Damn

Dam level figures released today for Cape Town’s ‘Big 6’ indicate that we’re 0.4% worse off than we were this time last week, teetering once again just above the magical 30% ‘CRITICAL‘ level, below which nothing actually changes.

Fullscreen capture 30-May-16 92448 PM.bmp

Oh then, to be in Sheffield (as I was a couple of weeks ago) where the dams are just about as full as they can be:

That total of 10,410,000,000 gallons is equal to 47,324,796,900 litres, in case you were wondering.

And what does a dam that’s 98% full look like? Like this.
And what does the other side of the wall look like when the dam is 100.1% full? Like this.

Going critical

Nothing good comes from things going critical. Nuclear power plants are probably highest up the list of things which are bad when they go critical, with toddler tantrums pretty close behind.
Critical isn’t good situation to be in. Critical is… well… critical.

Unless something rather remarkable occurs very shortly, when their capacities are measured again on Monday, Cape Town’s dams will have fallen below the “critical” level of 30%.

Fullscreen capture 2016-04-14 100229 AM.bmp

It’s obviously an arbitrary level that they’ve chosen to call “critical”, and quite what happens when we cross that threshold is unclear, although we have been told that we shouldn’t panic. But then, that begs the question, why bother having a “critical” level in the first place if nothing changes once you find yourself below it?

After all, very little happened when we crossed the legendary “careful now” 70% margin, nor the distinctly worrying “er… guys…?” 50% line.

I do hope that the city council have got this all in hand…

Barry Wood, the city’s manager of bulk water supply, told the council’s portfolio committee “We don’t have to be too concerned, provided that it starts to rain.”

Ah. That’s all ok then.
Colour me completely reassured.

Water lot we got…

Right. The general consensus seems to be that we’re done with winter now and the good news is that Cape Town’s dams are full to bursting. Not literally though, I hope.

dams

Yes, you read right: our dams are 104.5% full right now, with just Wemmershoek letting the side down with its paltry 99.9%. “Big Boy” Theewaterskloof is looking especially resplendent on 107.5%, no less.
How can this be? Well, it’s not like the water rises above the dam wall and is held there by a giant meniscus or anything (although that would be really cool to see). The extra 7.5% is due to the difference between the intended capacity of the dam and the actual amount of water it can safely hold, as described here:

Man-made dams are artificial catchment areas and, by definition, are storage areas for water. When the dammed water reaches a level that indicates the maximum water that that dam can hold (before being put under stress by additional water pressure), a drum gate opens automatically to prevent over-pressure. The drum gate is designed to keep the dam at the maximum of the storage level – the so-called ‘full’ level – but often the ‘full’ level is well below the dam walls max capacity, for safety. So, as the water level begins to rise, the water level above the ‘full’ level is marked, and typically ends at 10% (or 110% if you like) above ‘full’. At this point, the water pressure is considered to be dangerous and sluices are opened to let water out. These sluices are carefully controlled to make sure that the river below the dam wall does not breach it banks and ruin expensive weekend homes! I think that saying the dam is 107% full is meaningless, and misleading, even though it an engineering necessity.

Either way, despite the fact that we are closing in on 1,000,000,000,000 litres of stored water, it’s still sensible to use it carefully, as the City points out:

It is important to bear in mind that the time to save water is when there is water to save, and we should therefore not become complacent about our water saving efforts. Cape Town will never be in the position of having sufficient water to waste, and we must continue to be vigilant.

Right you are. Not that my garden will need any more watering for about a month given the last couple of weeks.

More water

If you have been in or around the Western Cape over the past couple of weeks, you can’t have missed the rain we’ve been having. It’s caused floods, landslides, death and misery. However, on the bright side, it has also filled up our dams nicely.

Hoorah!

But it was only when I read the City’s weekly dam level figures, published each and every Monday, that I realised just how much it had filled up our 6 local dams.

It’s the last two columns you want to look at – this week versus last week – indicating that the percentage storage in our dams has increased by 7.7% in just 7 days. That’s good news, as we need the water for our long dry summer (remember that?).

7.7% is quite a lot, incidentally. I’ve been doing some rudimentary calculations (while fuelled by Diemersfontein pinotage) and it appears to me that we have almost 69 billion more litres stored than we had this time last week. 68,609,000,000 litres to be exact.
That’s enough to fill 27,443 Olympic size swimming pools, although if you were to actually do that, I wouldn’t be allowed to water my garden in February.

So don’t.