Modeling Geomorphic Features in Levee Reliability Analyses

It has long been understood that subsurface geology affects internal erosion potential in levee foundations. Geomorphic features associated with fluvial deposition environments are known to concentrate or block seepage, resulting in sand boils and internal erosion. However, incorporating the effects of geomorphic features into either deterministic or probabilistic analysis has remained a challenge. Based on the assumption that the preponderance of the underseepage risk to a levee reach is due to the geomorphic features along that reach, a methodology is being developed to incorporate the combined effects of these geomorphic features into reliability analyses of underseepage risk. A Response Surface-Monte Carlo (RSMC) analysis that takes into account the uncertainty in the subsurface geometry and the soil properties of the geomorphic features is used to assess the flow regime associated with each geomorphic feature. The RSMC is followed by an event-tree analysis to assess the likelihood of levee failure based on the resulting flow regime. Three-dimensional finite-element seepage analyses will be used to develop the response surface in order to take into account the inherent three-dimensional aspects of the geomorphic features. Finally, the total probability of failure of the levee reach due to underseepage will be calculated by combining the probabilities of failure for the geomorphic features located along a levee reach. In this manner, the “length effects” of a levee reach in a fluvial environment can be assessed by combining the failure probabilities of the geomorphic features located along the levee alignment.