Eli5 – Drowning in floods but less than a foot of rain?


Whenever I read stories of people drowning in floods, it seems there was only a foot of rain or less. I can see how this would be extremely destructive but how does water up to your neck and higher occur?

In: Other

The rainfall received is received over the whole area, a foot of rain can actually be a lot if it’s received in a short space of time like an hour or two, and if it’s over a large region then it can be quite devastating. This is because the rainfall will drain through its local water shed, so anyone who is settled in a low lying area or flood plain won’t experience a foot of rain in height, but will be on the receiving end of the accumulation of rain water draining from the entire area. If they are in the flood plain that water can easily be a meter or two of rushing water, sweeping them away and getting pushed under the debris.

Usually it’s the sweeping motion of the water knocks you down, and then slams you into other things, and that knocks you out and leads to drowning; you’re not standing, you are horizontal.

You get swept into rivers, lakes, sea, etc.

Once you are in a large body of water, it makes it easier to drown.

For an example of how a foot of rain can become ten feet of floodwater, think about a city street. Most of the area is taken up by buildings, with occasional gaps of streets. When the rain falls, most of it’s going to be falling on the roofs of those buildings, but it will flow off and into the lower ground of the street. If there’s just three square feet of roof for every square foot of road ( and that’s probably a conservative estimate for a lot of places) then that’s already four feet of floodwater that will end up on the road. Add in hills and the like, as well as rivers spilling their banks with fast flowing currents, and that’s how floodwaters can get so high and dangerous

Towns tend to pop up near rivers, which are usually in valleys, low in elevation.

Drop one foot of rain across the whole land, where do you think it’s going to pool up?

The problem is that rain doesn’t just stay where it lands – it travels downhill.

So a moderate amount of rain up at higher altitudes will be collected and concentrated by the streams and rivers that flow past further downstream.

In fact it doesn’t even necessarily need to be raining downstream for a river to be overflowing and bursting its banks.

Most of the deaths you hear about do not occur in the large, relatively calm flooded areas (though there will often still be dangerous currents hidden there) that you see on TV, but at the points the water is concentrated and fast flowing – the person that got swept away trying to wade across a river, got trapped by rising floodwaters, fallen into the water as land has been washed away from under them, or caught in debris flowing past.

* If everything was completely flat, and there was no drainage, a foot of rain would mean water 1 foot high everywhere.
* However in the real world, things aren’t flat, and there is drainage, so things are much different.
* There will be lots of water concentrated in some spots and very little in others.
* And that drainage we talked about, it takes time.
* So if 1 foot of rain fell today….but 2 inches of rain fell yesterday and the day before, and the day before the ground will be saturated and that 1 foot won’t drain as quickly as it usually does.