A sustainability comparison of green infrastructure interventions using emergy evaluation

Green infrastructure (GI) has been touted as a more economically, socially, and environmentally sustainable option for urban stormwater management than more traditional gray infrastructure solutions. There are many types of GI, however, and there has been limited comparison of sustainability between types. This study examines the benefits and detriments of current GI designs (rain gardens, green roofs, porous pavements, and tree plantings) using emergy analysis, an environmental accounting system that evaluates the direct and indirect energy inputs used in the production of goods or services. Results of the analysis revealed inherent differences between the four GI typologies examined and identified system inputs that dominate emergy flows, and thus sustainability outcomes. Porous pavements performed the worst when evaluated using standard emergy-based environmental sustainability indices and the best when using economic indices. Indices calculated for green roofs and tree plantings indicated that these types of green infrastructure might inherently be more environmentally sustainable. Emergy inputs of stone and soil were dominant inputs for all systems, as was the emergy cost of disposal of excavated materials. Porous asphalt was a high emergy input for the porous pavement projects examined. Labor and equipment inputs were high for most projects, but were overshadowed by stone and soil inputs. These dominant emergy inputs show areas where efficiency of designs could be improved by practices such as recycling excavated sediments or utilizing construction materials that are less emergy intensive. In addition, the results of this study show that not all GI projects are created equally. Urban planners and other decision makers can use this information to improve standard design and implementation practices for GI projects.