Flood "exceeding 5,000-year frequency" at Ban Ve hydroelectric reservoir is a hydrological statistical model, has not happened in reality.

According to a hydrologist, the phrase “exceeding the test flood and exceeding the 5,000-year frequency” is not a data error but a hydrological term, referring to the extreme level of the flood. This is a number based on probability calculations and hydrological statistical models, not something that has ever been witnessed in reality.

In the report on the damage caused by storm No. 3 and floods as of this morning (July 23) by the Department of Dyke Management and Natural Disaster Prevention ( Ministry of Agriculture and Environment ), Ban Ve hydroelectric reservoir (in Nghe An province) had a flood peak at 2:00 a.m. of up to 12,800 m³/second, exceeding the flood control level and "exceeding the 5,000-year frequency".

The Department of Dyke Management and Natural Disaster Prevention also said that the entire region from the North to Ha Tinh has 4,487 irrigation reservoirs, most of which have been filled with water at high levels, especially the reservoirs in Nghe An which are basically full. The current water storage capacity is 56-85% of the design capacity, posing a potential risk of insecurity when there are more heavy rains.

Previously, on the night of July 22, the People's Committee of Nghe An province also issued an urgent notice to localities stating that 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%, meaning it occurs once every 5,000 years).

Heavy rains and floods caused many communes in Nghe An to be deeply flooded.

Ban Ve Hydropower Plant is the largest hydropower project in Nghe An province and the North Central region, with a designed capacity of 320MW and a basin area of more than 8,700km².

The information about "floods exceeding the 5,000-year frequency" has caused public concern and concern about the level of impact of the flood. According to a hydrologist, the phrase "exceeding the test flood and exceeding the 5,000-year frequency" is not a data error but a specialized term in hydrology, indicating the extreme level of the flood.

“5,000-year frequency” means that the flood has a probability of occurring once in 5,000 years (equivalent to 0.02% per year). This is an extremely rare flood level, requiring proper technical regulation to ensure safety for the project and downstream.

The Ministry of Agriculture and Environment said that Ban Ve Lake had to open its floodgates from the afternoon of July 22 to regulate. This morning, July 23, the lake's water level was still high (but the total flood water had decreased significantly) and was being discharged at 4,351 m³/second to ensure a balance between the safety of the project and flood control to help downstream areas.

Director of Ban Ve Hydropower Plant Ta Huu Hung also explained that the number “once every 5,000 years” is a technical statistical parameter. “The frequency of 0.02% corresponds to a cycle of 5,000 years. Similarly, the frequency of 1% is 100 years, 0.1% is 1,000 years. When constructing hydropower projects, people apply two frequency levels: Design flood and test flood. Particularly for Ban Ve Hydropower Plant - a special project - the test flood level is determined by the frequency of 0.02%”, Mr. Hung said. This is a number based on probability calculations and hydrological statistical models, not something that has ever been witnessed in reality.

Source: https://cand.com.vn/Xa-hoi/lu-vuot-tan-suat-5-000-nam-o-ho-thuy-dien-ban-ve-la-mo-hinh-thong-ke-thuy-van-chua-xay-ra-trong-thuc-te--i775720/