The Effect of Intraparticle Porosity and Interparticle Voids on the Hydraulic Properties of Soilless Media

An essential component of a building-integrated vegetation system, such as an extensive green roof, is the layer of lightweight planting medium that supports rooting and stores water. Predicting and describing the stormwater management performance of green roofs requires reliable data regarding the water retention properties of the planting medium. Ten materials proposed for use on green roofs, including four mineral components, three biological components, and three commercial blends, were characterized through measurement of their water release curves (WRCs). In combination with the particle size distributions, the resultant data demonstrate that some of the materials contain measurable intraparticle pore networks in addition to the interparticle void spaces described in classical soil hydrology. The WRCs were also used to model the maximum water storage under static equilibrium conditions throughout a 15-cm profile of each material. In freely draining, unsaturated green roof systems, the role of the intraparticle pores may be limited to increasing microscale roughness of particle surfaces, thereby reducing film flow under drier conditions. The highly organic, biologically derived materials-screened compost, bark fines, and shredded wood-demonstrated hydrophobicity when air dried, but wetting occurred within <30 min on all occasions, which would be within the time frame of many rainstorms. As with natural soils, the saturated hydraulic conductivity was lower in materials with a higher proportion of fines (<106 mu m).