ASCE Says NO Levees Remain in Question

The American Society of Civil Engineers (ASCE, www.asce.org) issued a public comment on the Interagency Performance Evaluation Taskforce’s second report on the hurricane protection system in New Orleans, raising concerns over the findings of the report and what they could mean for the "current and future safety offered by levees, floodwalls, and control structures in New Orleans." Established by the U.S. Army Corps of Engineers (www.usace.army.mil), the Interagency Performance Evaluation Taskforce (IPET) is comprised of government engineers and scientists from federal, state, and local agencies, as well as engineers and scientists from academia and private industry. The commission has been studying the performance of the New Orleans flood control system during hurricanes Katrina and Rita, and it has been providing status reports and recommendations on rebuilding efforts. The ASCE is serving as an External Review Panel tasked with evaluating the work of the IPET and helping the Corps of Engineers devise a viable strategy for flood control in southeast Louisiana.

According to the ASCE, the design calculations for the 17th Street Canal floodwall did not account for the possibility of a gap developing on the canal side of the floodwall as the hydraulic loading on it increased following Hurricane Katrina, effectively slicing the levee in half. ASCE says this calls into question the ability of other floodwalls in New Orleans to withstand design flood-level loading and requires that all such structures be reassessed.

In addition, the ASCE says the design calculations for floodwalls in New Orleans did not account for the lower shear strength of soils at and beyond the toe of the levee relative to the strength beneath the levee crest. Because this fact was not addressed in the design calculations at the 17th Street Canal, the ASCE says it may not have been taken into account in the design of other levees in New Orleans. Thus, the ASCE says the stability of levees founded on soft soils remains in question.

Based on its findings, the ASCE recommends the following actions:

A. All I-walls should be re-evaluated for current design loadings assuming a water-filled gap along the flood side of the wall.

B. All levees underlain by soft soils should be re-evaluated for current design loadings accounting for reduced shear strength of soil in areas at or beyond the toe of the levee.

C. All levees and floodwalls should be re-evaluated to identify those areas with a questionable degree of conservatism inherent in the design process, and those sections of concern reanalyzed for current design loadings employing an appropriate degree of conservatism.

D. A risk-based approach toward defining the design hurricane conditions is needed. The ASCE advises the Corps of Engineers to proceed as quickly as possible toward redefining the standard project hurricane using principles and practices similar to those used in establishing design criteria for other infrequent but potentially catastrophic natural disasters such as earthquakes and floods.

E. External peer review is an important component of design practice for all critical lifesafety structures. The ASCE recommends that the steps described above receive external peer review.