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In extreme weather, Viet Nam’s hydropower dams create ‘human-made floods’
17 August 2026 at 14:03 (Updated on 18 August 2026 at 10:26)
Rigid operating rules no longer effective to mitigate flooding as weather becomes increasingly unpredictable
Illustration: Deepika Bharadwaj
ĐÀ NẴNG, VIET NAM – Signs of last year’s devastating floods are still visible throughout Hồ Thị Đậu’s house: a broken television, a rusted refrigerator with a loose door and dried mud stains reaching as high as her neck, etched into the discolored walls.
Similar mud stains remain in nearly every house within a 30-minute drive of Đậu’s home. Last October, Đà Nẵng was hit by five floods in only 10 days. Water levels surpassed the historic 1964 record.
Đậu believes this was unusual. “Normally, the water rises slowly, but this time it surged in all at once,” said the resident of Thu Bồn commune.
“It had to be water released from the dams, not rainwater,” the 66-year-old argued.
Many villagers shared her conclusion. In their collective memory, they had never experienced an evening flood followed immediately by another the next morning.
“It’s according to the dam’s release schedule,” said Nguyễn Thị Hai of Vu Gia commune. “When that happens, we have to evacuate immediately.”
For decades, hydropower dams in the Vu Gia-Thu Bồn basin, where Đậu and Hai live, have helped rein in floods. Yet as weather becomes increasingly unpredictable and early warning systems struggle to keep pace, rigid operating rules have amplified flood impacts on the downstream, rather than reducing them.
A flood-prone basin
Vu Gia-Thu Bồn is one of Viet Nam’s largest and wettest river basins, known for its complex and extremely volatile hydrology.
Its two main rivers, the Vu Gia and Thu Bồn, branch into multiple tributaries before draining into the sea through three river mouths. A cascade of 24 hydropower dams ranging from 3 to 200 MW further complicates the flow.
These dams were not designed as flood-control infrastructure, yet they have undertaken this role for years, under an inter-reservoir procedure called Decision 1865. This document dictates how much water the dams can store, and when flood-control operations may start.
Before flood mitigation is activated, water levels in the reservoirs can neither surpass a prescribed ceiling, nor drop below a fixed threshold. These numbers are predetermined, and not adjusted to real-time rainfall forecasts.
In exceptional circumstances, the Chair of the Đà Nẵng People’s Committee may authorize adjustments to these limits to ensure downstream safety.
This system has largely worked as intended for decades.
A study in 2022 showed that reservoirs can reduce flood peaks and downstream flows, limiting inundation in the Vu Gia-Thu Bồn basin, as seen during the 2009 floods. The authors, however, noted that prolonged flooding cannot guarantee this result.
Seperate research into four decades of streamflows also showed that reservoirs reduced the annual average volume in the Vu Gia river, but augmented its flow in the downstream, particularly after 2011, when 12 cascade hydropower dams were built in the sub-basin.
‘Human-made flood’
Extreme weather patterns, such as the October event in Đà Nẵng, have now exposed the weaknesses of this system.
In recent years, rainfall has become increasingly extreme, with many storms far exceeding historical records, said Lê Hùng, a lecturer in Hydraulic Engineering at the University of Đà Nẵng.
Data from 2013 to 2025 show seven storms with rainfall intensities exceeding 80 mm per hour – a level far higher than those recorded in 1979-2012.
The October 26-31, 2025, event coincided with La Niña, exceeding historic levels in 1999, 2007 and 2009.
Forecasts have struggled to keep up with increasingly erratic weather, leaving dams out of sync. During the November 13-18, 2025, floods, for instance, Đắk Mi 4 retained water, while Sông Tranh 2 released it.
Made with Flourish • Create a chart
Earlier this year, the Ministry of Agriculture and Environment acknowledged that forecasting localized extreme weather remains a challenge, particularly during unusually severe events.
The present rules were designed for single floods or those within historical records, not today’s back-to-back extreme events.
They also fail to fully account for the impacts of repeated emergency releases on downstream areas. When heavy rains arrive in quick succession, reservoirs can already be full and forced to release water as the next storm hits, leaving little capacity to reduce flood peaks.
Đà Nẵng’s 2025 floods exposed this problem. Within the first half of the very first moderate flood, all the reservoirs were already full. Without any remaining capacity to absorb floods, operators had no choice but to release water to protect the dams.
Before reservoirs can regain their flood-storage capacity, the next storms may already arrive. If dam releases coincide with natural floodwaters from tributaries, flood peaks can become higher or arrive sooner.
A study in 2013 dubbed this phenomenon ‘stacked flooding’, or ‘human-made flood’. When and how fast water is released, therefore, can be just as important as how much is released.
Ngô Xuân Thế, deputy general director of A Vương Hydropower JSC, insisted that floods recorded at the A Vương reservoir since 2009 have remained within the dam’s limits.
Thế said the dam can only operate within the national rules and the decisions of local authorities. He acknowledged, however, that dam operators faced a constant trade-off between flood control and power generation, especially at the end of the dry and rainy seasons.
At the end of the dry season, if they release all the water, the downstream may face drought if the monsoon arrives late. On the contrary, if they hold back and the floods arrive early, they lose electricity revenue and may exacerbate damage on the downstream.
Similarly, they either risk economic losses or becoming a flood catalyst, if they miscalculate their operations at the end of the monsoon season.
Made with Flourish • Create a chart
In a letter sent to the Ministry of Agriculture and Environment in late March, the Đà Nẵng Department of Agriculture and Environment argued that the rules under Decision 1865 are no longer suited to today’s increasingly erratic floods, and should be revised.
At a meeting in early November 2025, Deputy Minister of Agriculture and Environment Nguyễn Hoàng Hiệp stressed that more effective reservoir management would require stronger accountability from both dam operators and government agencies.
Dak Mi 4 hydropower plant. VIDEO: Thanh Minh
Fixing outdated rules
Associate Professor Vũ Thanh Ca at Ha Noi University of Natural Resources and Environment said Viet Nam’s reservoir rules no longer keep pace with climate change, as hydropower dams were designed using outdated hydrological records.
For example, the Thác Bà hydropower dam was designed in the Soviet Union between 1959 and 1961, when little flood data were available for the Chảy River.
Today’s rainfall and floods have far exceeded those assumptions, pushing the dam into an emergency situation during the 2024 floods.
Ca urged a review of governance rules to give operators more flexibility, balancing dam safety with more effective downstream flood control.
Hùng agreed, arguing that as climate change intensifies, reservoirs need to maximize their flood-storage capacity to protect downstream communities.
Hùng proposed allowing reservoirs to be drawn down to the spillway-gate level before major storms, as long as forecasts are accurate.
He also proposed real-time reservoir management. As rainfall begins, authorities should rapidly forecast inflows, adjust reservoir operations, and produce downstream flood maps within 15-30 minutes to support emergency decisions.
“Rules alone won’t work. They need to be paired with real-time operations and ready-to-use flood risk maps.”
Hùng said inspection frequency should also be constantly reviewed, as situations shift quickly. He also urged operators to incorporate El Niño and La Niña forecasts into reservoir operations, noting that both can now be predicted well in advance.
For example, operators can hold water early during El Niño, and release most of the volume during La Niña.
This year, the US National Oceanic and Atmospheric Administration (NOAA) forecasts that El Niño is likely to develop soon and persist through the winter.
Hùng cautioned that El Niño’s low precipitation does not mean the absence of rain. Short, intense downpours can still occur and trigger flooding.
“Reservoir operations and flood preparedness must not be taken lightly,” he stressed.
These days, some 400 residents of Vạn hamlet of Thu Bồn commune brace themselves for this year’s flood season.
They expect the river will turn into a hungry beast, gnawing at the sandbanks and carving deep cavities, swallowing everything in its path.
“The floodwater swirled and grunted. As soon as the dams release water, the river banks will be flooded,” Tăng Trung, head of the Fatherland Front Committee in Tĩnh Yên village, told Mekong Eye.
“We ran the moment we heard the water, leaving everything behind. I slept with my eyes open. It was terrifying,” Trung said.
In the hamlet, the eroded bank is now only three meters from the nearest homes.
This story was supported by Earth Journalism Network. It was originally published in Vietnamese in Tia Sang magazine.