My little girl is 7 and just isn’t wrapping her head around this and I need to find a better way to explain this.
She is so so smart because she saw the coverage of Helene, and heard 17″ and hot confused.
I told her the water runs from the high places to the low places, but then she asked why the water doesn’t leave the low places and why is it still flooded.
I’m bad at dumbing things down.
In: Planetary Science
17 inches doesn’t just mean ‘on the streets’ it is EVERYWHERE. Your house, parking garage, etc. The rain doesn’t discriminate.
As far as why it doesn’t leave, pipes can only have so much water in them. They are ‘gravity powered’ so it isn’t like your sewage system is ‘pulling’ the water. Lawns and such can also pull in so much water so fast and once it is ‘full’ it is full.
Now add in that flooding tends to cause other problems to occur as well. Someone’s pipes may burst (and with no ability to access the shutoff valve there is now more water constantly being pumped out to add to the amount already there) and a fire hydrant being hit by a car/other thing swept away and hey now there is A LOT of extra water being added until someone figures out how to stop it
So it falls 17″ everywhere. All of this is going towards rivers (both above ground and underground. Usually a lot more water is transported underground through ground water, though these flows are a bit slower than water above ground. This is why rivers keep flowing when it hasn’t rained recently. There is still more water in underground reserves that haven’t been able to drain yet).
When these underground flows are being overloaded it starts to flow above ground. In hilly terrain this excess above ground water will quickly find its way to low ground, and this low ground gets water from ALL of the surrounding area uphill.
This water will try to find a river to flow to. But if the river is normally 3 meters deep and it’s now 1 meter taller than normal, that’s only 25% more flow than normal (ok, a little bit more since it’s usually flowing faster) but there is just so much water than rivers can handle and the groundwaters ability to soak up this water is overloaded.
It’s kind of like dropping a big ass bucket of water into your sink, it’s going to take a while to empty out even if the drain isn’t plugged.
A more accurate analogy would be if you put a lot of sponges to form an uneven covering a very slightly sloping half-pipe (simulating ground waters ability to absorb rainwater) and then dropped bucket after bucket of water in from the top. To some point the water is just going to soak up into the sponges, then get pushed downwards by gravity and it’s going to slowly drain from the bottom. That’s how a river normally works. Draining the water that’s slowly seeping out of the ground into a pipe at the lowest point. But pour down enough water and it will flow over the top, but it’s going to keep getting trapped in the “uneven parts”, which is where you get floodings and standing water.
It does drain from the low places, but its like draining a bathtub. The water can only flow away so fast depending on the gaps between high places and the kind of soil. If the rain is falling faster than it can drain,it will flood.
Run your kitchen sink at a trickle with semi-closed drain.
This is normal rain.
Run your kitchen sink on full with semi-closed drain. This is more rain than the drainage system can handle.
The water is running down faster from hills than it can run out into the ocean.
Think of if you had a bathtop, and you opens the plus and opensed the tap, everything would be in balance if there is only one tap open, as the water would run out the drain at mostly the same speed. Now imagine that you had 10 taps, and you opened them all, and the water coming in would now be 10 times as much, but the drain would only take some of it out and the bathop would slowly fill.
Same with the hillsides, there are so many more hill sides than there are river outlets to the ocean
Try explaining “storm surge” to her. Show her in the bathtub that you can push water and make its level go higher. Then explain that the water being pushed by the wind keeps the water draining from higher elevations from escaping to the sea.
It’s cool that she wants to learn this, and that you are trying to help her understand.
Science experiment time!
Grab a funnel, or if you don’t have one you can punch a hole in the bottom of a plastic cup.
Have your daughter hold it under the faucet and turn on the water. Start with just a small trickle. The water should have no problem flowing out the bottom at the same rate it’s coming in from the top, so the water level will never rise very high inside the funnel.
Now turn the faucet higher until it’s going full blast. Past a certain point, more water is coming into the large opening at the top than is flowing out the small opening at the bottom. When this happens, the level will rise and eventually it will spill over the top of the funnel.
This happens because the physical characteristics of water including incompressibility (meaning it can flow and change shape but can’t actually get smaller) and surface tension/friction (meaning it can only move so fast under the force of gravity alone) mean that there is a maximum speed at which it can move through a system. This is true even for large waterways like the French Broad River in Asheville. If water is coming into the top of the system faster than it can flow out the bottom, then it will fill all of the available space until it can find a new exit to spill out somewhere higher up. And 17 inches of rainfall is *a lot* of water being added to the system *very fast.*
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