Discussion

Optimizing the Urban Fabric Design

Table 22.5 shows the GWP of two street profiles, which were composed considering the best and worst designs identified in Sect. 3. The most environmentally-friendly design option for the street profile resulted in a carbon footprint of 1.1 tones of CO2 eq. The best design included concrete sidewalks, asphalt pavements, HDPE gas pipes, PVC, water pipes and concrete sewer pipe with CP1 trench. In general, the environmental impacts were reduced by 23 % with respect to the worst street profile. The use of granite in sidewalks, LDPE in water supply pipes and HDPE in the sewer system with PP2 trenches were the least recommended design options due to the increase in the GWP.

Table 22.5 Global warming potential (GWP) (kg of CO2 eq.) of the best and worst designs of a 1 × 8 m street section in a time frame of 50 years

When focusing on the different sub-systems, the asphalt pavement had the greatest contribution to the total GWP because the light-weight traffic road had the largest surface in the street profile (6 m2). Nevertheless, there are two elements that presented the greatest room for improvement. In both scenarios, the concrete sidewalk accounted for almost 30 % of the impacts. Generally, this material is widely applied to pedestrian sidewalks, although asphalt is also used given its low initial cost. Thus, the elements of the paved skins contributed the most to the urban fabric environmental profile, due to a more intensive maintenance of these exposed areas. However, this might depend on the traffic density, the type of mobility and the land use (e.g., pedestrian areas).

The second element that presented variations in the best and worst designs was the sewer network. In this case, the lifespan (i.e., required reposition) also played an important role: when concrete pipes (lifespan: 100 years) were included in the best scenario, they accounted for 10 % of the impacts; in contrast, the contribution of the sewer increased up to 23.3 % when plastic pipes were considered (lifespan: 50 years). Hence, the integration of life-cycle environmental data and service-life planning information is essential for urban planners for identifying long-term environmentally friendly constructive solutions (Mendoza et al. 2012b). Finally, it was detected that the installation of subterranean pipelines had a great contribution to the total GWP and this trend could be extrapolated to other subterranean systems such as telecommunication and electricity networks.

Another key issue is the variation in the street configuration. This study presented a standard street profile, which consisted of common pipe diameters and trench designs. Nonetheless, the population density and the configuration of the city might demand larger pipe diameters because of a more intensive gas and water consumption and wastewater production. Therefore, the contribution of these elements to the total impact of the urban skin might increase. As a result, urban planners must focus on the possible material and installation alternatives that best suit their case studies (Petit-Boix et al. 2014).

Towards Smart Grids and Self-sufficiency

Because networks present a relevant environmental contribution to the impacts of the street, the cities of the future should consider approaching smart grids and selfsufficiency. In this sense, decentralization of urban services is essential to improve the environmental performance of cities. This approach aims to reduce the required networks to supply services while increasing the independence of individual neighborhoods and buildings. From an environmental point of view, this would contribute to reducing the environmental burdens of the subterranean profile of cities. This strategy is particularly meaningful in low-density settlements where longer networks are installed. In the case of gas distribution, when an isolated house requires more than 69 m of neighborhood network, the installation of individual propane tanks becomes favorable in terms of GWP (Oliver-Solà et al. 2009a). Furthermore, the gas supply through district heating systems can be considered as an alternative at the neighborhood scale (Oliver-Solà et al. 2009b).

In the framework of decentralization, self-supply of endogenous resources such as water or energy contributes to the environmental improvement of urban areas. In this case, besides being independent of the central network, local and renewable resources substitute the consumption of nonrenewable ones. For instance, rainwater harvesting systems are increasingly implemented for nonpotable purposes and play a key role in countries dealing with water scarcity. In addition, wastewater recycling can also be integrated in the water metabolism of buildings.