The ups and downs of levees; GPS-based change detection, middle Mississippi River, USA

Changes in levee-crest elevations were measured along 328 km of Mississippi River levees between St. Louis, Missouri, and Cairo, Illinois, in 1998 and 2007. We also compared 1998 and 2007 survey data with 50 yr flood elevation profiles to assess levels of levee protection. Levee heights, stability, and safety are nationwide concerns, especially since Hurricane Katrina and floodwall failures in New Orleans, Louisiana, in 2005. The majority of surveyed levees were stable at the centimeter to decimeter level during the 9 yr measurement interval. Change was measured in other areas, including increases of as much as 1.49 m and decreases of as much as 1.26 m between 1998 and 2007. The increases corresponded to local crevasse repairs, small-scale road maintenance, and larger levee-raising projects. Decreases in levee elevations are interpreted as small-scale surface erosion and larger compaction or subsidence of levee and/or foundation materials. Levee crests in 2007 were locally as much as 2.0 m below the 50 yr flood-grade elevations. Levee degradation reduces protection levels for floodplain residents, often without easily visible symptoms. Intentional levee raising without necessary studies, engineering, or permitting has a range of potential negative impacts including both a false sense of security and the potential for exporting flood risk to neighboring levee communities. Regional surveying of levee elevations and change over time can provide a preliminary tool for assessing levee conditions, stability, and compliance with levee regulations.