Laboratory Experiments and Integrated Surface-Subsurface Hydrological Modeling to Evaluate a Permeable Pavement Performance

Permeable Pavements (PPs) are a type of Sustainable Drainage Systems that reduce runoff in urban areas and the related discharge to the drainage network, thereby decreasing the risk of flooding without the need of changing the end of use of the retrofitted areas. However, uncertainty affecting their performance and the lack of a comprehensive understanding of the physical processes governing their functionality are still current issues that limit their installation.

Several numerical models have been employed to describe hydraulic processes in PPs. Typically, these models focus solely on simulating flows through variably saturated porous media using 1D and 2D approaches. However, this approach neglects or oversimplifies surface processes and their interaction with subsurface flows. The water exchange at the surface-subsurface interface is intrinsically linked to the infiltration process in the underlying soil layer.  Surface runoff, when present, is influenced by the geometric and hydraulic properties of the surface as well as the local infiltration capacity.

These features can be adequately represented by Integrated Surface-Subsurface Hydrological models (ISSHMs) such as CATHY (Catchment Hydrology, Camporese et al., 2010), a spatially distributed and physically based model that jointly describes runoff and infiltration processes.

Here, the CATHY model has been used together with experiments developed in a lab facility to achieve a detailed understanding of the physical processes occurring in PPs.

The lab model was developed reproducing a 1:1 scale permeable parking lot section, 6 m long, 2 m wide and with thickness varying between 0.9 and 1 m (surface longitudinal slope of about 1.2%), enclosed within a 6×2 m² concrete box.

The CATHY model is used to simulate the hydraulic response of the PP subjected to artificial rainfall events generated through a rainfall simulator (Lora et al., 2016). No-flow boundary conditions are imposed at the bottom and lateral sides to reproduce the impermeable concrete walls surrounding the PP. A seepage face boundary condition is assumed downstream to simulate the subsurface flow through the porous wall on the downstream side of the facility.

Data regarding the water table evolution is continuously gathered through spatially distributed sensors (tensiometers and piezometers), along with surface runoff and subsurface discharge measurements collected at the downstream end via tipping bucket flow gauges. The dataset is used to calibrate the CATHY parameters, i.e., the hydraulic characteristics of the pavement tiles and of the aggregate materials forming the filter layer package. A first set of parameters are defined according to literature review and laboratory tests on the aggregate materials.

Despite difficulties encountered in the evaluation of the parameters, the calibrated ISSHM represents a useful tool to achieve a better understanding of the physical processes characterizing PPs.