Once soils data have been collected, soils performance can be incorporated into GSI design. This is done by accounting for soils variability, which in turn, impacts infiltration— how water moves through soils with differing textures.
Our Green Stormwater Infrastructure Sizing Tool allows planning process participants to explore how design decisions interact with soil variability to impact the size of the storm that you can reliability manage in the face of climate uncertainty.
The below offers broad recommendations for who might be involved in sizing of GSI. As always, evaluate participation in accordance with your specific project.
Green stormwater infrastructure design can be improved by incorporating native soils data into sizing decisions. The sizing tool can be used in community engagement to explore how native soils influence GSI performance relative to other design variables. A collaborative community engagement process benefits GSI design by incorporating community-driven design preferences, lived flooding experiences, and previous planning into decisions. This can be accomplished by actively involving community groups, decision-makers, and residents in the design process.
To learn more see the Engagement step of the design process.
Any stormwater planning process participant can follow the sizing tool instructions to explore site-scale GSI design options.
Soils texture data can be obtained from the Web Soils Survey (WSS); when WSS data is not available for the site, a site-specific soils data collection and analysis performed by expertly trained field soil survey personnel is recommended. The chosen design storm will benefit from knowledge of: relevant regulations, rain frequency data, flooding consequences (economic extent of damages, land use impacted, such as critical infrastructure), community engagement process outcomes, and design storm data from sizing tool feedback scenarios.
Assistance from a landscape architect is recommended to ensure the selected GSI design options best fit the site, and an engineering professional is needed to model performance to ensure that options meet project performance goals.
To learn more about professionals and their roles in GSI Design, see Who Might be Involved in GSI Design?
Local governments can help in setting objectives for GSI—one of those objectives may be meeting any local regulations or ordinances, with which they will be familiar. Local governments also provide familiarity with the site history, as well as current and future land-use planning context. Because local governments have land-use authority, considering GSI site design in the context of broader planning is critical in siting and scaling GSI features to catchment and sub-catchment scales.
To learn more about the stormwater management authority of local government, see Illinois regulations.
The GSI sizing tool requires gathering the following information: (1) design storm (0.1 to 5 inches), (2) storm duration, (3) soil texture, and (4) GSI type.
(1) Design storm and (2) Storm duration: The design storm choice (0.1 to 5 inches) and the storm duration choices (2 hours, 24 hours) for the sizing tool are typical of those in the greater Chicago metropolitan region. In addition, it is recommended that in selecting design storm and storm duration, consider available project funding, relevant regulations, program objectives (as determined in a community engagement process), rain frequency data (NOAA Atlas 14, Illinois State Water Survey Bulletin 70 and Bulletin 75), and flooding consequences (economic extent of damages, land use impacted, such as critical infrastructure).
(3) Soil texture: Soils texture data can be obtained from the Web Soils Survey (WSS); when WSS data is not available, it is recommended that soils texture data come from a site-specific soils data collection and analysis performed by expertly trained field soil survey personnel (e.g. Illinois State Geological Survey, USDA-Natural Resources Conservation Service, private geotechnical engineering firms).
(4) GSI Type: When choosing the type of GSI (bioretention or permeable pavement), consider factors such as site location, preferences informed by a community engagement process, and GSI benefits. Additionally, landscape architecture expertise is recommended to ensure the selected GSI design options best fit the site.
Access the Green Stormwater Infrastructure Sizing Tool to explore GSI design options. The sizing tool allows the user to input some initial information, including: design storm inches, storm duration, soil texture, and GSI type. The user can then generate GSI design scenario output results.
The GSI design scenario output includes a range of differently-sized options to meet an 80% reduction in stormwater runoff performance standard. These options vary in (1) design depths (12”, 18”, 24”, and 30” for planted and paved GSI), (2) site loading ratios (1:2, 1:3, and 1:5 for bioretention; 1:1 and direct infiltration for paved).
The most cost-effective GSI design scenarios are presented first, defined as designs with lower loading ratios and less depth. Users can also explore how different GSI design options impact the size of the storm that can be reliability managed.
This sizing tool is unique because it uses research from the body of knowledge about earthquake risk and applies it to GSI reliability (risk assessment). This approach allows for factors impacting GSI performance, such as design storm, to change over time, allowing stormwater planning process participants to more accurately evaluate the effectiveness of green stormwater infrastructure and set performance standards.
To learn more, explore the references listed in the Resources section below and watch these videos:
Reshmina William, PhD, Civil Engineering What are Reliability Curves and how can I use them in GSI design?
Reshmina William, PhD, Civil Engineering How is GSI modeling used to assess stormwater run off reduction?
- Mary Pat McGuire, David A Grimley, Andrew C Phillips, Ashlynn S Stillwell, Reshmina William, Jinyu Shen, Margaret Schneemann, Retrofitting urban land through integrative, subsoils-based planning of green stormwater infrastructure: a research framework, Environmental Research: Infrastructure and Sustainability, 10.1088/2634-4505/ac27bd, 1, 3, (035003), (2021). https://iopscience.iop.org/article/10.1088/2634-4505/ac27bd/meta
- Gabrielle M. Bethke, Reshmina William, Ashlynn S. Stillwell, Rain Garden Performance as a Function of Native Soil Parameters, Journal of Sustainable Water in the Built Environment, 10.1061/JSWBAY.0000967, 8, 1, (2022). https://experts.illinois.edu/en/publications/rain-garden-performance-as-a-function-of-native-soil-parameters
- William, Reshmina, & Stillwell, Ashlynn S. Stillwell (n.d.). Use of Fragility Curves to Evaluate the Performance of Green Roofs. Journal of Sustainable Water in the Built Environment, 3(4). https://ascelibrary.org/doi/abs/10.1061/JSWBAY.0000831
- William, R., Gardoni, P., & Stillwell, A. S. (n.d.). Reliability-Based Approach to Investigating Long-Term Clogging in Green Stormwater Infrastructure. Journal of Sustainable Water in the Built Environment, 5(1). https://ascelibrary-org.proxy2.library.illinois.edu/doi/full/10.1061/JSWBAY.0000875
- William, R., Gardoni, P., and Stillwell, A. S. (2021). Predicting rain garden performance under back-to-back rainfall conditions using stochastic life-cycle analysis. Sustainable & Resilient Infrastructure, 6(3/4), 143–155. https://experts.illinois.edu/en/publications/predicting-rain-garden-performance-under-back-to-back-rainfall-co