What are floods that "exceed the 5,000-year frequency"?

Ban Ve hydroelectric plant releases floodwaters. Photo: NGUYEN LONG

The Department of Dyke Management and Disaster Prevention ( Ministry of Agriculture and Environment ) has just released a report on the damage caused by Typhoon No. 3 and floods as of the morning of July 23.

Accordingly, Typhoon No. 3 (Wipha) and its aftermath caused widespread damage in Northern and North Central Vietnam, especially in the provinces of Nghe An, Thanh Hoa, Hanoi , Ninh Binh, and Hung Yen.

Initial reports of casualties and property damage from local authorities are as follows: 1 person missing, 1 person injured due to flash floods in Nghe An province.

Tuong Duong area (Nghe An province) this morning, July 23. Photo: Social media.

420 houses had their roofs blown off or were damaged by the storm, including 251 in Thanh Hoa and 161 in Nghe An. Over 119,000 hectares of rice fields were flooded, with Ninh Binh suffering the most damage at over 74,000 hectares. Local authorities are urgently operating pumps to drain the floodwaters.

Regarding dikes and embankments, six incidents have been detected so far, including two serious incidents in Hanoi: A 20-meter-long crack appeared on the right bank dike of the Cau River (Da Phuc commune) (local authorities have set up a checkpoint to prohibit vehicles from passing through). A 600-meter-long crack appeared on the right bank dike of the Red River (Phuc Loc commune) (a crack that occurred in 2024 and has now widened); the entire area has been fenced off.

In Thanh Hoa and Ninh Binh provinces, numerous embankment collapses and culvert failures have occurred: the West Cung River embankment, the Tam Diep canal embankment, the Nam Quan Lieu embankment, the Vuc Buu culvert, etc. Local authorities have initially addressed the issues by filling in the sinkholes with sandbags, driving bamboo stakes, and filtering out the collapsed sections.

Notably, the Ban Ve hydroelectric reservoir (in Nghe An province) experienced a peak flood flow of 12,800 m³/second at 2:00 AM on July 23rd, exceeding the flood control level and "surpassing the 5,000-year frequency".

Water levels exceeded 12,000 m³/second at Ban Ve reservoir early this morning, July 23rd. Source: EVN

According to a hydrology expert, the phrase "exceeding the flood test and exceeding the 5,000-year frequency" is not a data error but a technical term in hydrology, indicating the extreme nature of the flood. "5,000-year frequency" means the flood has a probability of occurring once every 5,000 years (equivalent to 0.02% per year). This is an extremely rare flood level, requiring proper technical regulation to ensure the safety of the structure and downstream areas.

The Ministry of Agriculture and Environment stated that the Ban Ve reservoir had to open its sluice gates from the afternoon of July 22nd to regulate water levels. This morning, July 23rd, the reservoir's water level remains high (but the total floodwater inflow has decreased significantly) and is being discharged at a rate of 4,351 m³/second to ensure a balance between structural safety and flood control for downstream areas.

Tuong Duong (Nghe An province) was submerged in floodwaters on the morning of July 23. (Photo shared on social media)

The Department of Dike Management and Disaster Prevention also stated that the entire region from Northern Vietnam to Ha Tinh has 4,487 irrigation reservoirs, most of which have accumulated water at high levels, especially those in Nghe An which are basically full. The current water storage volume reaches 56-85% of the designed capacity, posing a potential safety risk in the event of further heavy rainfall.

ANGEL

Source: https://www.sggp.org.vn/lu-vuot-tan-suat-5000-nam-la-gi-post805034.html