ABSTRACT
Subsurface flow wetlands are designed to provide contact between water, roots, and substrate. This type of system is variously called underground, gravel bed, or rock bed wetland, or rock reed filter. Spot determinations of water velocity in the gravel were made at several locations. Coarse and fine rocks were placed in large diameter pipe and the hydraulic conductivity measured via pressure drop and flow rate. The fine gravel gave 50,000 ft/d; the coarse gravel 210,000 ft/d. Correlations of the hydraulic conductivity with rock and bed properties exist in several forms. These predict the effect of reduced porosity and rock size on the hydraulic conductivity. These include the Ergun equation, friction factor methods, and gravel-specific correlations. On a more speculative level, there is an interesting observation to be made concerning flow distribution. In packed beds confined by vessel walls —an extremely common device in the chemical process industries —there are only minor deviations from plug flow behavior in long, slender geometries.
