Incorporating Climate Resiliency into Common Stormwater Control Designs
- Matthew P. Jones PhD, PE - Hazen and Sawyer
Stormwater and efforts to manage its impacts are intrinsically linked to climate. As climate change projections have advanced, there are increasing interests and needs to consider climate resiliency in the design of stormwater controls. To date, stormwater management projects along the coast and those that are especially large in scale have been a major focus of climate resiliency efforts; however, there are benefits in considering climate change across a wide array of stormwater improvements. The long-term functionality of stormwater controls focused on localized detention, retention, and water quality improvement can be impacted by changing rainfall patterns and other aspects of climate change. Although the design of these common stormwater controls are impacted by the same underlying climate changes as coastal and large scale projects where resiliency considerations are more common, the application of tools and analysis procedures to these more common projects differs substantially with regards to the type of climate change impacts considered and the level of analysis required.
The ability to consider sea level rise in the design of coastal stormwater projects is becoming more practical as modeling analyses improve and an expanding array of accessible analysis tools are developed. While these tools provide many benefits for long-term planning and design efforts in coastal areas, they offer limited utility in upland areas where sea level rise does not have direct impacts. In these areas, changes in annual rainfall volume and the distribution and intensity of storms are among the principal impacts on stormwater controls. Problematically, there are few widely agreed upon analysis protocols to account for changes in the intensity of design storms (Kang, et al., 2016). The analysis procedures that do exist often involve detailed evaluations of past rainfall records and climate projections, along with calculations and modeling efforts that are fairly complex in the context of the design of a typical stormwater control. Due to the level of effort involved in these evaluations, they may be justified for a large-scale project or area-wide planning initiative, but not necessarily the design of common stormwater controls. To address these challenges, an assessment of available analysis tools within the U.S. Climate Resiliency Toolkit was conducted to provide insight into how climate resiliency can be considered in an appropriate context for the design of common stormwater controls, such as bioretention, permeable pavement, and stormwater wetlands. Additionally, a presentation framework was developed to facilitate the inclusion of climate resiliency considerations in the design of stormwater controls and effectively communicate analysis results to a diverse audience.
The National Oceanic and Atmospheric Administration’s (NOAA’s) Climate Explorer is one tool that is readily accessible by stormwater design professionals and provides information on projected precipitation changes. Specifically, the Climate Explorer presents localized information on mean daily precipitation and the number of days in a year with precipitation depths in excess of 1 inch. Of these parameters, changes in the number of days with precipitation depths in excess of 1 inch is especially relevant to stormwater control design, as it serves as an indicator of the frequency of heavy precipitation events, as well as potential changes in the depth of the water quality design storm. Downscaled results are presented from Coupled Model Intercomparison Project Phase 5 (CMIP4) for Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5 for historical conditions through 2100.
EPA’s CREAT Climate Scenarios Project Map presents relative changes in annual precipitation depths and 100-yr storm intensities, which can inform the relative sizing of both water quantity and quality stormwater controls. EPA’s National Stormwater Calculator also provides projected changes in monthly rainfall and annual maximum daily rainfall, which can be applied in a similar manner to inform relative stormwater control sizing.
A series of design examples illustrates that the impact of climate change, informed by these tools, on stormwater control design depends upon the type of stormwater control, geographic area, climate modeling scenario, and time period of analysis. Unlike some other design considerations, climate change impacts on stormwater control design tend to be more qualitative and general in nature, due to the inherent uncertainty and limited detail of these analyses. This is especially the case when using readily accessible tools to serve as indicators in the absence of more thorough analyses.
Whenever considering climate change in the design of stormwater controls, it is important that the impacts on design be effectively communicated to project stakeholders given the nature of the impacts and varied understanding of climate change analyses. To facilitate this, a series of adaptable design fact sheets were prepared that touch on key aspects of climate resiliency impacts on stormwater design. Topics covered include background on RCP scenarios, potential climate change impacts on stormwater control functionality, the role of analysis time periods, and the effect of uncertainty on analysis results and utility. These fact sheets were developed to serve as templates that can be populated with information specific to a particular design and accompany a design submittal with background and justification for any climate considerations to an array of stakeholders.
Using existing tools accessible to typical stormwater design professionals, it is realistic and beneficial to consider general climate trends for a specific geographic area and utilize that information to inform the type of stormwater controls to implement, relative sizing, and other design considerations. Generally, the impact of these analyses on a design will be more general or qualitative in nature than may be preferred by some designers, due in large part to the inherent uncertainty of such analyses; however, consideration of climate change impacts, even in a limited fashion, is expected to provide more robust, long-term effectiveness for stormwater controls.
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