How does 10 inches of rainfall cause extreme flooding like what we are seeing in Kentucky?
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If the area has been *extremely* dry for a very long time, the soil is actually too dry to absorb the rainfall. At that point, the water still needs to go somewhere, so … It goes everywhere
Water can only be absorbed into soil at a certain rate. The variables here are soil composition, ground cover, current soil water content, and others.
When the rate of rainfall exceeds soil absorption limit, the water runs over the surface downhill.
Often this flowing water reaches a place where it is impeded and builds up to levels that are dangerous to people and property.
10 inches of rain is a lot of rain, the place where the rain is falling is not completely level. Rain/water will run downhill to the lowest point creating a surge so you get much more than 10 inches and it is often still moving downhill causing more damage.
all the water that falls on things that can’t absorb it, like houses, roads, pavement, hard dirt, etc. has to go somewhere
This is an *extremely* hilly area with steep slopes and very narrow valleys where the towns are located. A short time after the rain starts to fall the ground is saturated, and from there all the rain flows immediately downhill into streams and rivers. Rain falling on an area measuring tens of square miles converges all at once on a streambed valley only 100 feet or so wide, resulting in a fast-moving flash flood several feet deep.
A huge factor isn’t just the amount, but the rate. IIRC they got 8″ of rain in three hours which was too much for the ground to absorb, causing it to pool, flow from gravity, and collect in massive amounts along the way.
That’s 10” of water /everywhere/. Which means that water will flow. 90% of the ground will absorb like an inch or two, and have the other 8” flow downhill. Which means that the 10% that are downhill, they get 80+ inches of water flowing on to them. Even more for the major rivers that tend to be downhill of everywhere. And when you have a river running several feet higher than usual, it causes problems.
10 inches of rain might not sound like a lot, but it actually is. To put it into perspective, 10 inches of rainfall equates to roughly 56 gallons of water for every square yard of land it falls on, which is about ~~3 million~~ 173 million gallons of water per square mile.
Edit: Screwed my math up. There are roughly 3 million *square yards* in a square mile, and at 56 gallons each it’s more like 173 million gallons.
When you’re in the mountains there isn’t really any flat land. It’s like the roof of your house. The rain doesn’t stay on the roof, it runs down into the gutters. Even a little rain will cause a steam of water to pour out the drainpipe. In mountainous regions like Kentucky, most people don’t live on the side of the roof-like mountains, they live in the flat gutter-like valleys and lowlands since it’s easier to build there. So any significant rain will flood the low areas quickly.
Also, 10 inches is a lot of rain.
Go find the widest pan with raised edges in your kitchen and take it to the sink. Fill it with just a little bit of water, so that the whole bottom is just barely covered. Enough that a Lego guy could stand in it and be okay.
Now, tilt the pan a bit. Just a bit. It doesn’t take a lot.
All the water across the pan will now flow down to the lower side of it and collect there. All that water that just barely filled the entire pan has now created a deep pool on one side. If you chose a pan with very short walls, water probably started pouring out the side of it.
Kentucky is like the tilted pan. All that water runs off the hills and mountains and collects in the valleys and lowlands. It may have only been 10 inches of rain at any given location, but once it runs off and pools in one place, you have a real problem.
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