2008 Midwest Levee Failure: Erosion Studies
Bernhardt, Michelle Lee
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The United States contains an estimated 100,000 miles (160000 km) of levees in which erosion related issues are the top priorities. Proper documentation of overtopping induced erosion is a complicated issue involving the collection and analysis of timesensitive field data and personal observations. This thesis is a study of the performance of the Midwest Levee system during the 2008 flooding events. The goal of the Midwest Levee investigation was to gather and analyze perishable data in an effort to provide a comprehensive overview at each breach location. To predict how a site will perform during a particular flood event, there are three main inputs: the flood conditions, the site conditions, and the soil properties. Site geometry and imperfections can greatly affect the performance of a levee system. Any low spots or potential seepage paths can concentrate the flow and be detrimental to the levee. The vegetative cover is the single most important condition at a site. As seen in the Brevator case, vegetative armor can prevent failure of a levee comprised of less resistant soils subjected to long periods of overtopping. Recommended grasses include: Switchgrass, Smooth Brome, Reed Canarygrass, and Tall Fescue. It is also recommended that grasses are kept at least 0.5 m tall during the flood season and to limit the presence of trees to 10 m beyond the levee toe. The erosion resistance of the materials comprising the levee is also important. From the correlations in this study, it was determined that erodibility is influenced by grain size, relative compaction, clay content, and activity. Devices like the Torvane and Pocket Erodometer can also be used to get a quick field estimate of erosion. While these correlations and field devices give insight into an erodibility value, they are no substitute for site specific analysis with laboratory equipment such as the Erosion Function Apparatus. Soil behavior is highly nonlinear and the entire erosion function is needed to get an accurate measure of the erodibility of a soil. By combining these properties in an erosion matrix, a prediction of whether a site will withstand a given flood event can be made.