Why call alarm 1, alarm 3?

In the current flood warning system, river floods are calculated according to 3 levels: Alert 1 (abbreviated: BĐ I), Alert II (BĐ II) and Alert III (BĐ III). In the North, the flood warning system is measured at stations such as: Thao River (in Yen Bai ), Thao River (in Phu Tho), Cau River (in Dap Cau), Thuong River (in Phu Lang Thuong), Luc Nam River (in Luc Nam), Thai Binh River (in Thai Binh), Lo River (in Tuyen Quang and Vu Quang), Hoang Long River (in Ben De), Red River (in Hanoi).

Source: https://danviet.vn/toan-canh-he-thong-canh-bao-lu-song-hong-va-he-thong-cac-song-mien-bac-vi-sao-goi-bao-dong-1-bao-dong-3-20240911190816701.htm