Dual porosity effects in hydrological performance of extensive green roofs

Artificial substrates for green infrastructure have different composition and properties compared to natural soils. The admixture of light porous minerals such as pumice or expanded clay is often used to decrease the substrate weight and to increase water storage capacity. On the other hand, it could lead to dual porosity character of substrates and may affect their retention properties.

The dual-continuum model S1D is used to asses water flow in extensive green roof test beds with artificial substrate. The model numerically solves dual set of Richards’ equations. The soil hydraulic properties are described using van Genuchten-Mualem approach. Selected model parameters were optimized using Levenberg-Marquardt algorithm.

Two green roof test beds located at the University Centre for Energy Efficient Buildings of the Czech Technical University in Prague are studied. The test beds are filled with 60 mm of extensive green roof substrate, planted with sedum cuttings, respectively 40 mm of substrate, planted with sedum carpet. The substrate is a mixture of spongilit (55 %), crushed expanded clay (30 %) and peat (15 %). The outflow from the test beds is registered by tipping bucket flowmeter and the moisture content within the soil substrate by TDR probes. The test bed with sedum carpet is also weighted. For complete hydrometeorological characterization, data from the nearby meteorological station are available.

Dual-continuum model provides higher flexibility and overall better agreement between measured and simulated variables. Further investigation of hydrological regime of such substrates and possible hysteresis of their soil water retention curve is needed.

The research was supported by the Czech Science Foundation under project number No. 20-00788S. Experimental work has been supported by the Ministry of Education, Youth and Sports within National Sustainability Programme I, project number LO1605.