An extra 1.5 metres at
Masinga goes a long way
The Sunday Nation
Nairobi,
23 May 2010
As we celebrate the filling up of Masinga Dam, the main water storage for
the Seven Forks Hydroelectric complex, a small announcement was made to
the effect that the height of this reservoir will be raised by 1.5
metres. This will be done at a cost of $12 million and increase the
capacity by about 25 per cent.
This announcement has caused some confusion and several readers have
asked me whether this was a typo. Raphael
Munyao put it this way: “I read somewhere that the height of Masinga Dam
is 60m. How can raising it by 1.5m increase the storage by 25 per cent?
Did they mean 1.5m or 15m?”
True; the dam wall at Masinga is 60m high, but when you think about its
shape, you understand why a small increase in the height can produce
large increment in the volume of water.
Dams are normally constructed at a river valley that has a “V” shape in
two orientations: the first running across the front and the second
going upstream from dam wall along the course of the river. Therefore,
one meter depth of water at the bottom holds very little water compared
to the same height near the surface.
When full, Masinga dam covers an area of about 120 square kilometres.
Assuming that the 1.5m will be vertical on all sides, we can calculate
the additional volume of water as follows:
Volume is surface area multiplied by height. One kilometre is equal to
1,000m. Therefore, one square kilometre is 1,000m x 1,000m, equals one
million square-metres. So, 120sq-km is equivalent to 120 million sq-m.
Thus the volume represented by 1.5m is 120 x 1.5 = 180 million cubic
meters.
In case you are wondering, a cubic metre is equivalent to 1,000 litres,
so the additional water comes to 180 billion litres! Now that’s huge by
any standard: it is more than the capacity of Kamburu dam (123 billion
litres). But we have under estimated the answer. The actual amount
expected is about 400 billion litres. The reason for the greater volume
is that the dam’s surface area will also increase after raising the
height – remember it has a “V” shape.
At full capacity, Masinga holds about 1,560 billion litres, therefore
this additional water is about 25 per cent of the current volume. Since
Masinga holds about half of all the water in the Seven Forks complex,
the 400 billion litres will increase the total carrying capacity by 12
per cent.
And that’s not all: Masinga has two turbines that generate 20MW of
electricity each. At full throttle, the combined flow rate of water is
about 100,000 litres per second – yes, that’s not another misprint; it
is one hundred thousand litres per second!
At that rate it would take 4 million seconds to consume the additional
400 billion litres. That is about 1,100 hours or 46 days…assuming all
the rivers feeding the dam run dry.
But the greatest advantage is that once the water has gone through the
Masinga turbines, it has to pass through Kamburu, then Gitaru, followed
by Kindaruma and finally Kiambere before going to the
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