I know I’m missing something fundamental in my thinking on this.
Suppose an area is dumped on with 4 inches of rain in a relatively short time. That is a lot of rain, yes? Now we have flood warnings for an area the rivers and/or lakes may overflow causing major flooding.
But why is the flooding so major? We’re talking about 4 inches of extra water dropped into the river.
Let’s create an arbitrary flood area. Say the river takes up, I dunno, 10 acres of space within our defined area of 100 acres. The river obviously cannot handle the 4 inches of water, so it dumps it onto the surrounding earth. That’s 4 (additional) inches of water spread over 90 acres of land. How does that potentially create such major flooding and damage?
It’s not like the entire contents of the river or lake is dumped out onto the land. It’s just a few inches, which would be spread and absorbed over a much larger area of acreage.
Of course I don’t doubt that serious flooding happens; I just want to better understand how.
It’s not just rain falling directly into the river. It’s rain falling on the entire area drained by the river, which for some rivers, can be the better part of an entire continent.
It’s also wildly affected by the way human have altered the land. We build levees to contain a rising river instead of allowing it to spread out onto low lying marshland. At one time, no one built any structures on those marshlands so their occasional flooding wasn’t noteworthy. Today, those former marshes have been filled in and they’re now our subdivisions. When the levees fail, and they always fail, the river spreads out like it needs to and our homes, businesses and lives are flooded.
Just remember, Mother Nature has Tourette syndrome. Don’t try to fight her, you will not win.
The water that comes over the banks of the river would not spread evenly over your 90 acres of land unless the elevation of the surrounding area is completely level. That is not very likely, so the majority of the area would not flood, but the houses in the lowest, say, 10% of the area might well be under a foot of water while the vast majority are high and dry. It’s that 10% that make the news.
Or think of it this way: Suppose there were no river at all. In that case, 4 inches of rain over a region would mean that the entire region would be under 4 inches of water. That’s already a pretty bad flood.
And the river serves to concentrate that flooding, to areas near the river, while leaving areas far from the river unflooded. And humans tend to live close to bodies of water, where the flooding is worse, and not care so much about the unflooded areas further away.
I think your numbers are way off. Rivers themselves cover nowhere near 10% of land area. I bet it’s below 1%. (I tried googling but kept getting entangled with the areas of river watersheds, which of course cover 100% of the land.)
You are missing the fact that most of the water falling on the land drains fairly quickly into the river. So the river rises a LOT more than 4 inches. The river bursts its banks, and areas adjacent to the river are flooded.
Drainage Basins Everything flows down hill. And of course there are multiple ‘sub’ basis within the larger ones. In the county that I work for, we define the different regions by drainage basin. Upper Blue, Lower Blue, Snake, Ten Mile. And each of those can cause local flooding.
And note that in a significant rainfall event, especially in a wet year, the ground does not absorb the water. It can’t.
Another major problem for those who get flooded is that the sewers are the first thing to fill and then they overflow, together with everything that is in there. Even if you waterproof the outside of your house; if you don’t fit special valves, your basement and your downstairs rooms will be awash with sewage. This is one reason why it takes so long for people to move back in after a flood.
I sail regularly on the Chesapeake and have noticed over the recent years that the low and high tides are higher than they used to be. Places where I could expect to be inaccessible due to a deep draft are more accessible now. Marina docks that are not designed to be floating docks are ridiculously low at high tide. Just this past week, the rail of my boat was at sternum level during high tide as I was standing on the dock. There would be no way for most older, less physically capable boaters to get on and off their boat while in a slip.
Apparently, there is a Thermal Expansion phenomenon that goes along with warming bodies of water. The warmer water occupies a larger volume (becoming less dense), so even normal amounts of rain and normal tidal changes cause more flooding on coastal areas than they did in the past. Sections of Annapolis roads and parking lots next to the harbor now flood at high tide.
The good news is, I can anchor closer to shore. The bad news is, I might soon be able to tie up to someone deck.
I think the biggest thing you’re missing is that the land isn’t level. 4 inches across 100 acres is a lot of water. Of all that space, people that are topographically higher might just see it as a bad rain storm. The people that are lower (even just in local dips), might have several feet deep. Add to that all the momentum that water caries when it starts moving and it can easily take down structures.
Then to make it even worse, all that water has to go somewhere. It’s going to overwhelm the storm sewers which starts blowing off manhole covers. In areas where storm sewers and sanitary sewers are combined that rain is going to start coming up through drains inside houses flooding basements.
On top off all that, as the rain percolates down, it’s going to raise the water table faster than sump pumps can deal with it and come in through the foundation.
If you live in an area with snow, it may be easier to visualize 4 inches of rain as 4 or 5 feet of snow. Getting 4 or 5 feet of snow is enough to immobilize even the most prepared city for at least a few hours.
It’s mostly the concentration effect. The highlands don’t flood. (well, maybe a local lowpoint does, but not in general.) It’s the lowlands that all the water runs to that flood. It doesn’t even have to rain HERE to have a flood here, it could have rained upstream in the watershed.
They are talking about major flooding in the flood plain or flood fringe, not the entire land area… so it’s a measure of how much of that flood plain that’s dry 99% of the time gets wet for a few hours or days.
Nobody cares much about major flooding in unpopulated areas. But, people (especially before heavy earth moving equipment became so widely available) like building their houses and other structures on nice flat ground close to water sources, a lot of which are flood plains that might go decades or more with no “major” flood.
You can cram hundreds of millions of dollars worth of real estate along a few miles of river bank, and people can get surprisingly attached to a building location they know is bad… but keep on rebuilding anyway.
So flooding generally doesn’t actually affect much of the land mass by percentage, but we disproportionately build our stuff in the areas that do get flooded out. All that rain funnels down to the river which quadruples it’s width for a day and floods all the houses and streets.
I don’t know about mostly, but certainly around my location that is the case. Too many parking lots generating a lot of run-off is one factor. Development used to be nuts, and manditory retention ponds have helped but low lying areas like underpasses still tend to fill up. Some suburban cul-de-sacs have the same issue where run-off from the surrounding area floods a low bit with too much water to drive through. I took a trip to DC a while back and a summer thunderstorm flooded the national mall in the same way. We also have a problem of development upstream in the neighboring county. A storm ten or twenty miles north sends a flood downstream even if we had little rain ourselves. The river will be raised a couple of feet.
Yeah, and the effect downstream can be a day of more after the rainfall. You often hear of a river cresting well after the storm. My sister and I ride bikes on a path that parallels a stream. At one place near our homes, the path dips to go under a road, and often floods. One day we headed out for a 25 mile ride on a sunny day, a day or 2 after some big storms. On the way out, the stream was high, but the path was dry. Approx 90 minutes later, we could barely make it through water at least 4" deep.
While that’s one of the reasons, the main reason is that mold grows in formerly flooded buildings, making them unhealthy to live in. They have to replace the carpeting, drywall, and other stuff before it’s livable again.
The four inches of rain that fall directly on the river will raise the river only four inches, yes.
Four inches of rain, as others have said, presumably also fell on all the much larger area around the river. Very few soil types, even if they’re healthy soils in good ground cover, can soak up four inches of rain all at once. Some of it is going to flow off downhill.
Paved areas won’t soak up any of it. Roofed areas won’t soak up any of it. They are impermeable surfaces: effectively nothing will soak in. Heavily compacted soils, whether they’re heavily compacted due to large amounts of vehicle traffic or even foot traffic, or due to farming methods that reduce organic matter and thereby damage the soil tilth and/or that form hardpan layers in the soil (hardpan can be caused in many soils by heavy tractors/equipment, by the wrong type of or excessive tillage, and/or by tilling while too wet for the soil), are at best semipermeable surfaces: they can absorb some rain if it arrives slowly and gently, but in a hard rain very little will be absorbed and most of it will run off.
Take a cup and pour four inches of water into it and consider the results. Then take a large table with a hole in it (say a large picnic table with a hole for an umbrella in the middle, though I suggest doing this as a mental experiment only), fix a rim around the edge so water’s got nowhere to go but on the table or in the hole, fasten the cup under the hole, and pour four inches of water over the entire top of the table. The glass is going to fill up entirely, and there’s still going to be a lot of water on the table.
When you have not a flat table but uneven land surfaces, the water that runs off from the relatively high spots will fill up the lower spots, whether they’re rivers or basements or low spots in roads or entire low neighborhoods, just like water on the picnic table will fill up that cup.
yep. 'bout 5 years ago in Detroit we had the OP’s situation, 4+ inches of rain in one evening. The Clinton and Rouge rivers flooded pretty quickly. Not only were they taking on all the rainwater, but municipalities were running storm drains into the rivers just adding to the problem. doesn’t work too well when your pumps are trying to push storm water into a river that’s already higher than the drains.
