Liquid Migration Through Defects in a Geomembrane Overlain and Underlain by Permeable Media Available to Purchase

An analytical method is proposed to evaluate the rate of advective liquid migration through a defect in a geomembrane that is overlain by a permeable medium and underlain by a highly permeable medium (i.e. a medium significantly more permeable than the overlying medium). The rate of liquid migration calculated using the proposed analytical method approaches the rate calculated using the classical Bernoulli's equation for free flow through an orifice if the hydraulic conductivity of the medium overlying the geomembrane tends toward infinity. Graphs for typical sizes of geomembrane defects show that the rate of liquid migration calculated using the proposed method is always less than that calculated using Bernoulli's equation, and sometimes significantly less (when the liquid head is small and/or the hydraulic conductivity of the permeable medium overlying the geomembrane is small). For a given hydraulic conductivity of the overlying permeable medium, as the liquid head on top of the geomembrane increases, the rate of liquid migration calculated using the proposed method approaches that calculated using Bernoulli's equation. In the case of landfills, the paper shows how the proposed method can be combined with an equation giving the head of leachate on top of the geomembrane liner. The resulting equation makes it possible to select an optimum leachate collection material to minimize leachate migration through defects in the geomembrane liner. Design examples are presented to illustrate the analytical method.