Uncertainty in Household Behavior Drives Large Variation in the Size of the Levee Effect

Human-system responses to infrastructure projects are an important but overlooked driver of complex climate risks. For example, levees are commonly constructed to reduce flood hazards in low-lying areas and promote population and economic growth. Many studies show that levee construction achieves these goals but may also increase flood exposure to the point that overall risk increases beyond tolerable levels. Infrastructure planning practices tend not to account for this "levee effect," biasing decision-makers towards large structural defense projects. One reason planning practices do not account for the levee effect is that it is difficult to model the dynamics that emerge from levee construction. In this study, we examine how uncertainties in flood hazard, levee fragility, and household decision-making contribute to the occurrence and strength of the levee effect in coastal environments. We find that flood impacts from extreme events post-levee construction are highly sensitive to factors related to household behavior towards flooding and levee breaching. By accounting for the uncertainty in structural failure and household awareness of flood risks, city officials may be able to simultaneously promote sustainable urban development and improve coastal resilience.