9
International Stormwater Best Management Practices Database.1 3 Peer-reviewed studies have all
2 undergone a critical evaluation by several experts in the field, and only studies that are able to stand up to
3 this scrutiny are published. To assess the cost-effectiveness of green infrastructure, we used a literature
4 review, data from past research and a green infrastructure economic model called the Green Values®
5 Calculator, developed by the Center for Neighborhood Technology (CNT) to compare different urban
6 stormwater management technologies – both green and conventional – over their useful lives.
7 Since sedimentation, nutrient enrichment and flooding are among the three biggest stormwater problems
8 threatening ecosystem and human health, mitigation of total suspended solids (TSS), total nitrogen (TN),
9 runoff volume, and peak flow were selected for comparison between types of green infrastructure. It is
10 important to note that this analysis looked at green infrastructure practices separately, and not as potential
11 supplements to conventional stormwater management structures.
12
13 Green infrastructure categories analyzed include on-site stormwater filtration systems, bioinfiltration
14 infrastructure (including rain gardens, bioretention, biofiltration, bioswales, and grass swales), permeable
15 pavement, green roofs, and constructed wetlands. After examining 57 peer-reviewed journal articles,
16 some of which monitored more than one site (for a total of 173 sites), we found that green infrastructures
17 generally reduced total suspended solids and total nitrogen, and decreased runoff volume and peak flow.
18 We also identified in our recommendations emerging green infrastructure practices that have shown
19 strong potential.
20
21 Green infrastructure works on average at least as well as conventionally-engineered detention and
22 retention basins in reducing water pollution risks (see Figs. ES-1A and 1B, below). Green infrastructure
23 practices are effective in reducing both stormwater peak flows and runoff volumes which, if unmanaged,
24 increase flooding and sedimentation risks (see Fig. ES-1C). Among the significant benefits that green
25 infrastructure offers are the volume control and water quality improvements that almost all these practice
26 provide (and which may not be the case with conventional structures that focus on controlling stormwater
27 release rates). Treatment trains, which combine multiple infrastructures in series, and drainage-basin scale
28 approaches, which combine multiple infrastructures in parallel, may be even more effective than
29 individual green infrastructure practices.
30 In terms of cost, CNT‘s Green Values Calculator shows that green infrastructure is frequently 5-30% less
31 costly to construct and about 25% less costly over its life cycle compared with traditional infrastructure.
32 These cost values assume that recommended maintenance is conducted on schedule and that green
33 infrastructure is performing as expected; the same assumptions apply to gray infrastructure, however. In
34 addition, green infrastructure allows for more flexibility in adapting to changes in conditions and/or
35 knowledge, whereas once gray infrastructure is built, it becomes more costly to reverse or modify it.
36
37 III. FUNDING GREEN INFRASTRUCTURE
38
39 The Clean Water Act‘s state revolving loan fund (SRF) program has always been available for use in
40 helping to fund stormwater management projects, although the vast majority of SRF money has typically
41 gone to wastewater treatment projects. The American Recovery and Reinvestment Act of 2009 (ARRA),
3 http://www.bmpdatabase.org.
