A study finds that much of New Orleans is at sea level

[Kerry04]

http://www.nola.com/news/t-p/frontpage/ ... xml&coll=1

[NDG]

Oh wow, by a whole 51% at or above sea level, according to this study. I don't know how accurate this LIDAR system is, I thought that the most accurate way to detirmine elevation was to do it by satellite. Even if the study is accurate I don't know if that should feel the residents any better. The problem is when you a have a 20-30' surge overtopping or breaching through the levees, wheather you are just at or below sea level will not make that much difference, you are still going to get flooded, which is why you had a good 80% of the city flooded after Katrina, it might just mean that you will drain out quicker the higher you are in elevation once pumping out the water begins.

[Recurve]

Oh wow, by a whole 51% at or above sea level, according to this study. I don't know how accurate this LIDAR system is, I thought that the most accurate way to detirmine elevation was to do it by satellite. Even if the study is accurate I don't know if that should feel the residents any better. The problem is when you a have a 20-30' surge overtopping or breaching through the levees, wheather you are just at or below sea level will not make that much difference, you are still going to get flooded, which is why you had a good 80% of the city flooded after Katrina, it might just mean that you will drain out quicker the higher you are in elevation once pumping out the water begins.

Still, it puts a more scientific eye on the often-repeated claim that "New Orleans is below sea level."
I read a Louisiana geography book that came out recently showing 54 percent above sea level.
Fact is, some of it is quite a bit above sea level -- more than most of the Keys and many parts of other coastal regions. The ridges where the older parts of the city grew up, on the natural river levees, are quite a bit above sea level.

[MiamiensisWx]

I completely agree with the previous posts (and this aforementioned study). Recurve makes good points. While the region in general is low-lying, many smaller pockets of variations in elevation are a common feature throughout southern Louisiana. Even land close to Lake Pontchartrain locally is elevated five to seven feet above mean sea level. Some of the highest locations (and regions with different soil and land composition) may be a safer bet to place structures.

Although these variations may appear to be safe locations, this is only within a general sense. High storm surge events can still flood those locations. I would also advocate caution when development is brought to the table. Excessive Miami Beach-style development would be disastrous, but development oriented toward the local economy is an excellent choice for New Orleans and southern Louisiana.

[NDG]

Even land close to Lake Pontchartrain locally is elevated five to seven feet above mean sea level.

That is typically only found on land that was reclaimed from the Lake, or in another words land that was filled on the lake, for example Lakefront airport and the Lakefront park area. I lived only two blocks from Lake Ponchartrain in Kenner on the western side of the city and according to a flood map that I looked up my parcel was 6' below sea level, on my side of town I think the only land that was above sea level was the levee itself. It was very noticible when I would bike on the bike trail on top of the levee the difference between the lake's level and the streets.
Like you also said, I don't think that even the areas with the highest elevation like in the old river's ridges and close the banks of the MS river will make a difference once you are dealing with a storm surge so high as it is commonly seen in the northern Gulfcoast.

[aepp]

Oh wow, by a whole 51% at or above sea level, according to this study. I don't know how accurate this LIDAR system is, I thought that the most accurate way to detirmine elevation was to do it by satellite. Even if the study is accurate I don't know if that should feel the residents any better. The problem is when you a have a 20-30' surge overtopping or breaching through the levees, wheather you are just at or below sea level will not make that much difference, you are still going to get flooded, which is why you had a good 80% of the city flooded after Katrina, it might just mean that you will drain out quicker the higher you are in elevation once pumping out the water begins.

Agreed.

[Downdraft]

Above sea level perhaps but above Lake Poncetrain? Watching that water flow through the breached levees makes me believe otherwise. The other question I would have is if it's above sea level why do they have to pump it out?

[NDG]

Above sea level perhaps but above Lake Poncetrain? Watching that water flow through the breached levees makes me believe otherwise. The other question I would have is if it's above sea level why do they have to pump it out?

Lake Ponchartrain's level is at sea level, is not a true Lake, is more like a Bay where tides come in and out.
When the 17th street canal levee was breached the lake was several feet high from the storm surge.
I would say that 97% of the city is inside the levee system, even the sections that are at or above sea level. So once the storm surge overtops or breaches through the levees it will stay within the city, like pouring water in a bowl, it has no way to go out, so even the sections that are slightly above sea level will flood and will stay flooded until the water is pumped out and water levels start dropping. Also, every time it rains heavely in New Orleans the rainwater has to be pumped out, and if it rains at a rate that the pumps can not handle they get massive flooding, like the flooding of May '95.

[Recurve]

Above sea level perhaps but above Lake Poncetrain? Watching that water flow through the breached levees makes me believe otherwise. The other question I would have is if it's above sea level why do they have to pump it out?

Seriously, much of it is actually, truly above the level of the sea, Lake Pontchartrain, the Gulf of Mexico and the Atlantic Ocean. Why did the French Quarter not flood when some neighborhoods were under 12 feet of water? Because some areas are many feet below sea level and the French Quarter, the Garden District, The Esplanade Ridge, Uptown, and some other areas are as much as 12-17 feet above sea level. It doesn't matter how much the "bowl" part of the city fills, the areas near the Mississippi are high enough that they are not subject to levee-breach flooding.

The area where a friend of mine lives in New Orleans, at about 15' elevation right near the river levee uptown, is a much higher, more well-protected location than my neighborhood in the Keys. He had very minor structure damage in Katrina, no water in the neighborhood at all, just like most of uptown. The entire crescent between Magazine and the river was probably dry, and much of the area inside Clairborne was dry or had only a few feet of flood waters.

The reason for the pumps is because of the low areas, as the city expanded from the river levee and the natural ridges, there was more and more need to have pumps for drainage of heavy rains.

The floodwalls on those drainage canals are another matter. That was the biggest engineering mistake ever made. The canals were basically a pipeline with weak walls carrying the lake level all the way into midcity.

[NDG]

Above sea level perhaps but above Lake Poncetrain? Watching that water flow through the breached levees makes me believe otherwise. The other question I would have is if it's above sea level why do they have to pump it out?

It doesn't matter how much the "bowl" part of the city fills, the areas near the Mississippi are high enough that they are not subject to levee-breach flooding.

True, as long as the storm surge breaching through the other levees is only around 10-15' high, if the storm surge that hit N.O. during Katrina would had been as high as it was on the MS Gulfcoast, I guarantee you that even most of uptown and the French quarter would had been under water aswell had Katrina veered more to the west.

The reason for the pumps is because of the low areas, as the city expanded from the river levee and the natural ridges, there was more and more need to have pumps for drainage of heavy rains.

True, but I remember many times when Uptown and the French quarter flooded during a rain storm because their antiquate drainage system, rainwater has to be pumped out also even in higher areas because the terrain is generally so flat and they also have the natural sinking of land that has created bowls within the higher elevated areas. Which is the reason why the 17th street canal, the London canal, etc were built, so that pumping stations would be closer to uptown and the French Quarter.