster prevention planning, not to predict the specific time of flood occurrence.

“Misunderstanding this can lead to unnecessary subjectivity or confusion. Therefore, I hope that when communicating, the media must include a clear explanation of the probability,” Mr. Dung recommended.

VNA

Source: https://baohaiphongplus.vn/chuyen-gia-ly-giai-thong-tin-lu-tan-suat-5-000-nam-moi-co-mot-lan-o-nghe-an-417069.html