Site Location

Place matters for green stormwater infrastructure (GSI) performance—applying knowledge of local conditions can improve GSI performance

Overview

Green stormwater infrastructure site location selection can be improved by considering native subsoils properties. Using soils and related geologic data as a foundation for GSI siting will leverage the infiltration properties of native soils and will increase GSI performance.

The GSI design process benefits from an understanding of how human activities have shaped the surficial geologic landscape through time, starting with the natural, pre-development landscape and progressing to present-day surficial conditions.

Outcomes

As a result of integrating native subsoil data into the green stormwater infrastructure siting decision, the team identifies community areas to prioritize for GSI site locations.

Recommended Involvement

The below offers broad recommendations for who might be involved in the GSI site location. As always, evaluate participation in accordance with your specific project.

Community engagement can be used to build a common understanding between local officials, stormwater professionals, and residents to improve siting decisions. In addition, a collaborative process benefits the work of soils researchers and technicians in selecting soils sampling sites. For example, through the use of maps at community meetings or in surveys, residents can share their flooding experiences and can start a dialog with stormwater managers.

Siting can also benefit from participants’ engagement in previous, related planning processes (stormwater, open space, watershed, sustainability, land use) to ensure continuity in the community’s vision and efforts.

To learn more see the Engagement step of the design process.

Helpful expertise for performing green stormwater infrastructure siting tasks include: spatial analysis skills (data collection, analysis, and visualization; geographic information system (GIS) software; map overlay creation); community engagement skills, including the ability to effectively convene and engage populations experiencing flooding; and having an understanding of current stormwater plans and infrastructure. When using maps to inform GSI siting, it is important to have the ability to assess underlying data quality and to be sure you have good descriptions of the data and data products (metadata). The GSI siting process, including integrating soils data, benefits from the knowledge of geologic, hydrologic, soils, and anthropogenic processes that have operated in the past, or are operating at present. To visualize and communicate potential GSI site locations, it is helpful to have landscape architecture expertise to create or render drawings using AutoCAD or a similar software.

To learn more about professionals and their roles in GSI Design, see Who Might be Involved in GSI Design?

Land use and stormwater management authority are under local government control in Illinois, and so, local government involvement is crucial for green stormwater infrastructure siting decisions, including coordinating GSI with land use plans. Local zoning must be considered for siting. CSO consent decree could be prescriptive on the amount of GSI. Local authorities may be tasked with compliance responsibilities within MS4 or CSO permits.

Even when GSI is sited on private land, effective functioning of this infrastructure as part of a stormwater program strategy and requires incentives, policy, planning, coordination, and maintenance support from local governments. Because stormwater does not stop at municipal boundaries, involving multiple units of local governments in the same catchment or watershed is advisable.

To learn more about the stormwater management authority of local government, see Illinois regulations.

Tasks

The initial siting task is to gather data to use in a mapping analysis. To integrate native soils into green stormwater infrastructure site selection, in addition to other siting and site prioritization data, gather surficial geological maps, which show the distribution of materials on the land surface (soils and sediment). Use that data with topographic, soils, hydrologic, water table maps, and other maps to infer best siting. For instance, a sandy soil can be wet or dry depending on where it is on the landscape.

 

  1. Soils and geologic data:
    1. Surficial geologic maps: ISGS Quadrangle Maps, 1939
    2. Soils maps, hydrologic soils data: NRCS web soil survey
    3. Previous geotechnical studies and analyses (data will vary by community): Illinois Department Of Transportation, Illinois Water Well (ILWATER) Interactive Map
  2. Other siting and site prioritization data:
    1. Land use (zoning, public/private ownership): CMAP 2015, available from regional councils
    2. Land cover data: shows surface and ground conditions including pavements and compacted ground areas
      1. National Land Cover Database, Land Cover Data for Illinois, CMAP High-Resolution Land Cover, Cook County, 2010, CMAP High-Resolution Land Cover, NE Illinois and NW Indiana, 2010
      2. Historic aerial images: USDA Aerial Photography, USGS EROS Archive – Aerial Photography – Aerial Photo Mosaics, Mapping History at the University of Illinois Collection
      3. Contemporary aerial images: Cook County Viewer, Illinois county GIS Viewers, recommend searching for your own county GIS data viewer
      4.  UTM coordination system
    3. Flooding areas: gather previously conducted flooding susceptibility/vulnerability analyses, or conduct an analysis: Vulnerability Assessments, CMAP Flood Susceptibility Analysis) floodplain maps (Illinois Flood Maps), sewer inundation maps (proprietary, obtain from community)
    4. People, community organizations and character: (U.S. Census, University of Illinois Extension Community Survey Tool, CMAP Community Data Snapshots)
    5. Topography: Illinois Topographical Map, catchment delineation USGS Hydrologic Unit Maps, overland flow assessment ISWS, ESRI ArcHydro, CMAP ArcHydro Modeling Outputs, 2017
    6. Gray infrastructure: network and condition, inlet locations, bottlenecks or areas under capacity (proprietary data, obtain from community)
    7. Opportunities for social improvement, ecological connectivity, economic growth: Local and regional plans

After data has been gathered, use spatial analysis, including:

  1. Identify flooding areas.
    1. Urban mapping overlays
      1. Soils and related research data overlays/layers
        1. Surficial geological maps
        2. Hydrologic soils data
        3. Comparative analysis of hydrologic soils data with land cover data
      2. Typical stormwater siting and site prioritization data overlay layers
        1. Land use, to discern patterns and areas of green stormwater infrastructure opportunity (including  transportation infrastructure, residential-commercial-industrial zoning, green land, vacant land, civic-public land)
        2. Comparative analysis of land use and ground conditions using aerial maps and site visits
  2. Locate prevalent patterns of flooding due to ground conditions, noting where pattern locations are different than problem areas.
    1. Use CMAP’s FSI Index to prioritize areas that may be more prone to flooding, identify patterns of imperviousness and topographic relief contributing to flooding, and to discern GSI opportunity areas.
    2. Examined areas of depression using CMAP ArcHydro Modeling Outputs, 2017 and Topographic Wetness Index
  3. If possible, determine the source of flooding and the factors contributing to flooding. It can be helpful to look upstream of the flooding area.
  4. Create additional investigations where data or information is missing.
    1. Conduct interviews. (reports of conditions and patterns)
    2. Conduct site visits. (ground truth observations)
    3. Uncover findings and/or stories.

Create drawings that indicate possible site areas for green stormwater infrastructure design. Review the examples of GSI solutions from the case studies to aid in discussion and design direction for the project.

  1. Engage people in the process to understand community knowledge about flooding conditions.
  2. Create maps and diagrams, photos, and images.
  3. Look at urban land and flooding relationships.
  4. Talk through patterns and opportunities.
  5. Build consensus about sites of interest.

Soils and geologic data is incorporated into maps, photographs, and diagrams of the site, and existing conditions context.

Resources

What are ISGS Quadrangle Maps and why are they useful for understanding soils?
Discussion of USGS/ISGS Quadrangles, how to find them, how to read them, and how to use them to assess geologically-derived soil types in a given project area.