The "levee" failures
"As Katrina moved over land Monday the water it brought surged into Lake Pontchartrain. (Watch the video of water filling the city -- 1:52)
A day later, the straining levees could not hold back the additional water and they broke in three places -- along the Industrial Canal, the 17th Street Canal, and the London Street Canal -- allowing water to pour into the city. (Map: See where the levees' broke)"
Below is the pertinent map from the US Army Corps of Engineers (New Orleans District) showing the three canals and I have used green arrows to show the approximate locations labeled on the CNN satellite images.
This COE map happens to show some of the flood protection and I think what is shown is confined to that which was part of the Lake Ponchartrain, LA and Vicinity Hurricane Protection Project. I'm not sure, but presume, that what is shown is work completed, not what comprises the project.
The numbers on the COE map refer to the elevation of the flood barriers in those locations. The elevations are based on NAVD datum (1929). (NAVD is simply explained here and has a nice cross-section of New Orleans there too.) Anyway, elevation 0.00' is basically sea level. These elevations, I believe, represent to the flood elevation rating of the barrier which is 3 feet below the top of the barrier. The additional 3 feet (I don't know if that is a standard) is called freeboard, if I remember correctly, which is basically an allowance for water sloshing around for various reasons, e.g., wakes, waves and such.
In the case of the 17th Street Canal the barrier was breached where the barrier was about 10' above sea level.
In the case of the London Avenue Canal the letters refer tabulated ratings. The elevation at 'g' is 10.0'. (I am not sure whether that is for both sides of the river or only the west side. The Engineer in me says west side only since their are elevation noted for the east side but it seems odd that the east side (13.7'+/-) was the failure point rather than the west side (10') unless it was a structural failure, not a breach failure by overtopping.
There are pictures of the failure at the Industrial canal and it was on the east side. Since that location either was not a part of the project or not completed at the time of the last update of this map, no barrier protection is identified there on this map. I'll have to look around for more comprehensive maps.
What is really interesting is two of these canals, the 17th Street and London Avenue are dead ends. I wonder if these serve also as discharge points for pumping stations. In any event, it appears the weak spots in the system were not at either the Mississippi or at Lake Ponchartrain. They were in the canals jutting into the City from either the Lake or the River.
I had noticed floodgate information on other pages, including one currently being installed at the Harvey Canal. There is a lock on the Inner Harbor Navigational Canal (IHNC) at the entry from Mississippi. My gut feeling is the failure occurred here from high side, the Mississippi side, since it appears Lake Ponchartrain is lower than this point in the canal.
One thing to note. It is more likely, based on the locations and particularly the location on the 17th Street Canal and the London Avenue Canal, that floodwalls, and not levees, were what failed. Levees are earthen berms, which may or may not also have a floodwall designed for the top of them. A floodwall are structural concrete walls, steel walls or a combination of both. The walls are used where there is not enough width for a levee design.