A novel methodology for determining future rainfall frequency is described in this report. Isohyetal maps illustrate how heavy precipitation may change in the future, but the results have a high level of uncertainty expressed as very wide confidence limits. Uncertainty in possible future conditions is much greater than the uncertainty identified for current commonly used precipitation analyses. The resulting isohyetal maps do not replace existing sources, such as Illinois State Water Survey (ISWS) Bulletin 70 (Huff and Angel, 1989) or National Oceanic and Atmospheric Administration (NOAA) Atlas 14 (Bonnin et al., 2006). Presently, the ISWS is updating Bulletin 70 (Huff and Angel, 1989) for subregions of Illinois. Some of these updates will include projected rainfall frequency. The key objectives of this study are to i) design a framework to translate future climate scenarios into a product that engineers and planners can use to quantify the impact of climate change, and ii) demonstrate how climate model output can be used to inform and plan adaptive strategies for stormwater and floodplain management. The framework in this study is illustrated using the observed and projected rainfall data in Cook County, Illinois, providing a road map to evaluate climate change impacts on urban flooding and a plan for adaptation. Numerous studies attempt to identify the implications of climate change with respect to hydrologic extremes (e.g., IPCC, 2007; CCSP, 2008; Milly et al., 2008). These studies project future climate conditions with more frequent extreme precipitation events in many regions around the world, including parts of the United States. In particular, it has been projected that northeastern Illinois, including the Chicago metropolitan area, will experience more frequent and more intense rainfall events in the future (Markus et al., 2012), which 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 which are 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. In these efforts, effective communication of climate change impacts on urban watersheds/sewer sheds is needed. Data should be delivered at the watershed level in a form that can be incorporated in watershed planning at the community level. Delivery of useful climate change information is critical for community planning and adaptation to changing climate conditions. It is common practice that future climate projections, which are based on global circulation models (GCM), are 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. Furthermore, city managers and decision makers need quantifiable future risk to demonstrate the need for adaptive actions, such as retrofitting storm sewers and other water conveyance structures or adopting higher regulatory design standards within the community. This is not offered by the present climate modeling outputs. In this research, 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. In this method, future conditions climate data are analyzed to prepare precipitation maps for selected design storm frequencies which can be used to model future climate conditions of stormwater runoff and flood risk. This report presents a newly designed research framework to determine future conditions rainfall frequency maps, illustrating it in Cook County, Illinois, for the 24-hour duration rainfall event and for a range of recurrence intervals (also called return periods). Engineers commonly use these maps to determine the appropriate return period rainfall amount by interpolating between the isohyetals to evaluate options for storm and flood water management. Impacts of future climate conditions can then be convincingly demonstrated using conventional engineering to show changes in flooding frequency and extent, as well as damage comparisons associated with changing intense precipitation. Using standard and familiar models with future conditions precipitation scenarios facilitates communication of quantifiable future risk and supports community decision makers so they can plan, mitigate, and adapt to future conditions. This 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.
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