Correct understanding of the information 'once in 5,000 years flood'

(Chinhphu.vn) – “Floods occurring once every 5,000 years” is the frequency of floods (or the possibility of floods occurring), not reflecting the specific, fixed time when floods occur. The assessment of “100-year floods” and “5,000-year floods” is to calculate the typical magnitude of rainfall, water level, and flow in the design of dikes, reservoirs, and disaster prevention planning, not to predict the specific time of floods.

"5,000-year flood" is a statistical interpolation method based on a series of rainfall and flood data observed over many years - Illustration photo

According to urgent notice No. 604/TB-UBND dated July 22 of Nghe An province, information about the water flow upstream of Ban Ve hydropower reservoir reached 9,543 m³/s, nearly reaching the peak flood flow of 10,500 m³/s (frequency ~ 0.02%, meaning it happens once every 5,000 years). Before this information, public opinion raised the question of whether a similar flood would only happen once every 5,000 years?

Director of the Center for Hydrological and Oceanographic Research, Institute of Meteorology, Hydrology and Climate Change ( Ministry of Agriculture and Environment ) Luong Huu Dung spoke with the press to clarify this issue.

According to Mr. Luong Huu Dung, regarding the urgent notice No. 604 of the People's Committee of Nghe An province with the information "floods that only occur 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 that it only occurs once every 5,000 years. This refers to the possibility of occurrence and is not associated with time because floods occur randomly so they can happen at any time. With floods with a recurrence period of 5,000 years corresponding to a probability of 0.02%, it means that the possibility of a flood larger than or equal to this flood is very rare.

In more detail, Mr. Luong Huu Dung said that if a flood has a recurrence period of 5,000 years, that means the probability of it occurring is 1/5,000, or 0.02%. This is a probability value (or possibility of occurrence), not tied to time.

Conceptually, in hydrology, flood frequency (symbol P) is understood as the probability of exceeding. That is, the probability of a flood occurring with a peak flow greater than or equal to the flow corresponding to this probability. Because this is a statistical probability, not a rigid cycle. A flood with a probability of 0.02% does not mean that it must wait exactly 5,000 years to repeat. In theory, it can still happen next year, but the probability is very low.

Director of the Center for Hydrological and Oceanographic Research, Institute of Meteorology, Hydrology and Climate Change (Ministry of Agriculture and Environment) Luong Huu Dung

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

For example, a 2-year flood has a 50% chance of occurring each year. A 5,000-year flood has a probability of only 0.02% per year, extremely rare but not impossible.

In response to the question in the case of August, if there is another rain similar to July, will there be another announcement of a "5,000-year flood"?

Mr. Luong Huu Dung said: "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 "there has been a flood, so there will be another one in 5,000 years" or "Vietnam's history is only 4,000 years, so how can we know about a 5,000-year flood?" It should be noted that the lower the flood frequency, the greater the rarity and extremity of the phenomenon, but that does not mean it will not happen again.

It is important that the media and professional agencies make it clear that this is the frequency of floods (or the likelihood of floods), not a reflection of the specific, fixed time when floods occur.

The assessment of "100-year flood" and "5,000-year flood" is to calculate the typical magnitude of rainfall, 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 subjectivity or unnecessary panic.

Thu Cuc

Source: https://baochinhphu.vn/hieu-dung-ve-thong-tin-lu-5000-nam-moi-co-mot-lan-102250723165638181.htm