Was Nairobi Water Co. responsible for overflow at Seven Forks?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 May 2020

There is widespread belief that the name “Seven Forks” for the dams along the Tana is derived from the number of power stations that were planned to be built along that river. But that makes one wonder: why wasn’t it called Seven Dams? Why “forks”?

There are five dams along the river, namely: Masinga, Kaburu, Gitaru, Kindaruma and Kiambere. The section between Gitaru and Kindaruma has an interesting natural feature: the river splits into seven streams (also known as, forks) and then rejoins into one before flowing into Kindaruma dam. I don’t know why this is not a tourist site like the 14 Falls of River Athi are.

Nevertheless, this is the reason why the entire project was named the Seven Forks System. As for the number of dams envisaged; when the feasibility study was carried out in the early seventies, a total of 11 (eleven!) sites were identified along the Tana where hydroelectric dams could be constructed.

In other news, the dams in the Seven Forks System filled up two weeks ago and the spillways had to be opened to let out excess water into the river. Unfortunately, this action caused flooding in downstream areas of Garissa, Hola and others.

At around the same time, the Nairobi Water Company announced that it had shut down its main treatment plant at Ng’ethu due to heavy rainfall in the catchment area which brought in too much silt. Now, Ng’ethu gets water from rivers Chania and Thika. A total of about 430 million litres is fetched each day and supplied to the city.

Now these two rivers (Chania and Thika) join in Thika town and continue as one into Masinga dam. However, after usage, the waste water from Nairobi is treated at the Ruai Sewerage plant and then emptied into the river Athi.

Rivers Athi and Tana do not meet anywhere along their courses. River Athi flows to the 14-Falls near Kilimambogo and then onwards to the Indian Ocean. Therefore, under normal conditions, Nairobi fetches water from the Tana and empties it into the Athi. So, could the shut down of Ng’ethu have been responsible for the over-flow in the Seven Forks?

In the two days of the shut down, a total of 860 million litres was not fetched from rivers Chania and Thika rivers. All this “extra” water went to Masinga. So, by how much did it increase the dam level?

Masinga has a surface are of about 120 square kilometres. This is equivalent to 120 million square metres; or 1.2 trillion square centimetres.

Now one litre is 1,000 cubic centimetres; so, 860 million litres is equivalent to 860 billion cubic centimetres. If we divide this volume of water by the surface area of the dam, we get the height increment in centimetres.

The answer is 0.72cm; or 7.2mm (millimetres!). Is this a significant increment? Well, the dam is about 50m deep near the wall; so, do you think an additional 7 millimeters makes a considerable difference?