Urban hardscape as it currently exists and as it is expanded through new development and perpetuated through maintenance, rehabilitation and reconstruction requires mining and importation into the urban area of large quantities of materials and exportation of demolition waste out of the urban area. Standards that require extensive vehicle parking for new housing and other development increases the amount of hardscape, and street standards can increase that further depending on requirements for street width. Siting of new quarries for the crushed gravel and sand that are the primary materials for urban hardscape is increasingly difficult near many urban areas due to concerns about emissions, dust and noise from truck traffic, as well as the depletion of nearby sources. While some parts of the country do have long-term quarries nearby, many parts of California have only a small fraction of their needs to 2050 permitted. Importation of hardscape materials can be fairly low impact where water transport has access into the urban area. Transportation impacts increase where rail and especially truck transportation is required. New quarries can often take up to 10 years to permit, and with decreasing probabilities of success in getting approval. Consequently, heavy quarry materials3 are transported from increasingly farther distances to urban areas, ironically increasing the emissions from the additional ton-miles of truck hauling and also damaging more of the road network. For example, 12.5 billion ton-miles or 9.7 percent of the 127.6 billion ton-miles of in-state cargo hauled on California roads in 2016 carried crushed gravel and natural sand,vertical rack of which an unknown but large percentage were used for hardscape .

This can be estimated to equate to approximately 1.25 million truck trips for urban hardscape other than state highways for the seven urbanized areas in California with populations over 1 million4 . Most of these urbanized areas are in federal non-attainment zones for particulate matter and air pollution . All of these numbers are highly conjectural because of lack of data, illustrating the gaps in information needed to calculate impacts of hauling hardscape materials for urbanized areas. At the same time, rehabilitation, renovation and reconstruction of cities requires the demolition of existing hardscape much of which must be hauled out of the urban area for recycling at the quarries or to landfills. Concrete vertical infrastructure is also useful for hardscape after processing, which has become more common as part of building site renovation. For example, in California 4.6 percent of ton-miles hauled on California roads in 2016 were waste and scrap of different types of which an unknown portion was demolition from urbanized areas. Using the same assumptions as for sand and gravel, except assuming that four percent of waste and demolition is from hardscape, this equates to approximately another 0.23 million truck trips. As noted previously, the recently reaffirmed California tax measure on fuel to be used for street and highway repair is expected to approximately double the amount of money spent, significantly increasing the ton-miles of hardscape material hauling as well. State highway pavements are primarily intended to carry heavy vehicles at high speeds. Most current urban hardscapes are scaled down versions of pavements used on state highways. There has been considerable work towards reducing cost and environmental impact by minimizing the quantities of virgin materials used over the life cycle on state highways. These improvements have come through improved construction quality to extend life, improved materials and structure design, reclamation and reprocessing of existing pavement materials both in-place and off-site, reductions in the use of the highest impact materials through substation of other materials, and development of new materials and pavement structures .

However, the hardscape in urbanized areas has not been evaluated as a system, nor considering the differences between urban areas and state highways of construction constraints, material sourcing locations, functional requirements, and interactions with other urban systems. Urban hardscape also plays a role in storm water flooding, groundwater recharge and water quality. Urban development using conventional impermeable hardscape decreases the permeability of flat surfaces to zero, and because hardscape makes up a large percentage of urban surface area, urban hydraulic patterns are dramatically changed. When urban hardscape receives rain, water that previously percolated through the surface, or was slowed by vegetation and rough soil before reaching surface streams and rivers instead flows rapidly over urban hardscapes into storm water conveyance and then streams and rivers. Figure 1 shows qualitatively how urban development increases the speed with which storm water enters the storm water conveyance system, and how it increases the peak flow that the storm water system must be built to carry, or in the case of existing infrastructure what it must be rebuilt or expanded to carry. In many areas this requires large investments in drainage infrastructure to minimize risk to lives and property.Data from the U.S. National Climate Assessment shows that the U.S. is getting warmer and receiving more frequent and heavier precipitation events and overall precipitation due to climate change which is reducing infrastructure performance and life, and causing increased flooding . During the past 50 years, the average temperature across the U.S. has risen more than 1.11 ºC while precipitation has increased by an average of about 5 percent. Some extreme weather events have also become frequent and intense which include heat waves, regional droughts, hurricanes and temporary changes in sea levels with storm surges . Such events pose risks and challenges for human and natural systems such as the transportation infrastructure.

Hence, anticipating and adapting to the effects of climate change is required. Information in the Fourth National Climate Change Assessment shows that the intensity of rainfall for the most extreme annual events, defined as the 99th percentile storm event, has increased across the country and particularly east of the Mississippi River, as can be seen in Figure 2. Climate change modeling indicates that these intensities are expected to continue to increase significantly as shown in Figure 3.Increasing the permeability of urban hardscape can decrease total storm water flow and peak intensity and increase lag time. Fully permeable pavement that includes infiltration into groundwater can reduce total storm water flow. Such pavements can be designed and constructed to handle heavy truck loads,vertical farming hydroponic and can be used for parking areas, pedestrian and bicycle paths, and sidewalks and other urban hardscape applications. Fully permeable pavements can also be used to significantly slow storm water runoff where the native soils below are not sufficiently permeable for infiltration or there are other reasons to not infiltrate the water. The full life cycle material flows and impacts of fully permeable pavement relative to conventional pavement have not been sufficiently analyzed to make broad comparisons with conventional impermeable pavement . Partially permeable pavements involve using permeable materials for the surface of otherwise impermeable pavement, causing storm water to flow more slowly below the surface as it moves to the storm water conveyance system. Life cycle assessments of partially permeable surfaces show that they do not last as long as impermeable surfaces, with 25 to 50 percent shorter functional lives. However, impermeable surfaces are usually thin . In addition to flood control benefits, both fully permeable and partially permeable pavements offer important storm water runoff quality benefits by capturing significant amounts of pollutants in the pavement. Research indicates that the durability of permeable pavements can be improved through additional research and development. Both fully permeable and partially permeable pavements are not widely used in the U.S., primarily due to institutional issues between transportation, storm water quality and flood control agencies as well as gaps in the technical information available regarding performance and life cycle costs and impacts. There is also a need to consider reductions in storm water quality and flood control benefits when comparing permeable pavements with conventional impermeable pavements . Urban metabolism is a method for quantifying consumption and use patterns in urban environments and has typically been applied to accounting of energy and materials inputs and outputs for cities. The term “metabolism” is used because this method uses the analogy of living organisms to understand the consumption of resources and materials, growth, and waste generation. The primary approach for urban metabolism proposed by this white paper is the use of material flow analysis . In MFA, the material flows into and out of the urban area are inventoried, and changes in material, including degradation from burning which changes the mass and produces emissions and heat energy, or other changes are also tracked through mass balance analysis. The impacts of the material flows can be assessed in terms of energy flows divided into different sources of energy where possible or considered qualitatively in terms of the masses being moved and the distances they travel and the types of materials. In addition to material and resource flow accounting, UM can be expanded in ways that allow more comprehensive and integrated assessments of the patterns and processes of urban systems.

Life cycle assessment principles can be used to help define system boundaries and time horizons to MFA and can also be used to help quantify the impacts of the results of MFA. LCA is a standardized methodology that can be used to quantify the resource use, energy consumption, and air, water and land borne emissions of a system. Once quantified, the results can be used to identify strategies for reducing emissions and finite resource use. The strength of this methodology is that the systems analysis is done over the life of the system in a cradle to grave approach. The International Organization for Standardization standards 14040 and 14044 describe the process of performing an LCA for a product, system or a process . The process is comprised of four phases, namely goal and scope definition, inventory analysis, impact assessment and interpretation. The problem statement is defined in the goal and scope phase of an LCA and system boundaries and functional description of the system are set ensuring consistency in performing the LCA. The environmental inputs and outputs that are associated with the product or system are identified and quantified in the inventory analysis phase. The system related outputs are translated into environmental impacts in the life cycle impact assessment phase. In the interpretation phase, results and conclusions are to answer the questions posed in the goal and scope phase. Life cycle thinking refers to conceptual models that require consideration of systems over their life span and may also require consideration of the supply chains they rely on. Life cycle thinking approaches such as life cycle cost analysis , environmental life cycle assessment and social life cycle assessment can be used to evaluate life cycle costs, environmental impacts and socio-economic impacts of existing and new large systems or processes, respectively. Due to an increase of migration from rural areas to urban areas, the energy and resource demand of the urban areas is rising, and so is the consequent pollution, referred to as the “urban metabolism disorder” . Application of life cycle thinking to cities at the urban area scale combined with the urban metabolism perspective offers the potential for insights into improving the sustainability of urban areas and the quality of life of their residents. The proposed approach makes use of UM, MFA and LCA to model and evaluate current practice and alternatives for urban hardscape that may reduce important impacts on urban quality of life and the environment. Figure 4 provides a conceptual view of the proposed approach. The principles of UM and MFA would be used to define the urban surface area boundary and define the hardscape within the boundary by function. The current typical hardscape structures and materials would be identified, as well as typical life cycles including construction, maintenance, rehabilitation and reconstruction or demolition. As noted previously, at this time it is difficult to separate materials flows for hardscape materials moving into urban areas and hardscape demolition moving out of those areas from aggregated values that are available. The urban hardscape flows into and out of urban boundary would be calculated and checked with other data such as materials transport and economic data. The flows would be summarized over a future life cycle analysis period using LCA principles to be able to consider all stages of the hardscape life cycle, including material production, construction, maintenance and rehabilitation, and end-of-life.