The economic effect of green roofs on non-point pollutant sources management using the replacement cost approach

A green roof is one of the sustainable management alternatives for mitigating non-point source discharges which have increased along with expanding impervious areas due to urbanization. Although technically viable, it is also necessary to assess its validity by economic analysis because green roof projects typically require a substantial amount of budget. Four scenarios were established to (1) analyze the effectiveness of green roofs for alleviating non-point source pollution; (2) present the criteria of economic analysis; and (3) estimate benefits by using the replacement cost approach. The Cheonggyecheon watershed in Seoul, Korea is selected as a study area because of its large impervious surface, and XP-SWMM is used for rainfall-runoff and water quality simulations. Our analyses showed that 817,884 kg/yr, 683,781 kg/yr, 452,758 kg/yr, and 356,523 kg/yr of Biochemical Oxygen Demand (BOD) loading can be reduced in the four scenarios (S-1 > 65 m²; S-2 > 100 m²; S-3 > 200 m²; S-4 > 300 m²) based on building roof area, respectively. And the benefits for 30 years resulting from those scenarios were estimated to be United States Dollars (USD) 257.3 million, USD 228.8 million, USD 174.6 million, and USD 149.2 million with currency exchange rate 1USD = 1,182 KRW. We expect that this study will contribute to a more economically accurate assessment for the validity of green roof projects to sustain environmental health in urban areas.