Evaluating the life cycle net benefit of low impact development in a city

- Combines empirical data with local survey results for quantification and evaluation.
- Covers economic, environmental, and social benefits, and different LID practices.
- Positive life-cycle net benefit obtained with median social benefit in Hong Kong.
- Result is sensitive to peoples' willingness to pay and land costs of green roofs.
- Quantification and evaluation framework applicable to other cities worldwide.

Low impact development (LID) practices (e.g., green roofs, bioretention systems, and porous pavements) offer multiple benefits to urban eco-systems. They reduce the expenses associated with water treatment, grey infrastructure, and energy consumption and thus generate economic benefits. They also benefit the environment by mitigating air pollution and climate change, and they provide social benefits such as enhancing livability, urban green space, and educating and improving the health of the public. Many studies have attempted to calculate the monetary value of these benefits. However, few have considered all three types of benefits (i.e., economic, environmental, and social) or considered all of the different LID practices at a city-scale. This study develops a life cycle quantification framework to determine the monetary values of the three types of benefits and the life cycle net benefit of LID practices for a city. Applying the proposed framework to a case study of Hong Kong, the 30-year economic and environmental benefits are 5.3 billion USD and 1.2 billion USD, respectively. The mean and median social benefits are 35.1 billion USD and 49.6 billion USD, respectively. Subtracting the 30-year LID implementation cost (55.8 billion USD) produces a median positive net benefit of 2.3 billion USD with an annual unit value of 1.05 USD/m2 yr, and a mean negative net benefit of 12.2 billion USD with an annual unit value of −5.58 USD/m2 yr. Sensitivity analyses show that the net benefit is sensitive to the willingness to pay (WTP) of Hong Kong people, especially the WTP of the private sector, and the land cost of green roofs. Overall, this study provides a framework for quantifying and evaluating the life cycle cost, benefits, and net benefit of LID practices. The assumptions in the framework can be modified based on local information and applied to many other cities worldwide.