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statewide green infrastructure policy. And, if it does work as intended, is it still affordable when 1 compared to the costs of the conventional technologies used to manage urban stormwater? 2
To answer the first question, we examined data in published, peer-reviewed studies to evaluate the 3 effectiveness of green infrastructure. Such studies have all undergone a critical evaluation by several 4 experts in the field, and only studies that are able to stand up to this scrutiny are published. We also 5 looked at some data that were not rigorously peer-reviewed, including the U.S. Environmental Protection 6 Agency’s International Stormwater Best Management Practices Database.5
To evaluate the effectiveness of green infrastructure, we used four indicators that reflect pressing 12 stormwater issues and are commonly (and relatively inexpensively) monitored. Since sedimentation, 13 nutrient enrichment and flooding are among the three biggest stormwater problems threatening ecosystem 14 and human health, mitigation of total suspended solids (TSS), total nitrogen (TN), runoff volume, and 15 peak flow were selected for comparison between types of green infrastructure. It is important to note that 16 this analysis looked at green infrastructure practices separately, and not as potential supplements to 17 conventional stormwater management structures. 18 To assess the cost-7 effectiveness of green infrastructure, we used a literature review, data from past research and a green 8 infrastructure economic model called the Green Values® Calculator, developed by the Center for 9 Neighborhood Technology (CNT) to compare different urban stormwater management technologies – 10 both green and conventional -- over their useful lives. 11
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Green infrastructure categories analyzed include on-site stormwater filtration systems, bioinfiltration 20 infrastructure (including rain gardens, bioretention, biofiltration, bioswales, and grass swales), permeable 21 pavement, green roofs, and constructed wetlands. After examining 57 peer-reviewed journal articles, 22 some of which monitored more than one site (for a total of 173 sites), we found that green infrastructures 23 generally reduced total suspended solids and total nitrogen, and decreased runoff volume and peak flow. 24
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In general, we found that green infrastructure works on average as well as conventionally-engineered 26 detention and retention basins in reducing water pollution risks (see Figs. ES-1A and 1B, below). We 27 found that green infrastructure practices are also effective in reducing both stormwater peak flows and 28 runoff volumes, which increase flooding and sedimentation risks (see Fig. ES-1C). Perhaps the most 29 significant aspects of green infrastructure practices are the volume control and water quality improvement 30 capabilities that almost all these practice offer (and which may not be the case with conventional 31 structures that focus on controlling stormwater release rates). Treatment trains, which combine multiple 32 infrastructures in series, and drainage-basin scale approaches, which combine multiple infrastructures in 33 parallel, may be even more effective than individual green infrastructure practices. 34
In terms of cost, CNT’s Green Values Calculator shows that green infrastructure is frequently 5-30% less 35 costly to construct and about 25% less costly over its life cycle compared with traditional infrastructure. 36 These cost values assume that recommended maintenance is conducted on schedule and that green 37 infrastructure is performing as expected; the same assumptions apply to gray infrastructure, however. In 38 addition, green infrastructure allows for more flexibility in adapting to changes in conditions and/or 39 knowledge, whereas once gray infrastructure is built, it becomes more costly to reverse or modify it. 40
5 http://www.bmpdatabase.org.
