Permeable pavement made in porous asphalt: A sustainable solution to reduce floods impacts

Urban growth and infrastructure development in recent decades have led to a significant increase in the amount of impermeable soil in urban areas. This is attributed mainly to the construction of roads and buildings, resulting in the reduction of permeable surfaces, such as grasslands and natural soils, replaced by impermeable surfaces covered by asphalt and concrete. Urbanisation has a profound impact on the natural water cycle, in particular by changing management of rainfall. In a natural environment, rainwater is absorbed by the soil, recharging groundwater and maintaining the ecological balance. However, the expansion of urban areas and the proliferation of impervious surfaces disrupt this balance and cause other problems, such as the increasing of the risk of hydroplaning and flooding, and the concentration of pollutants.Hydroplaning (also called aquaplaning) occurs when the amount of water on the road exceeds the drainage capacity and causes a loss of traction between the vehicle tyres and the road surface. This can lead to dangerous situations, particularly during heavy rain or storms, increasing the risk of road accidents.Urban flooding is often a consequence of water accumulation due to loss of surface permeability. In fact, the drainage problems can cause water to pool in streets and low-lying areas, potentially causing damage to property and infrastructures.The surface runoff also causes an environmental problem in terms of leaching of the pollutants deposited on the pavement. In fact, the vehicular traffic, the smoke release from industrial plants and other production activities results in the release of pollutants that are finally deposited on the pavement and then transported by runoff to the sewer. The main pollutants deposited on the roads include total suspended solids, heavy metals (such as lead, copper, cadmium, chromium, and zinc), organic pollutants expressed as Chemical Oxygen Demand (COD), nitrogen and hydrocarbons.The emergence of these problems has led to the development of various solutions to convert a 'traditional city' into an environmentally sustainable city. The main solutions involve the construction of drainage systems that return the pavement to a state of permeability while, at the same time, it performs its function. These solutions have received different names such as: Sustainable Drainage System (SuDS), Low Impact Development (LID) or Best Management Practices (BMP), Water Sensitive Urban Design (WSUD). Permeable pavements (PPs) are a widely used LID in both urban and highway areas. They are manufactured like a traditional pavement, but with higher porosity and interconnectivity of voids. This increases their permeability and, therefore, their ability to be infiltrated by rainfall, but at some sacrifice of mechanical strength.PPs can be permeable in the surface course only or in all their courses. The former, also known as open-graded friction course (OGFC), consists of Porous Asphalt (PA) layer used to reduce the risk of hydroplaning. The second type collects and temporarily stores water in the deeper course, known as the storage layer, before water is released into the underlying ground or is discharged by drainage pipes to sewer or reservoirs for reuse. The surface layer can be made of PA, Pervious Concrete (PC), Grid Pavement (GC) or Permeable Interlocking Concrete Pavement (PICP); in all these four cases, significant infiltration is provided, resulting in a reduction of pollutants in the runoff.A practical way for modelling and sizing PP is the use of software such as the EPA SWMM (Storm Water Management Model) produced by the Environmental Protection Agency (EPA). This is a one-dimensional software used to model urban drainage networks. Recent versions have added tools for LID modelling and water quality assessment, noticeably improving its potential.The present work aims to investigate the properties of PA: in particular, it focuses on the effect of macrotexture as well as on the influence of roughness on superficial runoff, and on pollutant release/removal. All these aspects are typically investigated in different applied engineering disciplines, but they relate to each other and, indeed, they all provide a contribution to overall performance of the pavement, with a specific focus on its hydraulic performance and the consequent water quality. The work is organised as follows. Chapter 1 discusses flooding and hydroplaning issues, as well as the state of the art of PPs and the main methods for surface texture evaluation. Chapter 2 describes the EPA SWMM software and explains in detail how to carry out appropriate modelling and simulation of PPs. Then, Chapters 3, 4 and 5 focus on porous asphalt, each chapter dealing with a different experimental characterization, to gain a better understanding of the effectiveness of its use. Chapter 3 deals with strength testing by measuring stress, and hydraulic testing in a flume whose bottom was covered by asphalt slabs, for assessing resistance to flow and surface roughness. Chapter 4 deals with numerical simulation of two case studies where PPs are considered. A case study is a car parking inside the Palermo University campus, for which two different combinations of PPs are considered. The second case study concerns the whole Palermo University campus. The analysis of the results allows interesting observations on the use of PPs and their effectiveness in mitigating runoff and flooding. Finally, Chapter 5, instead, concerns water quality analysis of water passing through asphalt slabs.