When did 'Olympic-size swimming pool become' a unit of measure?

With all the recent flooding here in the northeast and storms battering the central US we’ve been hearing a lot about Olympic-size swimming pools. The water spill over dams is described as "the equivalent of X Olympic-size swimming pools per minute”. Water rushes over rapids at a rate of “Y Olympic-size swimming pools per second”. And, most ridiculous, I actually heard heavy  rainfall being described as so many Olympic-size swimming pools per hour dumped on city Z.

I understand that the media want to convey the idea that this is lots of water, but surely there’s a better way to communicate that message. I have never seen (outside of TV) an Olympic-size swimming pool. I know it’s big, but how big? I have no idea. Nor does the International Olympic Committee as no standard exists that limits the depth of such a pool, only it’s length and width. And even those dimensions are minimums only.

If they are going to use meaningless comparisons, why not 747s? “That’s the equivalent of 36 747’s full of water going over that dam every 13 seconds.” I can get a visual from that.

But, better yet, why not use an actual, legitimate measure? There’s the ton (either short or long), or tonne (1,000 kg, 2200 lbs)? Or even the homonymous tun (4 hogsheads, 252 Imp. gallons). All would be more meaningful, and certainly more accurate. Even gallons themselves, although I expect when reporters begin talking about millions, or even billions, of gallons eyes will glaze over and people will probably wonder “How many Olympic-size swimming pools is that?”

But then again, the average person doesn’t know when to properly use “number” versus “amount”, so perhaps we’re stuck with Olympic-size swimming pool.