1.  THE ERP REPORT: WHAT WENT WRONG AND WHY The levee and flood wall [on the 17th Street Canal] were built over a layer of organic soil called peat or marsh. This peat overlies a layer of very soft clay (Figure 1). To prevent failure of a levee or I wall, the resisting forces (the strength of the underlying soil) must be greater than the driving forces (the weight of the levee and the pressure of the canal water acting against the levee and the I wall). The engineers responsible for the design overestimated the soil strength. The soil strength used in the design calculations was greater than what actually existed under and near the levee during Hurricane Katrina. 1.  THE ERP REPORT: WHAT WENT WRONG AND WHY

2.  THE ERP REPORT: WHAT WENT WRONG AND WHY In the 17th Street Canal failure the critical zone for sliding was the soft clay between elevations -15 ft and - 30 ft. The soil strength chosen for design is the red line (Figure 2). The soil below the levee was actually weaker than that used in the I-wall design. Actual average soil strength in the critical stability zone is 0.13 tons/sq. ft. (12.45 kPa), compared with the engineer's design strength of 0.19 tons/sq. ft. (18.19 kPa), or approximately 32% lower. Data for soil tests were taken along the centerline of the levee, which is consolidated and strenghtened by the weight of the overlying soil. Orange line shows the average strength below the centerline of the levee. Blue line shows the average strength beneath and beyond the toe of the levee. 2.  THE ERP REPORT: WHAT WENT WRONG AND WHY

3.  THE ERP REPORT: WHAT WENT WRONG AND WHY The factor of safety used in the design is 1.3. This factor is at the low end of generally accepted engineering values. Key Corps design guidance documents specify 1.4 or 1.5. The design was simply too close to the margin of safety, allowing little or no room for uncertainties. Another critical engineering oversight that led to the I-wall and levee failure involves not taking into account the possibility of a water-filled gap (Figure 3). The water-filled gap turned out to be a very important aspect of the failures of the I walls around New Orleans. 3.  THE ERP REPORT: WHAT WENT WRONG AND WHY

4.  THE ERP REPORT: WHAT WENT WRONG AND WHY Figure 4 shows the critical failure surface without the water-filled gap. Figure 5 shows the critical sliding surface with the water-filled gap. With a water-filled gap, the sliding surface cuts through less soil, so less soil strength can be mobilized, and the factor of safety is about 30% lower. A late 1990s sophisticated computer modeling study concluded the following: "As the water level rises, the increased loading may produce separation of the soil from the pile on the flooded side. Intrusion of water into the tension crack produces additional hydrostatic pressures on the wall side of the crack." The design of the I walls was not checked for safety in light of this new information. 4.  THE ERP REPORT: WHAT WENT WRONG AND WHY

5.  THE ERP REPORT: WHAT WENT WRONG AND WHY The soil beneath [the London Avenue Canal south breach] is sand beneath the marsh layer. As the water level rose in the canal, water seeped into the highly permeable sand, flowing under the levee toward the land side of the wall. In well designed structures, the uplift water pressure is never allowed to come close to the weight of the overlying soil. In this case, the marsh was lifted up off the sand, and cracked open, which allowed water to rush through the cracks. The upward rushing water carried sand with it, scouring a hole that rapidly expanded and worked its way back under the levee, undermining it and destroying it. Calculations did not include the presence of the marsh layer, which was neglected in the design. Proactive measures such as extending the depth of the sheetpile wall, or installing relief wells, were not employed, leaving no redundancy in place. Failure was exacerbated or caused by the water-filled gap. Due to the water-filled gap, the elevated water pressures were brought much closer to the land side of the levee. 5.  THE ERP REPORT: WHAT WENT WRONG AND WHY

6.  THE ERP REPORT: WHAT WENT WRONG AND WHY Both the Industrial Canal east bank south I wall and the Industrial Canal west bank I wall were overtopped by Hurricane Katrina. The peak water level was estimated to be 1.7 ft (0.5 m) above the tops of the floodwalls and levees. The apparent failure mechanisms are shown in Figure 8. 6.  THE ERP REPORT: WHAT WENT WRONG AND WHY

7.  THE ERP REPORT: WHAT WENT WRONG AND WHY Earthen levees without I walls were overtopped by Hurricane Katrina. The majority of the 50 levee breaches can be attributed to overtopping and erosion (Figure 9). Levees constructed with properly compacted clay with a good grass cover appear to have withstood the storm the best. Levees with higher silt and sand content, or levees built with hydraulic fill, sustained the worst erosion damage, and in some cases, were completely washed away. 7.  THE ERP REPORT: WHAT WENT WRONG AND WHY

8.  THE ERP REPORT: WHAT WENT WRONG AND WHY The engineering failures were complex and involved numerous decisions by many people within many organizations over a long period of time. If the hurricane protection system had been treated as a major dam, the return period for design would have been 100,000 or 1,000,000 years. The risks associated with New Orleans' hurricane protection system had never been quantified. Hurricane Katrina had a return period of 50 to 500 years. The Corps defines the standard project hurricane (SPH) as a "reasonably characteristic event." The Corps did not evaluate the "probable maximum hurricane" (PMH) for New Orleans. The SPH was not strong enough to sustain Katrina. An accurate vertical datum is a necessary step for the design of all engineered structures. Some structures were designed and constructed relative to datums that were incorrectly assumed to be equal to the local mean sea level (Figure 10). 8.  THE ERP REPORT: WHAT WENT WRONG AND WHY

9.  THE ERP REPORT: WHAT WENT WRONG AND WHY The entire New Orleans region is subsiding, at rates varying from 0.2 in/yr to 1 in/yr. Subsidence was not taken into account in the design of the levee system. Due to subsidence and use of an incorrect vertical datum, levees were up to 3 ft lower than the original design. Freeboards have been completely lost to subsidence. The peak storm surges were only 1-3 ft above the levee tops; therefore, had the levee tops and floodwalls been at the proper elevations, fewer overtoppings would have occurred. Various federal, state, and local agencies are responsible for the construction, operation, and maintenance of segments of the hurricane protection system. No formal coalition of agencies is charged with providing strategic direction, defining roles and responsibilities, and coordinating critical construction, maintenance, and operations. There was no formal external peer review of the projects. The organizational culture (for instance: "it's how we have always done it") influenced behavior over the many years that the levee system was under design and construction. The pressure for trade-offs and low-cost solutions probably compromised quality, safety, and reliability. The project-by-project approach resulted in a hurricane protection system being constructed piecemeal, with lack of attention to "system" issues. The Corps focused on reducing the problem to one that can be solved with the given authority and budget. The issues of risk, redundancy, and resilience took a lower priority. The design and construction of critical life safety systems are beyond the expertise of the Executive Branch or Congress. However, Congress exercised tight control over spending and even over design criteria. The Corps accepted these controls without fully addressing the risks and trade-offs. The Corps and its local sponsors did not argue vigorously enough for adequate funding to provide a high level of assurance for the public safety of the people of New Orleans. 9.  THE ERP REPORT: WHAT WENT WRONG AND WHY

10.  THE ERP REPORT: WHAT WENT WRONG AND WHY Overestimation of soil strength. Failure to take into account presence of marsh or peat. The chosen factor of safety (1.3) was too low. Failure to account for the water-filled gap in the design of the I wall. Failure to provide for protective measures or redundancies. Failure to establish a reliable vertical datum. Inadequate appraisal of risk (choosing the SPH instead of the safer PMH). Lack of coordination between agencies. Piecemeal approach to design. Willingness to compromise on cost. 10.  THE ERP REPORT: WHAT WENT WRONG AND WHY

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