Infiltration Models for Engineered Media in Living Roofs and Bioretention

Abstract

Engineered media used in living roofs and bioretention plays a crucial role in determining stormwater treatment performance and sustaining plant growth. This research focuses on (1) the hydraulic properties and pore size distributions of engineered media with varying aggregates, compost ratios, and amendments, and (2) the application of the classic Richards equation with commonly-used hydraulic functions for modeling one-dimensional infiltration process in unsaturated engineered media. The hydraulic properties, including saturated hydraulic conductivity (Ks), water retention characteristic (WRC) and unsaturated hydraulic conductivity (K(θ)), were measured according to standard methods for 14 engineered media classified as poorly-graded sand. Specifically, there were constant- or falling head experiments for Ks, tension plate combined with pressure plate methods for WRC, and an instantaneous profile combined with steady state, steady-rate methods for K(θ). Based on well fitted WRC functions, namely the van Genuchten and Durner functions, continuous uni- and bimodal pore size distributions for tested media were derived. The modeling of the infiltration processes of seven engineered media (either 10-cm or 100-cm depth) subjected to simulated large inflows was realized in the HYDRUS-1D software, which can numerically solve the Richards equation for uniform flow modeling as well as offers a mobile-immobile model for preferential flow modeling. The magnitudes of Ks values range 10-2 to 10-1 cm/s. The measured WRC data show that four media with a marine sand exhibit a unimodal trend while ten media with pumice aggregates display a bimodal trend. The decrease in K is a thousand-fold for media with unimodal pore size distributions and four orders of magnitude for most media with bimodal pore size distributions between the saturated and low water contents. The majority of the pores in tested media have radii less than 1 mm. Modeling results indicate that uniform flow exists in most media, involving both uni- and bimodal pore size distributions. Preferential flow is also found in the two media with bimodal pore size distributions. The Richards equation, with well-fitted WRC function and predictive K function based on Mualem’s approach, can predict outflows from media reasonably well for uniform flow conditions. The occurrence of preferential flow is most likely due to the interaction between the bimodal pore structure and the initial water content. Further investigation is needed for the effect of plants on media properties and infiltration process.