The Canal District BreachesIn New Orleans, a series of canals run south from Lake Pontchartrain in t

The Canal District BreachesIn New Orleans, a series of canals run south from Lake Pontchartrain in the North. Those canals ruptured during Katrina, flooding large portions of the city. Yesterday I covered the ruptures of the Industrial Canal that flooded the 9th Ward area. Today, on the anniversary of the actual strike of Hurricane Katrina on New Orleans, I want to focus on the breaches in the Canal District, including the breach along the 17th street Canal that we see here.To this geologist, these breaches are particularly fascinating. These levees failed because of the local geology and because of the properties of the storm.Let’s start with the storm itself. Earlier this week, I covered how storm surges are offset – the storm surge is larger on the right hand side of the storm because the winds are blowing towards the shore (http://tmblr.co/Zyv2Js1szyZid). On the left hand side of the storm, the winds are blowing back out to sea.Hurricane Katrina’s eye passed to the East of New Orleans, meaning the strongest storm surges hit Alabama. However, the area itself had one other unique feature: Lake Pontchartrain.Hurricane Katrina hit to the East of New Orleans and blew wind north in Alabama. New Orleans mostly received winds aimed to the south, blowing the ocean away from the city. However, winds blowing to the south also passed over Lake Pontchartrain. The strong winds from Katrina created a smaller storm surge on Lake Pontchartrain by blowing its waters to the south – right into the city and right into the Canal District. So that’s the first interesting detail – the presence of Lake Pontchartrain moved a storm surge to an unusual, surprising spot – the north side of New Orleans.The other interesting geologic story in this levee failure is how the levees actually broke. In the previous case, the levees were overtopped; creating a waterfall that eroded the levee support (http://tmblr.co/Zyv2Js1t5g4O2). This levee behaved differently; see how it was completely knocked over? This levee was pushed back. It wasn’t overtopped; it failed because the water pushed on it too hard.Levees are not supposed to do this. These levees were built to sustain this surge of water but they broke. This was a geological engineering failure.These levees are concrete walls anchored into a pile of soil beneath them. These anchors were large, iron pillars driven into the dirt levee below. Those supports were supposed to transfer the force of the water down into the sediment below, counter-balancing the force of the water.That system was what failed. When this levee failed, the first reports suggested the contractor that built them may actually have committed fraud and not driven the metal supports deep enough. However, during the investigation of this failure some of the supports were pulled out of the ground and found to be at the depth that was promised.Instead, this failure was actually controlled by the sub-surface geology. The team investigating the failure took cores through the sediment and found that near the bottom of these pillars there was a thin layer of silt and sand that was extremely weak relative to stress. When the storm surge hit, this layer became saturated and water began passing through it from one side to the other, making it extremely weak and unable to hold weight. When the weight of the water was transmitted downwards by the pillars, this thin layer of sediment allowed the dirt above it to shift.Once the levee above it began shifting, failure became inevitable. The levee lost its support and the weight of the water pushed it over backwards. Almost 80% of the water that entered the city of New Orleans poured in through gaps that formed because of the sub-surface geology.When these levees were reconstructed they were upgraded with deeper supports to hopefully avoid this problem in the future. This failure is a testament to how important geologic engineering can be. The teams constructing the levees didn’t discover the weak layers in the sediment and even some of the initial investigations before the large, National Science Foundation project missed it.Large portions of New Orleans flooded because of a thin layer of dirt. If a geological survey had found that layer beforehand and enough money had been appropriated to build columns deep enough…it could have saved most of the city.-JBBImage credit: NSF reporthttp://www.ce.berkeley.edu/projects/neworleans/http://bit.ly/1Kq0xUkhttp://bit.ly/1KSaEx0 -- source link

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