Assessing Physical Processes of Permeable Pavements with a Large-Scale Laboratory Model

In recent decades, due to on-going urbanization and changes in rainfall patterns, urban drainage systems are facing increasing challenges. The expansion of impermeable surfaces and the increase of both frequency and intensity of rainfall events, are responsible for the augmented peak-flows and heavily polluted stormwater volumes conveyed by combined sewer overflows to water bodies. The need of assessing these challenges to mitigate the impact on water bodies’ quality has prompted International Authorities to develop standards and scientific communities to find solutions for an effective stormwater management.

Sustainable Drainage Systems are effective at-source stormwater management solutions designed for collecting, retaining, and infiltrating direct rainfall and runoff from impervious surfaces. When properly applied in the urban drainage system, they mitigate pollution coming from wash-off of impervious surfaces and reduce both volumes and flood peaks conveyed to the drainage system.

Among others, Permeable pavements (PPs) and infiltration trenches (ITs) are two solutions that can be easily retrofitted into the urban environment. PPs reduce surface runoff allowing direct infiltration of rainfall, whereas ITs collect runoff from nearby impervious surfaces. Both can temporally store relevant amount of water which is then slowly released to deeper native soil layers. Moreover, these systems act as filters trapping solids and pollutants onto or into the filter layers. However, physical clogging related to particle accumulation on the surface or inside the porous media reduce permeability of the system decreasing infiltration rates along time. This is a crucial aspect affecting both PPs and ITs effectiveness that must be accounted in the urban environment maintenance plans.

A large-scale laboratory model is currently under development to analyze the main physical processes and to assess the efficiency starting first from the PPs. To this aim, a laboratory facility (Lora et al., 2016), built in the Laboratory of Hydraulics and Hydraulic Works of the Department of Civil, Environmental and Architectural Engineering (University of Padova), is being rearranged. The facility consists of a reinforced concrete box 6 m long x 2 m wide, and the height varies from 3.5 to 0.5 m. It is equipped with 50 openings on each lateral side for the insertion of probes (e.g. water content reflectometers - WCR) to continuously collect long term monitoring data in different positions. The end side of the facility is made of porous bricks allowing subsurface runoff to drain into a V-notch stream gauge. Another stream gauge is installed to measure exceeding surface runoff. During experiments, steady rainfall intensities ranging from 50 to 150 mm/h will be produced with a specifically designed rainfall simulator.

Suitable materials for the filter layers package will be laid for 1 m total depth assessing filtration processes through the probes in three positions along the vertical. The rainfall simulator will be rearranged to guarantee uniform rainfall distribution on the PP surface characterized by a mild slope (about 2-3%).

In the first set of experiments, the characteristics of the investigated PP will be tested in clear water condition, thus without adding suspended solids, to define the maximum infiltration capacity.