Effect of Changes in Hydraulic Conductivity on Exit Gradient at Selected Levee Systems Using Numerical Models

ABSTRACT

The objective of the research was to provide a better understanding of the influence of hydraulic conductivity on the exit gradient for a rectangular block representing a root zone. A critical area of concern to design engineers is the exit gradient at the toe of the levee and other sensitive areas. If the exit gradient becomes too large, water flows too fast, thus creating the potential for piping and internal erosion. Seepage analyses using the finite element method were conducted where the hydraulic conductivity k within an estimated root zone was varied. These analyses were based on the assumption that a root system alters the k of a soil. Levee systems used in this study were located in Sacramento, CA; Burlington, WA; Albuquerque, NM; and Portland, OR. These sites were selected based on available data from previous investigations. Both two-dimensional (2-D) steady-state and transient computations were made. Three-dimensional (3-D) solutions were generated by extruding the 2-D cross sections from each study site to form a 3-D mesh and then running a parallel program. For each levee cross section, a root zone was placed at different locations on the levee profile. The root zone was estimated from geophysical surveys to be approximately 1.8 m × 1.5 m in size. The original (k) assigned to the root zone was multiplied by a factor, β, 0.01 β ≤ β 100. The 2-D computations were made with values of β = 0.01, 1, and 100 for the different root zone locations. Exit gradients calculated for root zones placed at the bottom of the dewatered drainage ditch in Albuquerque, NM, and on and just beyond the toe of the levee in the other levee systems showed changes in exit gradients when k was varied. At other locations of the root zone along the levee, exit gradient showed small or no change with different values of k. Field measurements were not a part of this study.

Keywords:

Exit gradient, seepage in levees, vegetation modeling, piping, finite element method, numerical models of levees, steady-state seepage, transient seepage.