Do Levees Redistribute Floodwater Risk? A Bayesian Assessment with the National Levee Database

Since the 1800s, levees, embankments that prevent water from entering a floodplain, have played a critical role in U.S. flood management.
While physics-based models are used by engineers prior to construction projects to limit adverse impacts, flood victims, law suits, and academic papers have asserted that in redistributing floodwater, levees redistribute flood risk up or downstream—to communities without this infrastructure. However, there has been little empirical research examining the actual effect of levees on stage (surface water level). We examine this question at a national scale, extending the simple statistical method of a small-scale study (in IL, IA) to levees in the National Levee Database, then generalize this approach to a fully Bayesian treatment using a multi-level model.
While the original small-scale study found that levee construction significantly increased stage upstream and adjacent to levees, we find that this result does not generalize nationally. However, the mutli-level model offers many advantages over the previous methods in this application, and allows us to study effect heterogeneity, where preliminary results suggest that non-accredited levees and those in regions with high silt soil composition may be more likely to exhibit stage increases.