The City of Phoenix wanted to weigh up the broader costs and benefits of implementing various Low Impact Development (LID) features across the city to help inform capital planning going forward. To do this, they enlisted the help of Stantec, Autocase, and Watershed Management Group (WMG) to perform a triple bottom line cost benefit analysis (TBL-CBA).

Using Autocase’s online TBL-CBA platform, the impacts listed in Table 1 were quantified for each of the LID features and then compared against a traditional concrete base case on a per-1,000 sq. ft basis.

At a high level, the results in the table below show that LID features generate positive triple bottom line benefits. On a pure lifecycle cost, we find that Infiltration Trench is the only feature that is cheaper than Concrete – costing almost $2,000 less per 1,000 sq. ft. However, compared to concrete, every LID feature (other than Pervious Pavers) generates enough social and environmental benefit to result in a positive TBL Net Present Value (NPV).

Since runoff does not get treated by a wastewater treatment plant, given that Phoenix has a separate storm sewer system, water quality was a major concern for the City. Instead, runoff passes through either a hydrodynamic separator or a filter catch basin insert before going in to the storm system.

LID can positively influence the water quality in a local area by reducing surface runoff of pollutants. We estimated the value of improved water quality by first modeling the reduced runoff that would be passing through these grey systems due to having LID present on the site. We then equated this resulting reduction in runoff to an avoided capital expenditure and operations & maintenance costs for the grey systems. The water quality results for each LID feature is given in the table below. It is clear that they contribute significant value to the overall TBL results.

Multi-account analyses like these not only answers the questions of “What are the benefits?” and “Who are the beneficiaries?”, but equally important, “How much do they benefit?”. Putting everything in dollar terms allows comparison on an apples-to-apples basis and helps build consensus to the delivery of projects while creating an evidence base to promote a shared responsibility to capital planning for these non-traditional projects.

Simon Fowell
Economist

Article also published on the APWA C4S (Center for Sustainability) Blog