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.
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.