Three-dimensional levee and floodwall underseepage

Abstract

Levee and floodwall seepage models based on two-dimensional (2D) conditions can underpredict landside vertical hydraulic gradients and uplift pressures due to excavations and convex bends. The Sherman Island levee system is used to calibrate a three-dimensional (3D) seepage model to evaluate the effect of finite landside excavations and convex levee bends on landside seepage. The model shows that a 3D analysis is required for a landside excavation with an aspect ratio (length to width) less than 1L:1.5W. For drainage canals and ditches that parallel a levee or floodwall and are wider than 15 m, gradients at the excavation center are essentially equal to 2D vertical gradients but greater than 2D gradients near the excavation sidewalls. The Sherman Island calibrated seepage model also confirms concave bends diverge seepage and yield lower vertical gradients than 2D models. Varying the degree of levee curvature (ω = 45°–100°) indicates that sharper convex bends (ω = 100°; axisymmetric radius, 150 m) cause vertical gradients that can be about 150% greater than 2D analyses.