The operators and designers of a New Orleans flood control system are faced with sophisticated design conditions (Christian 6). In the early 20th century, pumping stations were constructed along the ridge to take out floodwaters from the river basin. The newly inhabited areas north of the ridge were exposed to flooding from the nearby lake. Rather than constructing additional pumping capacity at the shores of the lake, the existing pumping stations were connected to drainage canals that emptied into the lake. The city also stretched towards the east (particularly in Lower Ninth Ward and New Orleans East regions).
All these areas towards the east lie below the sea level and thus, they must be protected or sheltered by a system of levees (Christian 7). Southern Louisiana was constructed out of sediments transported from Mississippi River interior. The crystalline bedrock is covered by tens of thousands of feet of soft sediments from the river. There is a basic pattern of subsidence sophisticated by additional settlement such that a levee can be placed on the sediments. Thus, the individual structures and the city settle by varying amounts, for instance, the tops of the levees can be actually low than the water levels they are required to defend against.
Additionally, because of the basic subsidence, it is not easy to establish reliable benchmarks that can be used to assess the location of the levees (Christian 7). The flooding in New Orleans occurred due to three phenomena and two of these phenomena are important in this context. The phenomena include overtopping of the levees and sections of the canals or levees failing even before the level of the water reached the overtopping level.
Some of the levees were overtopped because they were constructed withstand the small magnitude of storms than Hurricane Katrina. The levees were below their design elevations and the impact of the design storm was not sufficiently estimated. The levees that failed to work even before the overtopping could not adequately hold water levels for which they were constructed.
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