flash floods?

Question

How is it that, within minutes of heavy downpour, rivers can flood streets with 3-4 feet of water? It just doesn't seem like _that_ much comes down in so short a time, not to mention the water is spread out over a couple miles at least.

Answer 1

Kaj is correct. Let's look at an example.

Suppose we have a T-storm that dumps, say, 12 inches of rain in 24 hours (which is less than the 19 in we saw recently in Texas). That's one foot of rain.

Now let's say that rain falls on one square mile of dry hard soil that will not soak up the water. The volume of rain falling is V = (5280)^2 (1) ~ 30,000,000 cubic feet of water that has to go somewhere. So it flows toward the lowland surrounding an otherwise dry riverbed.

Now let's say the river trench is 30 feet wide and 10 feet deep. Thus, a mile of river volume in the middle of the 12 inch downpour is v = 300 X 5280 ~ 1,500,000 cubic feet.

So there we have it...V = 30,000,000 cubic feet of rain running off into 1,500,000 cubic feet of river bed in 24 hours. That is, there is 20 times more rainwater than river volume to carry it away. Thus, in 24 hours, the river needs to turnover 20 times its own volume in a day. The river cannot drain off all that rainwater in such a short time; so the collected water spills over the banks and we have flooding.

Answer 2

Look at a map, where does the flood occur, generally a part down the river, so the river have received water from a very large area, that gives a large quantity to leave the area in a very small spot, thus creating at flash flood.

Answer 3

All that water has to go somewhere.