Multi-level assessment of the environmental benefits of a permeable pavement: numerical analysis and experimental investigations

Abstract

The increasing frequency of flooding events in urban catchments related to an increase in impervious surfaces highlights the inadequacy of traditional urban drainage systems whose aim is to rapidly collect and convey overland flows to the treatment plants. Recently, scientific community has focused its attention on Low-impact developments (LIDs) techniques that have proven to be valuable alternatives for stormwater management and hydrological restoration, by reducing stormwater runoff by reproducing natural hydrological processes in urban areas. However, the lack of diffusion of adequate modelling tools represents a barrier in designing and constructing such systems. In general, Permeable Pavement (PP) represents a good solution to solve stormwater management problems both in quantitative and qualitative way. This thesis focused on assessing the hydraulic behaviour and water quality performance of permeable pavements based on laboratory experiments and developing a modelling approach for the water flow in order to assisting engineers and researchers in the design of these systems. In this way, an adequate hydrological description of water flow in the pavement system relies heavily on the knowledge of the unsaturated hydraulic properties of the construction materials. Although several modelling tools and many laboratory methods already exist in the literature to determine the hydraulic properties of soils, the importance of an accurate description of hydraulic properties of materials used in the permeable pavement, is increasingly recognized in the fields of urban hydrology. Thus, the aim of this study is to propose techniques/procedures on how to interpret water flow through the structural system using the HYDRUS model. The overall analysis includes experimental and mathematical procedures for model calibration and validation to assess the suitability of the HYDRUS-2D model to interpret the hydraulic behaviour of a lab-scale permeable pavement system. The system consists of three porous materials: a wear layer of porous concrete blocks, a bedding layers of fine gravel, and a sub-base layer of coarse gravel. The water regime in this system, i.e. outflow at the bottom and water contents in the middle of the bedding layer, was monitored during ten irrigation events of various durations and intensities. The hydraulic properties of porous concrete blocks and fine gravel described by the van Genuchten functions were measured using the clay tank and the multistep outflow experiments, respectively. Coarse gravel properties were set at literature values. In addition, some of the parameters (Ks of the concrete blocks layer, and α, n and Ks of the bedding layer) were optimized with the HYDRUS-2D model from water fluxes and soil water contents measured during irrigation events. The measured and modelled hydrographs were compared using the Nash-Sutcliffe efficiency (NSE) index (varied between 0.95 and 0.99) while the coefficient of determination R2 was used to assess the measured water content versus the modelled water content in the bedding layer (R2= 0.81÷0.87). The parameters were validated using the remaining sets of measurements resulting in NSE values greater than 0.90 (0.91÷0.99) and R2 between 0.63 and 0.91. Results have confirmed the applicability of HYDRUS-2D to describe correctly the hydraulic behaviour of the lab-scale system. Water quality performance aimed to improve the knowledge of the system to remove heavy metals (Copper and Zinc) from stormwater runoff. It was assessed by using batch and contaminant flow experiments. Batch experiments were conducted on each construction material of the PP and highlighted that, among the pavement materials tested, only concrete blocks had the potential to adsorb the heavy metals investigated. Results shown that the adsorption capacity of the porous concrete is higher in adsorbing Cu (70% ÷ 90%) than Zn (69% ÷ 75%). Flow contaminant experiment were performed under different inflow concentrations. Results show that removal rates of Cu and Zn of the lab-scale pavements range from 85% to 92% and from 65% to 82%, respectively