Discussion of "Overview of New Orleans Levee Failures: Lessons Learned and Their Impact on National Levee Design and Assessment" by G. L. Sills, N. D. Vroman,

The article concludes that the primary IHNC north breach failure was a result of “differences between actual conditions and assumptions used as design.” Specifically, the breach was attributed to elevation differences and rotational failure as a result of wall deflections and the formation of a “gap” on the flood side of the I-wall. The article also concludes that overtopping was the only possible failure mechanism for earthen levees. While these failure modes are certainly possible, they are neither exclusive, nor exhaustive. Additional failure modes exist and should be accounted for and incorporated into the failure analyses. There were two major breaches along the IHNC east bank: a short 50-m breach at the north end and a much longer breach of 250 m near the southern end. These two major failures of the floodwall were strongly influenced by deep excavations that had been completed adjacent to the outboard side of the hurricane flood protection system just prior to hurricane Katrina Bea 2006a, 2008a. These excavations were part of a site demolition and clearing operation removal of structures, utilities, underground storage tanks, and contaminated soils for a new navigation lock to be constructed in the East Bank Industrial Area EBIA located immediately outside of the floodwall levee Fig. 1. Multiple excavations were dug to depths in excess of 7.6 m 25 ft that were backfilled with loose sand as a result of sand placement below the water table and “native” soils from this area Fig. 2. These excavations penetrated the buried marsh layers that underlie this entire area down to depths in excess of 9 m Figs. 3 and 4. Since the sheet piling supporting the floodwall were driven to tip elevations above the bottom of the marsh layers of approximately 3 m, seepage and water pressures generated by the hurricane Katrina surge waters were able to rapidly communicate under the levee floodwall and destabilize the flood protection structures. The impacts of these excavations on the performance of the flood protection system at the North Breach were evaluated by the authors. The primary failure mode impacted by the excavation work was floodwall underseepage piping. Fig. 5 summarizes results from a two-dimensional finite-element analysis of the seepage gradients and pressures at the north breach Bea 2008a. The overlying less-permeable fill and levee soils act as a blanket on Fig. 1. Color Excavations for soil borrow pit on waterside of floodwall at Lower 9th Ward Bea 2008a