Professor Tim Stark at the opened Morganza Control Structure on June 2. The structure was diverting excess floodwater from the Mississippi River into the Atchafalaya Basin through its 125 gated openings.
This spring's flooding along the lower Mississippi River provided a full-scale test of the area's flood-control structures from which engineers continue to learn. As part of a Geotechnical Extreme Events Reconnaissance (GEER) team, CEE Professor Timothy D. Stark visited the area June 2-3 to study the effects of what was the largest flood event in that area since 1973, when the Morganza Spillway was last opened to control Mississippi River flooding.
The reconnaissance team toured areas and structures from New Orleans to north of Baton Rouge and included academicians and a local consultant. The study was sponsored by the GEER project, which is funded by the National Science Foundation.
Stark’s specific focus was examining the performance of levees and other geotechnical-related flood control structures. Overall, these structures performed well and as designed, he said. However, the U.S. Army Corps of Engineers (USACE) identified 432 “Inspection Hot Spots,” or areas of interest, where more frequent monitoring was being performed because of an observed deficiency. In these Hot Spots, the most common deficiencies were underseepage, sand boils—an indication of the severity of underseepage—cracking, and levee leakage. These deficiencies were being monitored and addressed by the USACE.
Large sand boils about 2,000 feet from the Mississippi River.
At the time of the GEER visit, approximately a month after the spillway was opened, the Mississippi River level was within three feet of the top of most levees, which is the design freeboard, Stark said. In some places the freeboard was less than three feet, due to levee settlement or construction. Even though the levees had been severely challenged by the design flood level for at least three weeks, no levee breaches had been reported. Some levee and foundation seepage was observed, as well as some seepage anomalies, but none of these phenomena threatened the stability of the levees.
The flooding provided an opportunity to investigate the performance of levees under the maximum loading condition, Stark said. He is using this information to quantify the migration of the phreatic surface—the free surface of ground water at atmospheric pressure—through levees during flooding, which is important for the design and stability of levees, and the zone of influence of levee underseepage. For example, he said, in some locations significant sand boils were found more than 2,000 feet from the river.
Stark is using what he learned during this reconnaissance for the analysis of levees in the Dallas Floodway System, which are undergoing recertification and upgrade to an 800-year flood event, and for a causation investigation for the U.S. Department of Justice of the levee failures along the Inner Harbor Navigation Canal (IHNC) during Hurricane Katrina that inundated the Lower Ninth Ward in New Orleans. In the Lower Ninth Ward more than 1,000 people were killed and about 4,000 homes were destroyed by the levee breaches along the east side of the IHNC.
“The 2011 flood observations are important to these and other projects because this once-in-a-100-year or less loading represents a full-scale test of the flood control structure,” Stark said. “It’s a unique opportunity for engineers to observe, monitor, and collect data under the full design condition.
In addition to Stark, GEER Team Members included: Professor Robert B. Gilbert (Team Leader), University of Texas at Austin; Professor Thomas L. Brandon, Virginia Tech; Professor Joannes Westerink, University of Notre Dame; and Michael Alfortish, Project Manager for Fugro Consultants in New Orleans.
The GEER Team was greatly aided by the briefings and information provided by the following U.S. Army Corps of Engineers (USACE) personnel: Walter Baumy, Nancy Powell, Mark Woodward, James Sefert, and Erica Johnson. Their information assisted in developing the observations and information included in the reconnaissance report, Stark said.