The picture above shows that the water stopped downtown at the natural levee created by thousands of years of spring floods along the Mississippi River. Until the late 1800's and early 1900's, downtown and the high ground along the river were generally the only populated areas of New Orleans. The rest of New Orleans heading northward to the lake was a swamp.
The cause for the breaches along the canals in New Orleans is being called “the greatest engineering failure in American history” (Marshall, McQuaid, and Schleifstein 2006). The floodwalls and levees along the 17th Street Canal and the London Ave. Canal were not topped. The walls gave way. Engineers had not correctly taken into account the unstable soils below the surface in which the floodwalls were imbedded. This mistake allowed water to flow through the porous “marsh soils”, undermining the floodwalls which are sheet pilings driven into the ground.
Boring data from the project along the 17th Street Canal showed a layer of porous “peaty soils” (the old swamp) extending from 11 ft. to 30 ft. below sea level. However, when the boring data was transferred to a cross section, it stopped the peaty layer at only 16 ft. This was a mere clerical error! (Marshall 2006) This led engineers to design a wall extending to only 17.5 ft. below sea level. As a matter of fact, the cross section also noted that below 15 ft. the soil was “fat clay” or “lean clay”-sticky impervious soils considered very good for resisting water.
J. David Rogers, a member of the National Science Foundation team investigating the failure said, “It's pretty obvious the depth of the organic deposit shown on that cross section is what determined the sheet piling depth for the whole canal. They saw this marshy deposition; they recognized it as potentially dangerous, so they specified sheet piles that went just beyond the bottom of that line.” So it's easy to deduce that if they saw that peat layer going to 30 ft., they would have placed the piling at least that deep. And it appears pretty clear, at this point, that the transfer of the boring data to the cross section is where the ball might have gotten dropped.”
Rogers continued, “It looks like the guys working on the floodwall design thought they were anchoring those sheet pilings in clays, and they were really right in the middle of this highly organic, highly permeable soil that ran another 10 ft. below the tips of those pilings” (Marshall 2006).
In addition, the -17.5 ft. depth below sea level was still 1 foot shorter than the actual depth of the canal. More recent information has come forward stating that in addition to the above miscalculations, that pressure from the water on the canal side caused separation between the floodwall and the levee it was imbedded in, allowing water to fill in the gap between the two. The water pressure in that opening put pressure on that weak peat layer below extending to the land side of the levee, causing it to slip and bring the levee and floodwall down. Tragically, The Corps of Engineers had done a test in 1985 that showed that this would happen.
These drainage canals, that were used to pump rain water from the city to the lake, were the downfall of the Greater New Orleans area. The pumping stations are several miles from the lake, having been placed at the edge of where the old swamp was back in the 1800's. Hence, Lake Pontchartrain's floodwaters were able to intrude through these canals deep into the modern day city, being contained only by the poorly designed levee-floodwall system explained above. Katrina finally brought out the weaknesses. Some called them “Trojan Horse” canals after the storm.
Part of the federal government's plan to repair the system is to move the pumping stations all the way to the lake, where they will be part of the levee system along the lakefront. Then, the drainage canals, extending deep into the city will no longer rise and fall with the lake level.
Shifting our attention further east, the lower 9th Ward was devastated by flooding from the Industrial Canal. This is a navigational canal which connects The Mississippi River to Lake Pontchartrain, as well as the Intercoastal and Mississippi River Gulf Outlet canals (which connect to the Gulf). Here, all ten miles of the levees and floodwalls were actually topped by the water. The Corps of Engineers also identified four breaches, two on the east side and two on the west (Grissett 2006). The two on the east side did the most damage as the water roared into the 9th Ward. The picture below shows the result:
As mentioned before, the Mississippi River Gulf Outlet (also called MRGO) and the Intercoastal Canal may have worsened the flooding along the Industrial Canal. On the map above, you can see how both connect to the Gulf to the east. Remember that a strong easterly flow helps fill Lake Pontchartrain in advance of a storm. The same situation occurs with MRGO and the Intercoastal, as water is pushed up these canals (which converge) into the Industrial Canal (as seen in close up below).