A simple method for life cycle cost assessment of water sensitive urban design

The traditional method of urban drainage design uses features such as large impervious areas, concrete drains and pipe lines to collect and dispose of as much urban stormwater runoff and as quickly as possible. This results in a greater runoff volume that generally requires expensive enhancement of drainage network to reduce severity and frequency of flooding in urban areas. This also results in a higher pollutant washoff from the urban areas leading to deteriorated water quality in the receiving water bodies. Water Sensitive Urban Design (WSUD) is a relatively new approach of planning and design of urban areas that offers a sustainable solution for integrating land development and urban water cycle. This uses on-site infiltration and bio-retention system to reduce stormwater runoff volume and capture pollutants at or near its sources and considers various water preservation and reuse techniques to reduce potable water consumption. In adopting WSUD, local authorities in Australia are facing a number of problems such as lack of agreed standards and performance data on WSUD measures to support planning and design. This paper presents a simple method to conduct life cycle cost analysis of WSUD. The method was applied to a hypothetical site in Brisbane City Australia and it is found that the WSUD method has a lower capital cost but due to higher maintenance cost, its life cycle cost is little higher than the traditional method particularly at smaller interest rates. WSUD method contributes towards a number of additional benefits such as reduction in main water consumption by recycling stormwater runoff and greywater, delay or avoidance of major downstream channel upgradation or protection work to minimize flood hazard, cleaner waterways, avoidance of algal blooms, and delay in building of water supply infrastructures. When these are considered, the WSUD method has much greater overall benefit than the traditional method. Thus, the little increase in life cycle cost associated with the WSUD method (for small interest rates) appears to be justified.