Degree of Turbulence of Flow in Geosynthetic and Granular Drains

Abstract

Flow in porous media, such as geosynthetic and granular drains, is often nonlaminar. Nonlaminar flow can vary in character from semiturbulent to turbulent, depending upon the flow velocity, which is related to the hydraulic gradient. An exponent on the hydraulic gradient is used in the relationship among the hydraulic gradient, the apparent flow velocity, and the saturated hydraulic conductivity or transmissivity to quantify the degree of turbulence of the flow in porous media. Numerical values of this exponent are established for both granular and geosynthetic drains based on an analysis of published and unpublished experimental data. It appears that for typical hydraulic gradients encountered in practice, the degree of turbulence for flow in granular drains depends essentially on the particle size, with flow through sand drainage layers being laminar and flow through gravel drainage layers being nonlaminar. In geosynthetic drains, the degree of turbulence depends on the applied compressive stress and the material in contact with the drain.