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9:38am on Thursday, 30th January, 2014:



I heard on the radio the other morning that 60 square kilometres of the Somerset Levels are submerged under water. I also heard that the amount of water sitting on them is the equivalent of 600 Olympic-sized swimming pools.

So ... what's the average depth of this water?

Well, although the width and length of an Olympic-sized swimming pool are specified as 25m and 50m respectively, the depth isn't specified. The minimum depth is specified, but most pools are deeper than the minimum. OK, so let's go with what Wikipedia suggests and choose an average depth of 2m. On this basis, an Olympic swimming pool has a capacity of 25*50*2=2,500 cubic metres, which is 2,500,000 litres of water.

So, this means 600 such pools would equate to 1,500,000,000 litres. Spread across 60 square kilometres, that's 25,000,000 litres per square kilometer.

OK, so a litre has a volume of 1,000 cubic centimetres. It's a cube with 10cm-long sides. A square kilometre has sides 1,000 metres long, so that's 10,000*10,000=100,000,000 squares of size 10cm by 10cm. Spreading out 25,000,000 litres of water over that would mean 25/100=0.25 litres per square. As the squares have the same dimensions as the width and length of the 10cm cube that makes up a litre, that means the depth of the water would be 0.25*10=2.5cm.

In other words, about an inch.

That's quite impressive. Of course, as some of it is 2 metres deep in places, that means elsewhere it would have to be just a puddle to compensate and still qualify as being "submerged". I suspect what they mean is that an area of 60 square kilometres is affected by flooding, rather than is under water, but being under water makes for a more dramatic story for the news.

Needless to say, if the rivers hadn't been allowed to silt up for 20 years, the average depth of the floods would be much easier to calculate: they'd be 0cm.

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