Researchers warn of California and Louisiana flood risks

COLLEGE PARK, MD. and NEW ORLEANS—An independent review panel chaired by Gerald E. Galloway, research professor of civil engineering at the University of Maryland’s Clark School of Engineering, said that the area between the Sacramento and San Joaquin River floodplains in California’s Central Valley faces significant risk of floods that could lead to extensive loss of life and billions of dollars in damages. In another recent report, researchers from Louisiana State University and NASA warned that sediments deposited into the Mississippi River Delta are contributing to ongoing, irreversible subsidence along the Louisiana coast, including New Orleans.

The review panel’s report, A California Challenge: Flooding in the Central Valley, was commissioned by California’s Department of Water Resources. The panel pointed out that many of the area’s levees, constructed during the last 150 years to protect communities and property in the Central Valley, were poorly built or placed on inadequate foundations. Climate change may increase the likelihood of floods and their resulting destruction. The panel recommends that state and local officials take swift action to reduce the risk to people and the environment. The comprehensive flood-risk abatement strategy the panel recommends focuses on land-use planning and integration with other basin water management activities.

"The challenges that California faces are widespread across the nation," Galloway said. "The recent failure of a levee in a Nevada irrigation canal points out growing infrastructure problems."

Galloway is also co-author of an article in a recent issue of Science magazine—"Aging infrastructure and ecosystem restoration"—which calls for targeted decommissioning of deteriorated and obsolete infrastructure to support restoration of degraded ecosystems.

"As we move forward with infrastructure enhancement, we must consider how, in the process of carrying out these activities, we can restore and enhance the natural and beneficial functions of the floodplain, which can at the same time reduce flood losses," Galloway said.

Galloway, a former brigadier general with the U.S. Army Corps of Engineers, also reviewed the levee system around New Orleans after Hurricane Katrina devastated the area and has been part of the state of Louisiana review team looking at long-term plans for restoration of the Gulf Coast.

In an unrelated study, researchers reported that sediments deposited into the Mississippi River Delta thousands of years ago when North America’s glaciers retreated are contributing to the ongoing sinking of Louisiana’s coastline. The weight of these sediments is causing a large section of earth’s crust to sag at a rate of 0.04 to 0.3 inches a year.

The sediments pose a particular challenge for New Orleans, causing it to sink irreversibly at a rate of about 0.17 inches a year, according to data from a network of global positioning system stations and a model of sediment data collected from the northern Gulf of Mexico and the Mississippi Delta.

Hurricanes Katrina and Rita in 2005 focused national attention on the Gulf coast’s vulnerability to hurricanes because of a loss of wetlands and sea level rise. These new findings, researchers said, add another factor for scientists, government agencies, and the public to consider when assessing the vulnerability of the region to hurricanes and large storms.

A science team led by Erik Ivins of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., hypothesized that earth’s very slow gravitational flow response to the weight of the sediments, combined with a 427-foot rise in sea level that took place thousands of years ago, are contributing to the sinking of a 77,000-square-mile section of coastal Louisiana.

To test their theory, the team developed a physical model of sinking caused by both the weight of the sea level rise and the flow of glacial sediments into the Gulf of Mexico following the retreat of the great ice sheet that covered much of North America about 22,000 years ago. The model spanned the past 750,000 years.

Results were compared with global positioning system measurements and other geophysical data for Southern Louisiana and the Gulf, collected from multiple sources during the last 60 years. The scientists found the model results were in good agreement with the geophysical data, predicting sinking of between 0.04 inches and 0.3 inches a year. The highest sinking rates were observed where coastal land loss is greatest, near the center of the Mississippi and Atchafalaya River Delta complexes.

The scientists say when these results are combined with sinking totaling about 0.12 inches per year caused by other factors such as compaction and oxidation of sediments, pumping of oil and water by humans, faulting, and sea level rise, the overall outlook isn’t bright.

"Louisiana is slowly losing its battle with the Gulf of Mexico," said Ivins. "Our model predicts this rate of sinking will continue for hundreds of years. Continued sinking, along with the sediment starvation of the coast caused by construction of flood control levees along the Mississippi River, will ultimately lead to the drowning of the coast."

Co-author Ron Blom of NASA added that New Orleans is particularly vulnerable. "When the effect of this sinking near New Orleans is combined with a potential 0.35 inch annual sea level rise that could result should ice sheet melting accelerate as projected by many climate models, it is possible New Orleans could see a relative sea level rise of roughly 3.3 feet in the next 90 years," Blom said.

The good news, the authors say, is that, with refinement, their model may help the region prepare better for future large storms and the gradual inundation of the coast.

"Our model gives civil engineers and disaster preparedness managers very precise predictions of how the landscape is changing so that they can better mitigate the effects of this sinking," said Ivins. "Understanding all of the processes affecting the coast is essential for engineering effective solutions."