Unsaturated Flow Functions for Filter Media Used in Low-Impact Development-Stormwater Management Systems

Moisture retention relationships for coarse, high-infiltration soils are difficult to empirically determine and estimate. Present day software models for stormwater management (SWM) that are used as sizing and performance prediction tools for filtration Low Impact Development-Stormwater Management (LID-SWM) systems typically assume that these systems function under saturated flow conditions. This directly impacts prediction of system drainage and hydrographs, as well as the estimates of physically-based water quality improvement. Yet real-time monitoring of these systems demonstrated that saturation of the filter media is rarely achieved. This article presents a framework for obtaining the moisture retention curves (MRC) and relative hydraulic conductivity K-r(theta) function for engineered filter media and other hydraulic control soils used in four LID-SWM systems: pervious pavement, sand filter, subsurface gravel wetland, and bioretention. These functions needed in routing water through the filter media with unsaturated flow functions are developed from easily measurable soil properties like porosity and particle size distribution, and can be integrated in current available stormwater design software. The framework consists of a sequence of physically based equations: Arya-Paris for the theta(psi) function, Bower for gravel content adjustments along with an extension of the theta(psi) function proposed in this article, and Mualem for the K-r(theta) function. This sequence is combined with the Van-Genuchten fitting equation for soils with irregular particle size distributions. (C) 2014 American Society of Civil Engineers.