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Tin-roofed homes marooned by knee-high flood water with no one in sight.
Homes marooned by rising flood water at Muhoro in Tanzania’s Rufiji District village in April, 2024. Photo by -/AFP via Getty Images

Tanzania’s dams: flood risk depends on how they’re planned and operated

The Rufiji River, which drains into Tanzania’s south-east coast, experienced a major wave of flooding in April 2024. The flooding caused tragic loss of life and affected at least 88,000 individuals. More than 28,000 hectares of crops were damaged.

There has been much debate in Tanzania on the causes of this disaster, particularly the presumed role of the new Julius Nyerere Dam, which is built on the river. Barnaby Dye has studied the development and funding of dams, including those in Tanzania. He provides some insights into the potential risks and solutions.

What are the large dams in Tanzania and what were they built for?

Tanzania has a long history of dam building, from its early independence days in 1961. The country’s founding leader, Julius Nyerere, inaugurated the small Nyumba Ya Mungu hydroelectric plant in 1968. A steady programme of large dams followed. These included the dual Kidatu and Mtera dams completed between 1975 and 1988. The New Pangani Falls dam and Kihansi were completed in the mid to late 1990s.

The primary goal for all these dams was hydropower, which has historically dominated Tanzania’s electricity mix.

The 20th century also saw the dominance of an ideology trumpeting the power of these dams – and their electricity – to transform Tanzania’s economy into an industrialised society. The long-planned Stiegler’s Gorge Dam, in particular, which was recently renamed the Julius Nyerere Hydropower Project, captured these development dreams. They were part of Nyerere’s socialist vision for creating a so-called modern developed country.

However, a reliance on hydropower in the 21st century has plunged the country into repeated power cuts during droughts. Hydropower is also being questioned, given long build times, and environmental and social costs. There was a fall in dam building as the government prioritised quicker-win, and sometimes deeply corrupt, gas and oil plants.

This changed with the arrival of President John Magufuli (2015-2021), who decided that the Julius Nyerere 2.1 gigawatt megadam was the answer to Tanzania’s development and electricity needs. He refocused stagnant planning efforts and construction started in 2018.

Six years later, the dam is nearing completion, with the main dam wall and reservoir in place and first turbines operational.

Do any of these dams pose particular risks in the event of flooding?

Dams can prevent floods, storing water in large reservoirs and slowly releasing it downstream. But they can also make flooding worse, or trigger a disaster.

Dam collapses caused by poor maintenance, incorrect operation, or inadequate planning and construction quality are among the worst human-made disasters. The 2019 collapse of a Brazilian dam, for example, killed at least 250 people. China’s 1975 dam disaster killed 240,000 people after heavy rainfall overwhelmed a series of dam walls.

None of Tanzania’s dams have been built primarily for flood control. Most 20th Century dams operate more like run-of-river projects, meaning that they are built to constantly produce electricity and not to store significant volumes of water from the rainy season for drier spells. Therefore, with the exception of Mtera Dam, Tanzania has not historically had the storage reservoirs to prevent significant flooding.

The Julius Nyerere Dam could be different given its large reservoir. However, some media reports blamed the Julius Nyerere Dam for the 2024 floods,, as the new hydropower project sits directly upstream of the area that flooded in April. Other reports argued that it prevented a worse flood. It’s difficult to judge as little has been released about the current design and operation of the nearly-complete dam.

Earlier versions of the design envisaged a large storage dam. So it’s plausible that the dam is benign, as the government has claimed. Official spokespeople insisted that it prevented flooding in 2023 when the reservoir was being filled.

Without the necessary information, though, it’s impossible to reject arguments that the dam caused the destructive flooding. Tanzania has endured painful and constant power cuts. Thus, it is plausible that the government sought to maximise electricity generation from the new dam. Such a strategy would involve keeping the reservoir at its highest level over time. This could leave authorities ill prepared to store water from abnormally heavy rains like those experienced across east Africa in 2024.

As the reservoir approached dangerously high levels, dam operators would need to suddenly release as much water as possible to prevent it from overflowing and breaking the dam wall. Such actions, while preventing a worse dam collapse, would have caused severe flooding. Indeed, officials from the state-owned electricity utility reportedly stated that a release from the Julius Nyerere Dam caused April’s floods.

Thus, Tanzania’s dams, like others around the world, constitute a flood risk whose likelihood depends on how the dams are planned and operated.

What are some solutions to the flood risk?

Climate change models predict increased rainfall variability, and therefore more floods, in Tanzania’s future. Given the inherent risk of emergency dam releases in the short term, the government needs an effective early warning system to alert those downstream when water releases occur. Such a system seems to have failed this year.

Longer-term solutions should focus on slowing water and addressing the ultimate cause of flooding: having too much water in too short a time.

The government’s proposal involves construction of more dams. In my view, this approach to flood control seems shortsighted. These dams could worsen, rather than solve, extreme floods. And planned dams are designed only for hydropower – they leave little storage for flood prevention.

New dams on the Rufiji River come with major trade-offs as they pose a risk for other economic mainstays:

Natural infrastructure that slows water movement, like wetland or groundwater capture, holds the best potential. It is a cheaper, more effective solution, with economic opportunities for livelihood diversification. Equally, adaptation may hold the key, as researchers Stéphanie Duvail, Olivier Hamerlynck and colleagues found in thier participatory study. Changing housing and agriculture to cope with periodic flooding would allow Tanzania to enjoy the economic benefits that natural river floods bring.

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