Where does all the water for a flood come from?

August 28, 2005 8:53 PM

Explain flooding to me So they're expecting up to 20 inches of rain in New Orleans. And the city may be under 10 feet of water. Huh?

Ok, so figure if the city is 100 square Kms (10x10) it would take 300 million square metres of water to put it 10 feet under. If 20 inches of rain falls on those 50 million square metres, that's still only 225 000. So where does all the water come from in situations like that?

I know that in this case, part of it will run-off from higher elevation areas and the ocean coming in over the levees and getting caught in the city, but it seems like anytime there's flooding there's this same situation (the level of flooring reported exceeds the amount of rain that falls). And most of those places aren't shut off from the ocean by levees.

So what's the deal?

posted by duck to Science & Nature (16 answers total)

You are assuming that the flood is all from rain water. The majority is from a tidal surge. Winds will blow the water off the ocean over land and tidal surges can be very high. New Orleans is particularly susceptible due to the fact that it is at sea level. If water breaches the levies, those same levies will keep the water from going back to see. Mucho problemo.
posted by qwip at 8:57 PM on August 28, 2005

Response by poster: er...in my care to double check my calculations, I forgot to remove one of the errors...scratch that 225 000 reference.
posted by duck at 8:57 PM on August 28, 2005

don't forget the city is located between two levee's... dangerous...
posted by joshgray at 9:01 PM on August 28, 2005

Storm surge. Basically, as this huge storm moves it pushes the ocean up in front of it (like if you moved heavy furniture across loose carpet)[1]. I think they are estimating a surge of 20 to 30 feet plus another 20 or 30 feet due to wave height[2]. This is more than enough to push the ocean/Lake Pontchartrain over the levees.

It isn't all due to rain fall. In fact, most sites I've read today don't seem to be worried about the rain fall nearly as much as the levees breaking.

[1] I got this from a graphic/video somewhere. The actual phenomenon is more complex.
[2] There's been so many numbers thrown around today, but I think these are correct if not on the conservative side.
posted by sbutler at 9:02 PM on August 28, 2005

This giant jpg from the Mefi thread has some explanation of the bowl effect.
posted by jessamyn at 9:03 PM on August 28, 2005

Response by poster: Hmm...ok...going to look up the links and videos, but my question is also more general since it seems every floor you hear about (including those not near the ocean) seem to have this discrepancy between rainfall and water levels. For example, there was flooding in Toronto a couple of weeks ago, and I don't think it rained enough to explain the amount of water. (Obviously not flooding on the magnitude NO is likely to see, but my point is about the discrepancy). What explains the discrepance when the ocean isn't involved?
posted by duck at 9:06 PM on August 28, 2005

New Orleans is also right next to Lake Pontchartrain, which will overflow into the city.

But the main problem is that, under the best of circumstances, New Orleans needs to pump water constantly to avoid being flooded. It has one of the most sophisticated pumping systems in the world. This pumping system will not be able to deal with the expected flooding from Katrina, however, and so the flooding from Katrina will be added to the city's natural flooding. Insult to injury.
posted by cerebus19 at 9:13 PM on August 28, 2005

Well, Toronto is a far cry from being flat. I don't know the incident you're referring to, but I'd imagine the reporters meant "up to x feet" of flooding in some areas.
posted by sbutler at 9:13 PM on August 28, 2005

In Toronto, a lot of it was storm drains being unable to handle the amount of water in the short amount of time they had (that was a LOT of rain in a very short period), as well as rivers flooding (cops were rescuing people from the roofs of their nearly-underwater cars on the Don Valley Parkway, because of the Don River).
posted by biscotti at 9:18 PM on August 28, 2005

The Toronto flood downtown was caused by the heavy rainfall to the north of the city being concentrated by the Don Valley watershed. That's how a 10 cm rainfall ended up pushing the Don River a couple of metres higher than normal.
posted by Dipsomaniac at 9:26 PM on August 28, 2005

duck: while everyone else is right, and the issue in NOLA is not the rain but the storm surge and the nature of the city.

In general, however, 20" of rain can lead to 5 feet of water because the rain does not sit still. It flows downhill. Thus, after the soil is saturated, the high ground has no standing water, from the 20" that fell. The low ground has 5 feet, or whatever, from the run off from the high ground.

The answer to this, while not satisfying, is very obvious: the flooding is usually quite concentrated. The rain is not.

NOLA is a major exception, and that is one of the reasons why the situation is so dire there. Lots of water everywhere, nowhere for it to go if it breaks the levees.
posted by teece at 9:41 PM on August 28, 2005

But the main problem is that, under the best of circumstances, New Orleans needs to pump water constantly to avoid being flooded.

I know Nawlins had these pumps, but are they really running all the time? Is that how New Orleans stays dry?

How did it manage to exist in the first place, then? I mean? did it used to be a lake that was drained? Did the ground sink? What happened I don't get how you can have land below sea level so close to the ocean?
posted by delmoi at 10:26 PM on August 28, 2005

New Oreleans is a doomed city. It's built on sediment, deposited by the Mississippi, which is continually settling.

Once upon a time, the river would flood the site and deposit new sediment, but as people built more an more permenant strutures, they increaced effort into keeping the floods out. They've succecced, for the most part, and so the old sediment compacts without being backfilled and the city sinks further and further.
posted by Good Brain at 10:39 PM on August 28, 2005

delmoi, scroll to the right on the jpg jessamyn links to - erosion and lack of sediment renewal is what has brought the land below sea level
posted by Sangre Azul at 10:39 PM on August 28, 2005

Another way of looking at a storm surge is as a slow motion tsunami caused by the storm rather than an earth quake. One of the consequences is after the water comes in it'll flow back out and the rivers aren't going to be able to handle it. So you see structures being washed away in a similiar way as happened in the big tsunami.
posted by Mitheral at 11:48 PM on August 28, 2005

From AP:

"For years, forecasters have warned of the nightmare scenario a big storm could bring to New Orleans, a bowl of a city that's up to 10 feet below sea level in spots and dependent on a network of levees, canals and pumps to keep dry from the Mississippi River on one side, Lake Pontchartrain on the other.

The fear is that flooding could overrun the levees and turn New Orleans into a toxic lake filled with chemicals and petroleum from refineries, as well as waste from ruined septic systems."
posted by CunningLinguist at 3:23 AM on August 29, 2005