Measuring the Cost-Effectiveness of LID and Conventional Stormwater Management Plans Using Life Cycle Costs and Performance Metrics

This paper outlines a methodology to quantitatively measure the cost-effectiveness of stormwater management plans by linking runoff pollutant load removal and volume reduction performance rates to long term life cycle costs. A step by step approach to calculate cost-effectiveness is provided and one case study presented to demonstrate the methodology. The EPA has applied this methodology to four case studies, each of which compares a built low impact development (LID) project to an alternative conventional design approach. The conventional designs make heavy use of two land cover surfaces that have become dominant in many of today’s existing developments: surface parking lots and turf grass cover. The results are reported in units of annualized cost per pound of nutrient removal ($/lb/yr) and annualized cost per cubic foot of runoff volume reduced ($/cf/yr). Findings show that the LID plans analyzed were four to six times more cost-effective, on average, than the alternative conventional designs. A key finding was that LID stormwater systems exhibited higher performance rates at a lower cost, thereby realizing a ‘compounding effect’ when compared on a cost per performance basis. The results are compared to 43 additional projects identified in a literature review. In addition, a total of eight case studies found that the LID projects cost 19% less on an average capital construction cost basis, and could provide up to twenty additional benefits beyond water quality, than the conventional plans.