Finite Element Analysis of Levees with Woody Vegetation

The impact of woody vegetation on levee stability is of significant concern, yet the soil-root zone processes that dictate much of this impact are not fully understood. The root system can create locally anisotropic, heterogeneous modifications to material properties related to subsurface hydrodynamics and structural mechanics. The complexity of the physical processes, material heterogeneity and inherent three-dimensionality put such problems outside the range of traditional methods of analysis. Specifically, seepage and stability analysis for engineered levees is typically carried out on vertical cross-sections using steady-state subsurface flow and limit-equilibrium or semi-empirical based approaches for soil mechanics. However, vegetation can generate local modifications in the root zone that lead to genuinely three-dimensional behavior, while the temporal scale of flooding events and range of soil deformations possible make steady-state or equilibrium approaches inadequate. In this work, we consider the processes of variably saturated flow and elastic-plastic deformation using fully three-dimensional, nonlinear continuum mechanical models. We present mathematical model formulations and three-dimensional finite elements for simulating levee seepage and soil mechanics. In particular, we present factor of safety calculations for levee stability under various surface loads and seepage conditions and a verification and validation test set. The test set allows comparison to field data and traditional analytical methods as well as inter-comparison of various finite elements in terms of accuracy, efficiency, and robustness.