Theewaterskloof dam (Cape Town's primary water supply) in August 2017, 24% full

Mythbusting solutions to Cape Town’s water crisis

Here’s an only half serious take on some of the suggestions I’ve heard to deal with Cape Town’s water crisis. These are solutions that I’ve seen proposed to magically, quickly save us from the catastrophic water shortage we are facing in Cape Town, usually by people reluctant to adjust their own water consumption to face the new normal. They are mostly ridiculous, and fun to mythbust using an extremely low-level bastardisation of Fermi estimation. (They’ve put me fondly in mind of Prof Hahn’s first year applied maths lecture in which he instructed us to estimate the volume of a cow by assuming – to make the maths easier – that it was spherical in shape.) I digress:

Can we tow an iceberg from Antarctica to melt for fresh water?

PhD student Neil Malan wrote a detailed explanation of why this is an infeasible (I’m being polite here) idea. Here’s a highlight…

Icebergs are large, and heavy. Therefore, in order to get a decent-sized (say 20 km long) iceberg it is estimated that some twenty large oceangoing tugs would be needed to move the iceberg the 6 000 odd kilometres from the Southern Ocean to Cape Town. This would be done at a speed of approximately one knot, thus making a journey of 250 days to reach the Cape and losing about 40% of its mass along the way.

… and here’s the whole article, which I urge you to read in order to get a sense of some of the scales (distance, volume, time) involved in such a proposal.

Does anyone sane and respectable think it can be done? Well, yes, actually – salvage master Nick Sloane (who righted the Costa Concordia, and who is a singularly impressive individual), thinks it is a feasible solution. Judge for yourself who is right – listen to his comments on Cape Talk radio and weigh them up against Neil Malan’s reservations.

Has it been done anywhere else? No, but some people in Dubai are talking about it, and expert consensus is that it’s pretty much impossible.

Theewaterskloof dam (Cape Town's primary water supply) in August 2017, 24% full
Theewaterskloof dam (Cape Town’s primary water supply) in August 2017, 24% full

Can we supply water to Cape Town using water tankers from other parts of South Africa?

Let’s do some maths. At current water usage rates, Cape Town is using more than 500 megalitres (million litres) of water per day. Let’s round it down to 500 for ease of calculation. If we divide 500,000,000 by 24 (to get usage per hour), and then by 60 (to get usage per minute), and again by 60 (to get usage per second), we arrive at a figure of 5,787 kilolitres per second. (Just pause on that for a moment – it’s a big number.)

If a tanker truck can carry 20,000 litres, it’ll take one of these trucks arriving about every three seconds, around the clock, to provide enough water for the city. Let’s generously assume that such a truck can be filled, drive to Cape Town, offload its water, and return to its point of origin (which would have to be somewhere far away that has spare water – an entirely mythical place in South Africa at present) in 24 hours. If we multiply 24 hours by 60 minutes by 60 seconds divided by three (we only need a truck every three seconds, remember), we’ll need 28,800 tanker trucks running continuously to provide Cape Town’s water.

I can tell you, with great certainty, that there aren’t that many tanker trucks in South Africa, let alone unused tanker trucks with nothing to do other than drive back and forth bringing water to the Cape. (Who’s going to pay for this? Where will the water come from? Where will they park? Another story.) One water tanker costs in the ballpark of R1.3 million. A few tens of them are usually procured at a time, not thousands.

Think I’m being greedy asking for 500 megalitres per day? Let’s halve our water usage; then we’ll only need 14,400 tankers on the road. Think we should also rather calculate using 40,000 litre trucks? Then we’ll only need 7,200 tankers. STILL TOO MANY.

Can we build a long pipe from the Orange River to Cape Town, to bring us water?

Assuming that there was spare water in the Orange River (there’s not – almost all of it is allocated to agriculture), could we build a pipeline to bring it to Cape Town? We’re talking a distance of 600-800 kilometres here.

Pipelines are expensive and it’s hard to pull numbers out of the air, so let’s look at a South African example. The Gariep pipeline, from the Gariep dam to Mangaung in the Free State, was proposed in 2015. It was to be 180 kilometres long and to cost R2 billion. It would transport 130 megalitres of water per day (about 20 percent of Cape Town’s current daily water usage).

By 2016, it was a “multi-billion Rand” project, hadn’t been started yet, and was projected to take more than five years to complete. The engineers’ website says it is to cost R4.5 billion.

If we multiply R4.5 billion by three, because our imaginary pipeline is at least three times the length of the Gariep pipeline, we arrive at a figure of 13.5 billion ZARs. For a mere 8.5 billion ZARs, we could build a desalination plant that would supply 450 megalitres per day. This is about two thirds of recent usage, and our actual target usage under level 6B water restrictions, using water that is definitely there (the sea) instead of water that isn’t (spare capacity in the Orange River). We could buy some chocolate with the R5 billion left over.

Could we build a 600 kilometre pipeline quickly enough to help our current situation? These things take a long time, and in South Africa we don’t have a track record of speedy project completions, corruption-free tender processes, and trouble-free execution of projects. We can look to this 40 kilometre pipeline near Durban for an idea of how long a big project like this could take; phase two of the project was commenced in 2012, and completion “was expected” by mid-2017. So for a 600 kilometre project, are we looking at 75 years to completion?

Can’t we dig the dams deeper so that they store more water?

Cumulative annual rainfall at our weather station in Sun Valley
Cumulative annual rainfall at our weather station in Sun Valley

The problem is not that the dams are not deep enough, it’s that there has not been enough rain to fill them. Here’s a helpful interview with the regional head for the Department of Water and Sanitation.

While we’re talking about this, when building a dam the engineers don’t typically go out and dig a big hole to fill with water. Dams are typically structures that block river valleys, allowing the river to flood the land behind the dam wall as the natural course of the water flow is obstructed. Whatever used to be on the land – farms, homes, wildlife – can’t be there any more.

A cubic metre of soil weighs at least 1.5 tons (obviously depends on the type of soil), and removing this would make space for one kilolitre of water. Theewaterskloof  Dam has a capacity of 480 million cubic metres (480 million kilolitres) of water.

What does this much water look like? Well, if we put our Fermi estimation hats on and approximate the dimensions of just the flat-topped part of Table Mountain by a right rectangular prism with dimensions of 1,000 metres (height), 1,000 metres (length) and 200 metres (width), we arrive at a volume for the iconic flat bit of Table Mountain – minus the skirt that sprawls towards Camps Bay, Rondebosch, and the City Bowl, and the Twelve Apostles and the rest of the chain that spreads down the peninsula – of 200 million cubic metres. We could thus hide Table Mountain twice over (broken into bits, obvs) in Theewaterskloof dam and it still wouldn’t be full.

Theewaterskloof dam was built at the head of a valley where farmers once grew grape vines. You can see the dead vines sticking out of the sand now, when you visit the dam. There isn’t one big river that runs into the dam, but numerous small streams as well as the general runoff from the catchment area around the dam, which is about 500 square kilometres in extent. If the total volume of the dam had to be excavated, rather than using a natural valley, you’d be left with at least 720 millon tons of earth to dispose of. A Table Mountain-sized problem.

But the United Nations says that water is one of my human rights, so the taps can’t run dry!

Ignoring the fact that if there is no water, then your “human right” can’t be catered for, it is instructive to read what the UN actually says about the right to water and sanitation. I refer you to this media brief (pdf), which sets it out in some detail, with examples.

The amount of water you are entitled to is not unlimited. Between the UN and South Africa’s Constitutional Court the recommended amount is somewhere between 25 and 50 litres, and it is not required to be free (it simply has to be “affordable”). Page 7 of the media brief corrects some of the common misconceptions around this human right.

Anyway – my view of human rights, particularly in a country as thoroughly damaged as South Africa, where I am emphatically not one of the most vulnerable or disadvantaged members of the population, is that my rights are to whatever I can provide for myself. There are far more needy and less able people than I, and in terms of state or municipal or welfare assistance, their needs have to come first.

Enough already

This is ridiculous, and I’m tired. Solutions to water crisis? Use less water. No, less than that. Practise radical personal responsibility. No one is coming to help, except perhaps – if you are extremely lucky – friends and neighbours. Collect rainwater in your personal capacity so that you rely less on municipal supply. Recycle water in your home (grey water for flushing, hand washing, gardening), and continue to do this even if things return to some semblance of the way they were three years ago. Think about how you will cope with Day Zero.

In my inexpert opinion the city should immediately start to build capacity to treat and re-use waste water. In the long term, solutions such as desalination on a medium to large scale, and (if carefully managed) tapping into the aquifers will become very important to ensure the city’s water resilience as the climate in the Western Cape becomes drier and windier. Desalination as a short term, small-scale, temporary solution is laughable. So is drilling into the aquifers without knowledge of their capacity, without proper plans to recharge them (fascinating witchcraft), and without a scientific understanding of how much water it is reasonable to abstract on an ongoing basis. Peace out.

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Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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