There has been a big misconception that dams cause floods, but the reverse is true, in many regions around the world, dams have purposely been constructed to mitigate floods that are among destructive natural disasters that occur as a result of excess water in flood-prone regions.
In Kenya, a case study to demystify the role of dams in controlling floods can be cited from the Seven Forks Dams that are in the Eastern part of the country.
For decades, the Seven Forks Dams in the lower Tana region of the Eastern part of Kenya have been known for power generation by Kenya Electricity Generating Company (KenGen). However, what many people are not aware of is that as much as the dams are used to produce electricity, they also play a key role in mitigating flooding downstream during extremely rainy seasons.
How does the Seven Forks Dams in the eastern region work to mitigate against flooding?
The concept around dams controlling flooding is simple: the reservoirs hold huge amounts of water that could have naturally flown downstream, therefore filling rivers and breaking banks. For the case of the Seven Forks cascade, five such dams are involved in this concept whereby once each dam reaches its maximum levels, the excess water naturally overflows from one dam to the next through the natural river channel.
Masinga is the main reservoir and the first dam in the cascade with a capacity of 1.56 billion cubic metres. The dam, commissioned in 1981, has a power plant that generates 40MW of electric power and besides that its reservoir occupies a surface area of 120km2. With this amount of storage space, the dam can hold a lot of water, thereby lessening excess flows in the downstream region.
Water from Masinga dam is conveyed to the next dam - Kamburu which was commissioned in 1974. Kamburu generates 94MW and has a capacity of 150 million cubic metres with its main sources of water being Masinga Dam and River Thiba. With the addition of water from the River Thiba, more water is likely to spill downstream, but it is conveyed to the next power station (Gitaru) through a 2.9-kilometre tailrace tunnel.
Gitaru dam, with a water capacity of 20 million cubic metres holds more water from going downstream. Here, this water is harnessed to generate 225MW, making this plant the biggest hydropower station in East Africa in terms of installed capacity. Water from this dam is then passed on to Kindaruma, the oldest power station in the cascade via an underground 4.7-kilometre tailrace tunnel.
Water from Gitaru and other seasonal rivers feeds the Kindaruma dam which was commissioned in 1968. Currently, this dam hosts the first major power station in Kenya with an installed capacity of 72MW and once it overflows, water is conveyed to the last dam in the cascade which is the Kiambere dam.
Kiambere dam was commissioned in 1988 with an installed capacity of 168MW and has a water holding capacity of 585 million cubic metres. As a way of mitigating floods, the dam has a natural spillway, a low-level outlet at the main dam, an emergency spillway and intake at the saddle dam. The emergency spillway is provided to mitigate against extreme floods. After this dam, water flows naturally downstream to River Tana, then to the Indian Ocean.
That said, these dams have over the years continued to hold huge volumes of water that would have otherwise caused major floods in the lower region if allowed to flow freely. After KenGen Hydro-electricity dams, the Government through its development agenda has proposed to build more multipurpose dams in the lower parts namely: Mutonga and Low Grand Falls dams that will further combine forces with the current five dams in the Seven Forks to help in mitigating floods in those areas. This is the way to go.