Hanoi Flood (1971)

Vietnam, which is located in Southeast Asia, has a monsoonal climate in the north with a hot, rainy season from May to September. Hanoi, the capital of Vietnam, is situated in the Red River Delta of North Vietnam. The Red River at Hanoi comprises three major tributary systems: the Da, Thao, and Lo Rivers. The flood season in the Red River lasts from June to October, with particularly serious flooding in August when the water level is generally up to 13–20 feet higher than ground level.

Hanoi is protected from river floods by a 1,000-year-old dike system, which is regularly maintained and upgraded. In addition, several other measures have been taken to protect Hanoi from flooding. They include reservoirs built upstream, established flood diversion and retention areas, and floodwater drained by dredging and extending the river flood plain.

Despite all efforts, in August 1971, a flood of historic proportions occurred in the Red River Delta. This was the most serious flood in the 20th century, and caused four provinces, namely Hai Hung, Ha Bac, Vinh Phu, and Ha Tay, to be heavily flooded. In addition, three other provinces, Nam Ha, Ha Noi, and Tuyen Quang, were partially flooded.

Since the flood happened during wartime, information about this flood was poorly documented and preserved. Moreover, statistical figures vary by source. According to a report by Vietnam's General Statistic Office, there were 1,062 communes flooded, with 2,879,000 people affected in 586,846 households, in which 594 people died. A report by the Disaster Management Center revealed that the total flooded area was approximately 618,000 acres, about 2.71 million people were directly affected, and total direct loss was estimated at about $1 billion in 1971.

According to Vietnam's Hydro meteorological Agency (now under the Ministry of Natural Resources and Environment) and the Ministry of Water Resources (now under the Ministry of Agriculture and Rural Development), reasons for the 1971 flood in the Red River Delta included a combination of factors: especially large flooding on the Lo and Thao Rivers, a rare combination of flooding on Thai Binh and Red (Hong) Rivers, and a combination of upland flooding with lowland inundation due to heavy and prolonged rains. Particularly for Hanoi, rain, upland flooding, and upland broken dikes contributed to the flood in August 1971.

Prior to the flood, riverbeds were full of water and the river basins were saturated. From early July to August 12, rains and floods occurred continuously in the Red and Thai Binh Rivers. From August 12 until August 21, some areas in the upper area of the Red River Delta gauged rainfall from 16–24 inches for several days.

The flooding in the Red River Delta in 1971 can be divided into two stages. The first stage took place from August 13–16. During this time, a small flood occurred in [Page 264]the Da, Thao, and Lo Rivers. As a result, the water level in Hanoi increased to 36.7 feet by noon on August 17. The second stage was August 17–21. Within the first two days of this stage, heavy rains rapidly raised the water levels in all three Da, Thao, and Lo river systems, which affected the water level in Hanoi on August 19. An average water level speed during August 18 and 19 was 1.6 inches per hour. On August 19, several dike sections of the Da, Lo, and Red Rivers were broken, and the Hanoi water level reached 41.5 feet by noon and continued to increase with speeds of 2–3.5 inches per hour. On August 20, a dam was opened and other dikes were broken, and the Hanoi water level reached 45.4 feet by evening. More dikes continued to be broken on August 21, and the Hanoi water level reached its peak of 45.93 feet by the early morning of August 22. On that day, dikes in the lower part of Hanoi were broken, and the Hanoi water level decreased.

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