Locating and evaluating the costs and benefits of urban LID retrofits

Low Impact Development (LID) is a sustainable approach to land development and stormwater management (SWM) that aims to mimic natural hydrologic cycle. The City of Edmonton identified LID as one approach to make significant progress towards their water quality goal of no-net increase in pollutant loads as the city grows. The study assessed the potential costs and benefits of widespread implementation of LID in the developed areas of the Edmonton. First, opportunities for LID retrofits, such as rain gardens and green roofs, were identified spatially using remote GIS-supported methods. The siting process used data layers provided by the City—such as impervious surfaces, trees, buildings, and land use—to screen the entire study area for suitable LID retrofit opportunities. Additional steps were taken to refine the siting, such as processing topographic information to assess landscape position. Opportunities were categorized into three scenarios based on their location: public, residential, and other private lands. Identified LID retrofit opportunities were simulated in a hydrologic/hydraulic model developed in PCSWMM/SWMM5 environment to evaluate their impact on runoff and combined sewer overflows reduction. The modelling component of the project involved creating a base case scenario calibrated with the City’s sewer monitoring data and simulating various scenarios to evaluate implementation of different LID controls as well as different adoption rates within the study area. Then, a spreadsheet model was developed to estimate the life cycle costs and benefits of LID implementation scenarios, including reductions in runoff volume and runoff pollutant load. The cost-benefit ratios indicate that certain practices, such as land cover conversion from impervious to pervious, are more cost-effective than other practices, such as green roofs, for pollutant load reduction. The final inventory of LID retrofit opportunities totaled approximately 85,000 sites with a cumulative pollutant load reduction potential achieving the City-wide targets for long term total suspended solids and medium term total phosphorus reduction.