*WINNER* Unsaturated soil model effects on the propagation of the saturated zone in levees during flooding

Abstract

Design of waterside slopes for rapid drawdown typically assumes an initial state of steady seepage prior to drawdown. However, levees built from low permeability soils are unlikely to reach this state during a flood, and a method has been developed to evaluate the degree of seepage propagation based on a linear approximation of the phreatic surface at end of flooding. For impervious foundation conditions, the degree of seepage propagation can be related to a time factor that is a function of the flood hydrograph, the soil coefficient of compressibility, and levee geometry. In addition, the post-flood saturated zone in levees depends on the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF), which are quite uncertain for unsaturated soils. This study parametrically compares results obtained using the Slide v. 8.0 Simple model to the Fredlund and Xing, and van Genuchten models for SWCC and HCF for levees with different flood, geometric and soil properties. The results show that the type of SWCC and HCF model selected has a significant effect on the predicted degree of seepage propagation, or the start-of-drawdown phreatic surface. For the same set of levee scenarios, the Simple model predicts significantly larger saturated zones within levees compared to those predicted using the Fredlund and Xing, and van Genuchten models. For more realistic assessment of levee seepage, van Genuchten is recommended. However, for cases where unsaturated soil properties are unknown or very uncertain, the Simple model may be appropriate.