Rune Storesund

Simulated Wave-Induced Erosion of the Mississippi River–Gulf Outlet Levees during Hurricane Katrina

This case study on the Mississippi River Gulf Outlet earthen berm/spoil bank (EBSB) illustrates a method to quantitatively analyze wave-induced lateral erosion and breaching through the flood side of coastal flood protection levees with grass armor prior to overtopping. Lateral wave-induced erosion was evaluated based on a three-step process: (1) velocity profiles were generated using a general purpose transient dynamic finite element program; (2) resistance to wave-induced erosion as a result of grass cover (turf) on the flood face of the EBSB was estimated and no wave-induced erosion was applied until the grass cover protection failed; and (3) estimated soil erodibility characteristics were used, in combination with the estimated wave-induced flood face velocities, to estimate the magnitude of lateral erosion. This case study identified that soil erodibility characteristics directly impact coastal flood protection levee performance, grass/turf armoring provides erosion protection for a limited duration,...

Investigation of the performance of the New Orleans regional flood protection systems during Hurricane Katrina: Lessons learned

The recent flooding and devastation of the greater New Orleans region during hurricane Katrina represented the most costly peace-time failure of an engineered system in North American history. Extensive investigations and analyses have been performed by several major teams in the wake of this disaster, and some very important lessons have been learned. Many of these have very direct and urgent applications to levee systems in other regions throughout the U.S., and the world. Lessons include the importance of proper evaluation of risk and hazard; so that appropriate decisions can be made regarding the levels of expense and effort that should be directed towards prevention of catastrophe, and the levels of post-disaster response capability that should be maintained as well. The making of appropriate decisions, given this information regarding risk levels, is then also important. Also of vital importance are numerous “engineering” lessons regarding analysis, design, construction and maintenance; hard-won lessons with applications to flood protection systems everywhere. We must now do everything possible to capitalize upon these; and to prevent a recurrence of this type of catastrophe in the future. 1 Professor, Dept. of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, California 94720. Email: seed@ce.berkeley.edu GSP 161 Embankments, Dams, and Slopes Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org

Mississippi River Levee Failures: June 2008 Flood

During the spring and summer of 2008, record rainfall in the Midwest United States led to severe flooding as water overtopped the levees bordering the Mississippi River and its tributaries. The erosion associated with the overtopping resulted in levee breaches in many places. After the flooding, a field reconnaissance team was sent to collect time-sensitive data and provide a comprehensive overview of the performance of the levees during the flooding. Two locations where levee overtopping occurred are particularly interesting because of their differing site conditions and performance. This paper presents the levee overtopping case histories of the Winfield-Pin Oak site which was overtopped and severe erosion led to failure, and the Brevator site which was also overtopped but did not fail. Included are a hydrological investigation, documented site conditions, geotechnical soil properties, a soil erodibility analysis, and the documented levee vegetative cover. Levee performance is influenced by the flood conditions, the site conditions, and the soil properties. Both sites in this study experienced large levels and durations of overtopping water, but it is proposed that the Brevator site survived because of its vegetative cover and more erosion resistant soils. Erosion is a very complicated phenomenon that cannot be described by any one parameter, but in all cases, dense and consistent native vegetative cover can greatly improve the overall levee performance.

Erosion Tests on Samples from the New Orleans Levees

Twenty three samples were retrieved from eleven locations at the top of the levees around New Orleans. Thirteen were samples from Shelby tubes while ten were bag samples. These samples were tested in the EFA ort Erosion Function Apparatus. The results of such an erosion test is an erosion function which links the erosion rate in mm/hr to the water velocity in m/s. The results obtained show a large variation of erosion resistance among the soils tested and point to the need to evaluate the remaining levees. New ideas for such an evaluation and for improving the redundancy of such long linear systems are presented. GSP 161 Embankments, Dams, and Slopes Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org

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, R. E. Wahl, and N. T. Schwanz

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

Investigation of levee performance in Hurricane Katrina: The inner harbor navigation channel

The Inner Harbor Navigation Channel (IHNC) sits at the heart of the three main populated regions of New Orleans. As shown in Figure 1, a number of breaches of varying severity occurred along the shores of the IHNC during Hurricane Katrina, contributing to the flooding of all three of the most heavily populated protected areas flooded in this event. This paper presents the results of geotechnical and geo-forensic investigations performed to determine the causes of the various failures along the shorelines of the IHNC. This includes the two massive breaches at the west end of the Lower Ninth Ward, and a number of lesser breaches and partial breaches (distressed sections) along other sections of the canal. There are many lessons here, not just for the greater New Orleans region, but for the nation and for the Profession as well.