*3.3. Life Cycle Cost Analysis*

There is little published data on the life cycle cost analysis (LCCA) of permeable pavements that include actual costs and performance. Most studies are limited to comparative initial cost analyses for permeable pavements compared to conventional pavements, which indicates that the cost of permeable pavements is greater than the cost of conventional pavements; however, some studies indicate that the initial total costs are similar or lower because permeable pavements do not require stormwater drainage systems [19].

According to Mei et al. [56], rulers face the increasingly difficult task of planning water managemen<sup>t</sup> systems in urban areas, especially in relation to uncertainties of climate and socioeconomic changes, which requires decision-makers to plan the water managemen<sup>t</sup> infrastructure from economic and adaptation points of view. For a specific area, considering draining a region, several green infrastructure options are possible within the scope of planning. However, the systems have different impacts and hydrological costs, making assessments necessary to integrate the sustainability and cost-benefit of these systems.

Wang et al. [19] conducted a life cycle cost analysis to understand the cost implications of building and maintaining permeable pavements. The input data for the models were obtained from laboratory research and computer performance modelling. A detailed life cycle assessment could not be performed due to insufficient available data on the construction, long-term performance, maintenance and salvage value of permeable pavements and alternative Best Management Practices (BMPs) currently used for stormwater management. Two scenarios were considered in the study: a shoulder retrofit of a high-speed highway, and a low-speed highway or parking lot/maintenance yard. Both scenarios compared conventional pavements with conventional treatment BMP versus the use of permeable pavements. The results indicate that permeable pavements are potentially more cost effective than currently available BMP technologies. These results were used to prepare preliminary paving projects for pilot studies of permeable pavement in California and to identify under what conditions they are appropriate for use. Although a more comprehensive life cycle assessment should be undertaken after the completion of the pilot studies.

Kluck et al. [57] point out that, in Holland, pavements with permeable surface are used in order to reduce noise produced by the traffic. However, permeable pavements have a shorter lifespan than traditional pavements, causing frequent maintenance and supposedly increasing costs and thus causing economic and environmental damage. The study conducted by the authors aimed to replace the traditional binder used in the permeable pavement by synthetic binders in order to increase the lifespan of the system. Considering the net present value of the investment, it was concluded that the permeable pavement produced with the synthetic binders costs the same as the conventional pavement, but with a life cycle up to ten times greater, which brings environmental and economic benefits for the drainage system of the Dutch urban areas.

The economic benefits of permeable pavements can be appreciated when life cycle cost analysis is performed. However, due to the lack of large-scale testing, long-term performance data, and construction and maintenance cost data, life cycle cost analysis has been difficult to perform, requiring several assumptions. Wang et al. [19] compared permeable pavement systems with conventional stormwater managemen<sup>t</sup> systems used at the road shoulders. Permeable pavements reduced life cycle costs by up to 30%. In another study, conducted by Terhell et al. [58], based on data obtained from several agencies, it was found that permeable pavements can save up to US\$64,649, considering installation costs, and US\$3,788,856 considering stormwater treatment benefits over 25 years for 1/2 acre area compared to conventional pavement. The reduction in the cost of construction is attributed mainly to the fact that permeable pavements do not require side drains, overlays and so on.

To compare flood control efficacy and cost-benefit of green infrastructures, Mei et al. [56] evaluated the implementation of permeable pavements, green roofs, wetlands and bioretention basins in China. The increasing order of effectiveness of flood control was: green roof, permeable pavement, wetland and bioretention basin. This sequence is related both to the characteristics of the study area and to the properties of the specific practices of the green infrastructures. Implementation of the combination of the four practices would result in a peak flow reduction of 80.62%. The study also contemplated the life cycle cost of the systems, considering the phases of design, planning, construction, operation and benefits brought by the strategies. The increasing order of life cycle cost was wetland (US\$31.72/m2), permeable pavement (US\$98.48/m2), bioretention basin (US\$186.90/m2), and green roof (US\$317.10/m2). As a conclusion, it was found that the combination of permeable pavements with bioretention basins and wetlands is recommended as the best strategy for flood control and cost-benefit for the study site.

Chui et al. [59] verified that the life cycle cost of drainage systems depends on the place where they are implemented, and in the case studied, the life cycle cost of the systems were lower in the city of Hong Kong (China) when compared to Seattle (USA). The effective costs for the reduction of runoff were 0.02 L/10<sup>3</sup> US\$, 0.15 L/10<sup>3</sup> US\$, and 0.93 L/10<sup>3</sup> US\$, for green roof systems, bioretention basin and permeable pavement in the city of Hong Kong, while in the city of Seattle, the figures were 0.03 L/10<sup>3</sup> US\$, 0.29 L/10<sup>3</sup> US\$ and 1.58 L/10<sup>3</sup> US\$, respectively. It is noted that the results found by Chui et al. [59] show an opposite cost-benefit order when compared to the study published by Mei et al. [56]. Chui et al. [59] concluded that the relation between the reduction of the stormwater runoff and the cost of the permeable pavement forms an "S" curve; that is, the permeable pavement ideal design tends to have a smaller area and a thinner pavement surface. However, for more intense rainfall events, it is cheaper to expand the surface than to increase depth. The permeable pavement obtained the best cost-benefit for the reduction of the runoff between the three structures studied. Therefore, this type of pavement is recommended for places where stormwater managemen<sup>t</sup> is the main objective.
