Land and Hydrologic Analysis for Stormwater Detention and Volume Control Trading Exchange in Cook County, Illinois

Numerous studies have attempted to identify the implications of climate change with respect to hydrologic extremes (e.g., IPCC, 2007; CCSP, 2008; Milly et al., 2008; USGCRP, 2017). These studies project future climate conditions with more frequent extreme precipitation events in many regions around the world, including parts of the United States. The U.S. Global Change Research Program (2017) indicates that “heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence).” There are important regional differences in trends, with the largest increases occurring in the northeastern United States (USGCRP, 2017), followed by the Midwest (Karl et al., 2009). USGCRP (2017) also states that “mesoscale convective systems (organized clusters of thunderstorms)–the main mechanism for warm season precipitation in the central part of the United States–have increased in occurrence and precipitation amounts since 1979 (medium confidence).” Climate model projections also indicate that northeastern Illinois, including the Chicago metropolitan area, will experience more frequent and more intense rainfall events in the future (Markus et al., 2012, 2016). These increases will lead to more intense and more frequent urban flooding events and to increased human, environmental, and economic risks. Thus, various planning and management measures need to be considered by urban communities responsible for administering ordinances governing the construction and maintenance of stormwater management systems and for floodplain management to address public safety concerns, property damage, and economic interruption from intense precipitation. Future climate projections based on general circulation models (GCM) are typically downscaled to finer temporal and spatial scales using statistical or dynamical downscaling models. However, watershed-scale climate data generated by climate models still do not provide precipitation data in a format useful for community engineers and planners to prepare, mitigate, and adapt to future conditions. In this study, a method is designed to analyze and express climate data in a format that can be readily used to assess future extreme precipitation events in models commonly used for sizing stormwater infrastructure and identifying flooding potential. This report presents a newly designed framework to determine future condition rainfall frequency maps for 24- and 48-hour duration rainfall events and for a range of recurrence intervals (also called return periods). This framework directly supports climate adaptation and mitigation by providing an understandable method for community engineers and planners to demonstrate the impact of climate change at the local level and develop specific adaptation strategies that will reduce future risk. The Cook County Watershed Management Ordinance (WMO) allows detention trading as an option for providing off-site stormwater management for a given project as an alternative to on-site detention as part of an individual site development. Volume control (also referred to as retention) trading is proposed in the WMO Draft Amendment. As part of the effort to evaluate a potential stormwater detention and volume control trading exchange, the Illinois State Water Survey (ISWS) was contracted to assess potential opportunity areas for developing stormwater management facilities (detention and/or volume control) in suburban Cook County, excluding the City of Chicago. The ISWS land and hydrologic analyses compose one of three coordinated projects that explore the feasibility of stormwater trading in the study area. The ISWS was contracted by the Metropolitan Water Reclamation District of Greater Chicago, which led the land and hydrologic analyses team. A real estate demand analysis was performed by a team composed of Teska Associates, Inc.; Hey & Associates, Inc.; and Orion Planning and Design. The Nature Conservancy and the Metropolitan Planning Council led the policy analyses team. Studies and analyses were coordinated, and each team prepared a report. This report describes the analyses performed by the ISWS. The primary objective of the land and hydrologic analysis component of the feasibility study is to identify and quantify opportunity areas with favorable characteristics for stormwater detention or volume control. The analyses were performed on a geospatial platform, building on Geographic Information System (GIS) tools. Opportunity areas could be used to provide trading credit when a development cannot meet the requirements on-site. The analyses used to identify opportunity areas are based on existing and available geospatial data that determined the precision of the results. Desirable characteristics differ for detention and volume control facilities; thus, two sets of data compilations were prepared, one showing opportunity areas for detention and one for volume control. This feasibility assessment study showed there are ample opportunity areas for trading. Supply of opportunity area is clearly not a limiting factor in a potential market. However, these areas have not been developed before and incentivizing creation of stormwater detention and/or volume control sites for trading will be important to avoid supply constraints. The GIS tools developed as part of this study will help to identify potential sites to assist with catalyzing this trading initiative. Site development that uses off-site stormwater detention or volume control will meet the basic precepts of the WMO. Off-site storage upstream of the development site could reduce peak flows upstream, providing additional benefits to the area. Likewise, off-site volume control holds great promise in spurring the creation of the natural, multifunctional sites that enhance the study area and provide ecological and economic benefits. Stormwater landscapes, sometimes called “landscape as infrastructure” become highly important public and environmental spaces in municipalities. Recent research has revealed that stormwater landscapes create multiple benefits to communities visually, educationally, socially, and environmentally. Therefore, the relative “value” of given sites and their characteristics may differ among different subwatersheds and communities depending on issues and priorities. The methodology for site selection can be adapted for use by watershed or community planners to locate sites with the greatest potential to serve their development priorities and achieve the desired ecosystem benefits.