Reliability Underseepage Assessment of Levees Incorporating Geomorphic Features

Abstract

Levee foundations along meandering rivers are often modeled in seepage analyses with simplified models that allow for use of simplified reliability methods. Due to the complex geomorphic environment that is often encountered in the fluvial environment and curvature alignment, levee foundation geometry can range from simple to very complex. Geomorphic features in the soil layers underlying a structure often have a significant effect on the underseepage behavior and the potential for initiating internal erosion. Based on the hypothesis that levee underseepage susceptibility comes from localized subsurface geomorphic features that interrupt the characteristic profile along that levee reach, a methodology has been developed that assesses the hydraulic effect of geomorphic features in levee underseepage reliability. The methodology consists of a response surface-Monte Carlo analysis that takes into account the uncertainty in the subsurface geometry and soil properties in assessing the seepage regime associated with the feature. The method utilizes three-dimensional steady-state finite-element underseepage analyses to develop a response surface representing the relationship between soil properties and the three-dimensional levee foundation. The response surface then serves as the driving function for reliability analyses by means of Monte Carlo simulation analyses, resulting in cumulative probability functions for either hydraulic exit gradient or factor of safety against heave. These computed probability functions represent an assessment of conditional probability of initiation of internal erosion. Results can be adjusted for curvature effects when needed. The analysis of a crevasse-splay, an abandoned channel, and a meander scroll feature found in the Sacramento River (east side) levee system in California are presented as application examples.