(no subject)

[drydem]

So, here's why the flooding disaster hit after the hurricane passed N.O. If you look at the path once it hits land, Katrina is passing over numerous rivers and streams, dropping at least 1/2 inch of water on them(that's what we got here, so I assume that down south of us they got more). These swollen streams rush faster than normal and all of them, every single one, feeds eventually into the mississippi. As ALL of the water of the tropical storm gets fed into the water system, the Mississippi swells.
Here's the thing about the Mississippi. For decades now, the Mississippi has been trying to change its banks, to run along a different path, which all rivers do from time to time as they fill up their channels with silt. The US army corps of engineers has been trying to stop this, to tame the river by keeping it caged with levees and dikes. The appropriate metaphor for this is a fierce dog, trying to go running being put behind a fence. Now, imagine that dog gets shot full of methamphetamines and steroids, that is what happened with the Mississippi.
Man tries to control nature, nature simply builds up its power and releases it all at once. The worst part about this is that the ones in the path of the water are never the ones who are trying to control it. The army corps of engineers doesn't live downhill of the levee, poor people do. Poor people who the government has assured of the safety of the levees and dikes.
And we see what happens.

Comments

[kuribeh.livejournal.com]

Speaking as a geologist I would say that you are almost completely correct.

It's not the streams necessarily are rushing "faster" the velocity of the streams increase, but they are also deeper too meaning the over all discharge of the stream/river (meters squared of water per second or m2/s) increases. This is mostly due to what we call runoff. Runoff is basically water flowing overland and into the river or stream system. There is another factor with is called through flow. This is water that percolates into the soil and then underground downgradient to the stream/river.

Now, normally there is a lag time between when there is a massive discharge of water and when a flood reaches it's highest flood stage. This is because it takes a while for water to percolate through the soil and travel downgradient. When this occurs in natural areas that have had a minimum of development floods tend to be less high and occur over a longer period (several days to weeks). If a flood occurs in a heavily urbanized area with lots of concrete and developed land you get higher flood stages in a shorter period of time (hours to days). This is because of the increase in Runoff due to most of the soil and dirt being covered by concrete. Runoff gets water to stream systems faster than through flow, because it takes time for water to soak into soil. Thus more water faster to the rivers making for higher and more destructive floods.

Another thing to think about: New Orleans has been a sinking city for sometime. Part of this is due to natural subsidance of the delta, but another factor is from over pumping of aquifers. If you pump too much water out of the aquifer, there is no water to support the overlying rocks and soil and the land sinks to fill the void.

Thus ends my brief lecture on hydrology and the nature of floods.

[gollumgollum.livejournal.com]

...although the important thing to remember is that the levee that breached that's filled the city was on Lake Pontchartrain, not the Mississippi. which is probably a good thing (or at least a better thing--i don't think anyone could call this a *good* thing), because the river is several feet higher than a)most of the city (if not its entirety) and b)Lake P. (which is still higher than most of the city, but not nearly so high--much closer to sea level).