Why did storm Yagi cause flooding in 20 northern provinces?

A series of records for heavy rain appeared

As of the evening of September 15, storm No. 3 (storm Yagi ) had left 330 people dead or missing (292 dead, 38 missing); 1,908 injured, 168,253 houses damaged; 73,248 houses flooded; 183,394 hectares of rice, 44,071 hectares of crops flooded, damaged...

20/25 Northern provinces flooded after storm Yagi

Assessing the unusual heavy rain of this storm's circulation, the National Center for Hydro-Meteorological Forecasting said that from September 7 to 12, the North and Thanh Hoa will have heavy to very heavy rain with common rainfall of 250 - 450 mm, some places over 550 mm. At 83/84 measuring stations, rainfall is 4 - 6 times higher than the average of many years in the first 10 days of September.

Typically, at Sa Pa station (Lao Cai) recorded 517 mm, 440% higher; Luc Yen ( Yen Bai ) 503 mm, 461% higher; Dinh Hoa (Thai Nguyen) 545 mm, 677% higher; Son Dong (Bac Giang) 386 mm, 488% higher than the average of many years in the same period.

The meteorological agency assessed that storm No. 3 moved deep inland and weakened into a tropical depression in the northwest region. However, the heaviest rain caused by storm No. 3's circulation was mainly in the east of the Hoang Lien Son mountain range, although it was not in the storm's path and was not directly affected by the storm's winds. Most storms with similar trajectories in the past often caused heavy rain in the west of the Hoang Lien Son mountain range.

Very heavy rain over a large area (including many provinces) in the basins of Thao River, Chay River, Lo River, Gam River with high intensity (over 200 mm/day), lasting for many days after the storm dissipated; in which, there were areas with rainfall over 200 mm within just 2 hours (in Yen Bai City, night of September 9).

Red River flood in Hanoi exceeds 20-year record

Mr. Mai Van Khiem, Director of the National Center for Hydro-Meteorological Forecasting, said that heavy rain after the storm caused water levels on many rivers and streams in the North to rise rapidly. In particular, water levels on Thao River, Lo River, Thuong River, Gam River, Thai Binh River, the lower reaches of the Red River, Luc Nam River, Hoang Long River... all exceeded alert level 3 (BĐ3), some rivers exceeded BĐ3 by 3-4 m.

In particular, the flood on Thao River in Lao Cai and Yen Bai has exceeded the historical flood peak that existed for 53 years. The flood peak in Yen Bai reached 35.73 m, 3.73 m above BĐ3 level and 1.31 m above the historical flood level in 1968.

Along with that, the flood on Thac Ba hydroelectric reservoir also reached historical levels, threatening the safety of the dam. Specifically, Thac Ba hydroelectric reservoir was surveyed and designed from 1959 - 1961, hydrological data measuring the flow at that time was limited, so the spillway design had a maximum discharge capacity of only 3,230 m ³ / s. Meanwhile, the actual maximum flow to Thac Ba reservoir was 5,620 m ³ / s at 9:00 a.m. on September 10, exceeding the designed flood peak by 0.01% (5,100 m ³ / s) and far exceeding the maximum discharge capacity by 74%.

According to the operating procedure, when the water level of Thac Ba Lake reaches 59.60 m, it will switch to special operating mode to ensure dam safety. In fact, from 5:00 p.m. on September 10, the water level of Thac Ba Lake reached 59.62 m and switched to special operating mode, then reached its highest level of 59.84 m at 5:00 a.m. on September 11. In theory, the test water level of Thac Ba Lake is 61 m and if the actual water level reaches this threshold, measures must be taken to ensure dam safety.

In Hanoi, the water level of the Red River also recorded its highest level in 20 years. Rivers in the Red River and Thai Binh basins - the largest river system in the North - also experienced floods. Flooding on a large scale with many levels exceeding the threshold is also rare. According to statistics, 20/25 northern provinces and cities experienced serious flooding.

Thanhnien.vn

Source: https://thanhnien.vn/vi-sao-bao-yagi-gay-ngap-lut-cho-20-tinh-mien-bac-185240915184844325.htm