GSI Impact Hub: Capturing the Multiple Benefits of Green Stormwater Infrastructure

Many cities throughout the U.S. and Canada are adopting green stormwater infrastructure (GSI) and nature-based approaches to reduce polluted stormwater runoff and meet water quality-related regulatory requirements. Unlike conventional or 'gray' infrastructure, GSI can provide many additional economic, social, and environmental benefits that advance a community's livability and resilience to climate change.

While GSI has proven effectiveness as a stormwater management strategy, it is not always the least cost option. GSI may also represent a significant departure from 'business-as-usual' and can be difficult to implement in municipalities or utility service areas with competing resource needs and funding priorities. To further GSI implementation, stormwater practitioners have expressed a need for information to help them better understand and evaluate its multiple benefits.

Over the past three years, The Nature Conservancy, One Water Econ, and utility representatives from the Green Infrastructure Leadership Exchange (GILE) have developed a suite of resources to meet this need. This has culminated in the creation of the GSI Impact Hub, a comprehensive online resource of tools and guidance that allow users to capture the co-benefits of GSI so that practitioners, policymakers, and leaders have the evidence-based information they need to design and implement projects that provide the greatest benefits to their communities. The GSI Impact Hub is in the final design phase and will be officially launched in mid-February 2025. It includes:
- An online GSI co-benefit calculator. Members of the GILE identified a need for a co-benefit calculator that can be used to inform the development of stormwater management projects early in the planning phase, with minimal inputs. The GSI Impact Calculator is designed for this purpose. It allows users to quantify and monetize 12 co-benefits of GSI and compare those benefits to costs over a specified timeframe. The Calculator is designed for use at the block-scale; it is location-specific and calculates benefits based on city- or region-specific data using standard economic valuation approaches.
- Five in-depth co-benefit guides related to flood risk, urban heat, job creation and economic development, urban habitat and biodiversity, and transportation-related benefits. Based on case studies, interviews, and in-depth research, these guides include a review of academic studies related to each benefit, describe processes/key GSI design elements necessary for achieving the benefit, provide guidance for quantifying and monetizing the benefit, and identify relevant funding sources and financing strategies.
- A compendium of existing 'co-benefit' tools. An increasing number of tools are designed to evaluate, quantify, and/or value GSI co-benefits; however, navigating these various resources can be a daunting task for stormwater practitioners. Tools require different inputs, incorporate different methods, and require varying levels of stormwater management and/or economic expertise. The compendium is intended to help stormwater practitioners navigate the suite of available tools and utilize those that best meet their needs.

Our presentation will include a review of the GSI Impact Hub resources, including highlighting key research findings and data, with a focus on the GSI Impact Calculator. We will apply the Calculator to assess the benefits, costs, and return on investment of a real-world block level application of GSI in Cleveland, Ohio. This case study calculates the benefits of an 11.1-acre site undergoing significant redevelopment. The 30-year ROI for the project is 415%, with an annual monetized benefit value of $57,709 per year. For this project, which includes bioretention cells, impervious area removal, and extensive tree plantings, the top three benefits are community livability, avoided infrastructure costs, and urban heat stress reduction. The ROI for this project is high in part due to lower costs of implementation because the area is already being redeveloped.

In addition to the Calculator, we will share findings from our research on specific co-benefits, including highlighting the ability of GSI to provide climate resiliency benefits to communities in the form of urban heat and flood risk reduction. For example, we found that at the site, block, or sub-catchment level, targeted GSI can reduce localized flooding associated with small to moderate storm events (e.g., up to the 10-year storm return interval with sufficient storage). When distributed throughout a watershed or catchment, GSI can reduce riverine flooding caused by smaller precipitation events (e.g., typically less than 1.3 inches); however, the effect of GSI on downstream flows decreases for larger storm sizes.

The GSI Impact Calculator and related co-benefit guide also provide a methodology for quantifying the cooling effect of GSI in urban areas. At sufficient scale, GSI practices can decrease ambient air temperatures by 0.5 to 1.8°F. Even a 0.5°F decrease in urban temperatures can significantly reduce heat-related impacts, including illnesses and deaths caused by extreme heat events. The cooling effect of GSI is influenced by several factors, including baseline conditions, scale of implementation (e.g., converting between 6% to 31% of a study area will result in GSI-related cooling benefits), and the type and location of GSI installations within the urban environment.