Unsaturated 2D modelling of subsurface water flow in the coarse-grained porous matrix of a green roof

The analysis of the interactions between hydrologic processes and green roof installations in the urban environment requires to improve the understanding of the unsaturated water flow in the coarse-grained porous media usually employed in such technological solutions. In order to base our research on experimental evidences, a monitoring program was carried out at the green roof experimental site of the University of Genoa (Italy). A continuous acquisition system is available at the experimental site for rainfall intensity and subsurface water flow measurements at 1 min resolution. Soil water content is also measured at the same resolution in time by four TDR probes located along a vertical profile. The SWMS_2D model, based on Richards’ law and the Van Genuchten–Mualem functions, is here applied to simulate the variably saturated flow within the green roof system. The model was calibrated and validated using rainfall–runoff events observed at the experimental site. The calibration and validation strategy involved comparing predicted and measured outflow hydrographs. The model adequately reproduces the hydrographs, as demonstrated by the limited relative percentage deviations obtained for the total discharged volume, the peak flow and the hydrograph centroid. Furthermore the predicted water content closely matches the observed one at various depths along the vertical profile where measurements are available, thus confirming that the model correctly describes the variably saturated flow field within the green roof. The mechanistic model, based on a single porosity approach, here employed to describe the variably saturated flow within the thin stratigraphy, is demonstrated to suitably describe the hydrologic performance of a green roof, which is typically realized with high hydraulic conductivity and coarse-grained porous media.