Overview
Green stormwater infrastructure designs are modeled by professional engineers to estimate how rainfall is converted into runoff (hydrologic modeling), as well as how runoff is then transported through the natural and built stormwater infrastructure system (hydraulic). GSI performance results lead to modification of the design through an iterative process to meet specified performance targets, community preferences, and budgets.
Outcomes
The GSI design scenario’s runoff reduction performance assessment provides an estimate of the design’s effectiveness in mitigating runoff, and identifies areas where additional mitigation may be needed. The results will lead to further design performance evaluation or will suggest that design modifications are needed, leading to a final design decision.
Recommended Involvement
The below offers broad recommendations for who might be involved in assessing runoff reduction achieved with the GSI design. As always, evaluate participation in accordance with your specific project.
After GSI design performance has been evaluated, the design scenarios need to be weighed against the initial GSI visioning set out in Step 1 (Engagement), the design and engineering studies, and community preferences and support. Community stakeholders should provide feedback, which is then incorporated into the GSI design scenario, with initial project priorities revisited and reset. This iterative evaluation and/or community engagement process can be repeated until a final GSI design decision is reached. At this time, other planning decisions can be discussed, including further operationalizing the design, developing a funding plan, hiring consultants (designers and engineers) to create specification documents, and further engaging the community in moving into project implementation.
To learn more see the Engagement step of the design process.
Engineering expertise is needed to model GSI design scenario performance and estimate implementation costs. While anyone can use the Green Stormwater Infrastructure Sizing Tool (which has preloaded engineering estimates) to obtain initial planning level estimates, the performance should be confirmed by engineering modeling prior to implementing. Landscape architect expertise is useful to modify the design to meet performance targets and other specified project metrics. Facilitating, planning, and community engagement expertise can be leveraged to ensure that community preferences and feedback is adequately incorporated into the development of GSI design scenarios. Local government as well as property owner or manager participation is important to understand regulations and other opportunities or barriers to the placement of GSI installations. If the incorporation of co-benefit values is desired, economists and/or social science expertise can be of use. In considering the community capacity to support the GSI design, the participation of public utility experts, groundskeepers, property managers, and agency personnel is important. Other expertise may be needed to evaluate project metrics, depending on the project objectives.
To learn more about professionals and their roles in GSI Design, see Who Might be Involved in GSI Design?
Local governments provide guidance by: collaborating on modeling and evaluation; setting performance and other project goals in concert with other relevant stormwater managers (such as regional or state agencies); and helping with decisions on final GSI scenarios and the strategy for advancing to implementation. Local governments may have: preferred consulting engineers for estimating GSI design scenario performance; existing community engagement processes that can be leveraged; and land use/planning/economic development knowledge critical to informing final design selection.
To learn more about the stormwater management authority of local government, see Illinois regulations.
Tasks
Engineers evaluate GSI performance using hydrological and hydraulic analysis software for overland flow modeling and/or sewer network modeling. Numerous modeling software packages are available with varying modeling capabilities, but there is no one-size-fits-all tool. A review of stormwater models is available in the Resources section and here. For GSI at scale, performance is first modeled individually and then at nested scales to identify areas that do not meet performance targets. While the sizing tool contains built-in engineering performance modeling, it should be used for conceptual planning purposes. An engineer should confirm GSI design performance estimates as the project advances to the implementation stage.
To learn more, explore the references listed in the Resources section below and watch this videos:
Reshmina William, PhD, Civil Engineering How is GSI modeling used to assess stormwater runoff reduction?
Important GSI performance evaluation questions:
- If the design is based on setting and meeting targets, did it meet those targets?
- If the design was adjusted for site limitations, was performance still achieved?
- If performance targets weren’t met, how will implementation decisions be made?
- Are there any unexpected evaluation outcomes that need to be taken into account?
Engineers estimate planning-level GSI costs using cost analysis software, often in concert with assessing the runoff reduction project benefits. The table below outlines some important cost categories to consider.
| Cost | Metrics |
|---|---|
| Financial Costs | |
| Capital cost | Design and planning costs Land purchase/lease Initial construction costs – Materials – Equipment – Labor – Plants |
| Operations | Administrative costs Labor Material Monitoring costs Rehabilitation/replacement costs |
| Opportunity Costs | |
| Land cost | Value of foregone development Value of foregone resource use Value of foregone output Value of foregone socio-economic opportunities |
| Generated value | Income generated for third-party investors (design firms, construction firms, maintenance contractors) |
Green stormwater infrastructure process participants have the option of including analysis of additional or co-benefits identified by the community as important, such as enhanced property values, calmer traffic, and improved human and ecosystem health. This can be accomplished by using co-benefits valuation software and/or original co-benefits valuation studies, in consultation with an economist.
Use engineering modeling and evaluation to decide whether to proceed with implementing the GSI design scenario or to modify the design. To learn more, watch this video:
Reshmina William, PhD, Civil Engineering How is GSI modeling used to assess stormwater runoff reduction?
Understanding the scale effects of GSI is an ongoing area of research, especially regarding the placement of GSI installations in larger catchment areas and watersheds. Therefore, this process may involve adapting the design as new information is learned.
- Community engagement and outreach present the modeling and evaluation results and gather feedback about next steps in the process, which may vary by community.
- Review modeling and evaluation outcomes against community-defined criteria, such as:
- Community capacity to undertake GSI (including long-term care).
- Community engagement and buy-in.
- Measurable targets, including performance.
- Coordination/balance of gray and green infrastructure design.
- If the GSI design scenario does not meet the community-defined project criteria, project priorities can be revisited and reset.
- If the GSI design scenario meets the community-defined project criteria, the design can be advanced to implementation.
Resources
Reshmina William, PhD, Civil Engineering How is GSI modeling used to assess stormwater runoff reduction?
- Minnesota Stormwater Manual: Available stormwater models and selecting a model
- SWMM – Hydrologic and hydraulic modeling
- MIKE URBAN – Hydrologic and hydraulic modeling covering all urban water networks, including water distribution systems, stormwater drainage systems, and sewer collection in separate and combined systems
- U.S. Green Infrastructure Portal
- EPA National Stormwater Calculator – Hydrologic model developed by EPA for computing small site hydrology for any U.S. location
- Community-Enabled Lifecycle Analysis of Stormwater Infrastructure Costs
- Green Values National Stormwater Management Calculator
- The Green Infrastructure Leadership Exchange Green Infrastructure Co-Benefits Valuation Tool
- Cost Estimation Module for the U.S. Environmental Protection Agency National Stormwater Calculator
- WERF Performance and Whole-Life Costs of BMPs and Sustainable Urban Drainage Systems