Triple Bottom Line Cost Benefit Analysis of Green Infrastructure/Low Impact Development (GI/LID) in Phoenix, AZ

by | Jul 12, 2018 | Economics, News

The City of Phoenix Green Infrastructure Cost-Benefit Study is now available online. You can access the report here.

Given the importance of heat stress in Phoenix, instead of using historical temperatures Autocase incorporated future climate change in to its analysis. “More than 155 people died from heat-related causes in the Phoenix area last year, a new record in a place where the number of such deaths has been on the rise” (Phoenix Tries To Reverse Its ‘Silent Storm’ Of Heat Deaths, NPR,
July 9, 2018)

Source: US Climate Resilience Toolkit Climate Explorer – Phoenix

If using custom climate scenarios is of interest to you, contact us ( as we are busy making this available across North America in Autocase for Sites.

The Triple Bottom Line Cost Benefit Analysis (TBL-CBA) business case was conducted in Autocase – a cloud-based software tool, to provide insights into the value of costs and benefits of the projects to the City, as well as the broader societal and environmental impacts. TBL-CBA is a systematic evidence-based economic business case framework that uses best practice Life Cycle Cost Analysis and Cost Benefit Analysis (CBA) techniques to quantify and attribute monetary values to the Triple Bottom Line (TBL) impacts resulting from an investment. TBL-CBA expands the traditional financial reporting framework (such as capital, and operations and maintenance costs) to also consider social and environmental performance. TBL-CBA provides an objective, transparent and defensible economic business case approach to assess the costs and benefits pertaining to the project being analyzed.

This study provides information for City projects and private development that may want to implement and incorporate GI/LID facilities. The costs and co-benefits of GI/LID features in the Phoenix environment need to be evaluated to identify the benefits and aid in potentially identifying to which stakeholders they accrue.

Apart from the urban heat island, Autocase for Sites was used to value:

  • Financial costs and benefits;
  • Carbon emissions and air pollution;
  • Water quality improvements;
  • Flood risk reduction; and,
  • Property value uplift.

What can we learn from this type of analysis?

  1. If the City were to take a purely financial perspective, only infiltration trenches are cheaper than concrete, all other features are more expensive.
  2. In terms of social impacts, swales and bioretention basins stand out as winners.
  3. Environmentally, all features perform better than Concrete.
  4. In terms of the TBL-NPV, all but pervious pavers generate positive TBL-NPV, with swales and bioretention basins as the clear leaders.

We also learn where that value comes from and the uncertainty attached to the numbers. Swales, for example, generate an estimated $15,026 (95% confidence interval of -$2,151 to $33,600) in triple bottom line net present value over a 50-year time horizon relative to concrete, with -$2,400 created through financial impacts, $10,000 through social benefits, and $7,500 through environmental benefits. The figure below shows a waterfall chart of the breakdown of these values. On the chart, blue represents value being created, whereas red represents a cost, relative to concrete. We can see that swales have almost no incremental capital expenditure (CapEx) but do have higher operations & maintenance (O&M) costs compared to concrete. We can see that varying amounts of value are created across the social and environmental spectrum of impacts, with the most significant being heat island benefit, flood risk, water quality, and avoided carbon emissions from concrete use. The 95% confidence intervals shown in the report allow us to see the uncertainty in some of these figures. For example, CapEx and replacement costs could be higher or lower than concrete. There is a large spread in heat island benefits as well as water quality, and when all impacts have been assessed it creates a large spread in overall TBL-NPV but reveals only a small chance of generating a negative TBL-NPV as compared to concrete.

Three sites were also analyzed as case studies in the report.

  1. Primera Iglesia at 701 S. 1st Street, Phoenix installed in November 2011 and included 15 new trees requiring no supplemental irrigation after the vegetation was established, 4,500 sq ft bioretention basin/rain garden, and curb cuts and cores. This project provided the first Phoenix area GI/LID site demonstration.
  2. Glendale Community Center is located at 14075 N. 59th Avenue, Glendale. The project installation date was March 2016 and included 8 new trees, two bioretention basins/rain
    gardens totalling 6,000 sq ft, which is expected to harvest 10,000 gallons of rainwater per year, and curb cuts.
  3. A combined project encompassing Central Station, Civic Space Park, and Taylor Mall includes a transit center, public park, and pedestrian improvements generally located around 444 N.
    Central Avenue in Phoenix. The traditional features include landscaping and one new retention basin of 0.33 acres and one existing retention basin of 0.147 acres. GI/LID features include 680 shrubs, 52,000 sq ft of pervious pavers, 13,000 sq ft of vegetated swales with trees, 1,600 sq ft of tree planters, 30,000 sq ft of porous concrete, 243 new trees, and one underground stormwater storage cistern with a capacity of 9,600 cf.

See the report for the results of these case studies.

Multi-account results not only answer the question of “Who benefits?” but equally important, “How much do they benefit?”. By thinking of which stakeholders would benefit from each impact, it allows the City to:

  1. Assess what existing policies can be leveraged to support GI/LID, as well as how GI/LID may promote the goals of those policies, and
  2. Communicate results in a way that gets maximum buy-in from various agencies and external stakeholders. By showing that these projects are aligned with the broader goals of each respective stakeholder, the potential hurdles that often come with more cost-intensive projects can be addressed early.
  3. Finally, these types of analyses give visibility into which features are providing the greatest benefits in terms of the city’s priorities.

Given the heat stress Phoenix faces, users can utilize these types of results to prioritize projects that have the largest impact on that element. Ultimately, assessing projects across a spectrum of impacts and valuing them in dollar terms allows the City to map benefits and costs to various stakeholders and is an important step toward consensus building and developing a business case in a way that everyone can understand.


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