Bioretention Cell Performance Under Shifting Precipitation Patterns Across the Contiguous United States

Poster 221 – Click on poster below to view presentation from author.

Click on poster to view presentation from author.

Matthew Weathers

University of Tennessee, Knoxville

Co-Authors: Jon Hathaway, Andrew Tirpak, Anahita Khojandi

As climate change produces shifts in precipitation patterns, communities will need to understand how the performance of green stormwater infrastructure (GSI) may be impacted. Bioretention cells are one of the most commonly implemented forms of GSI for their ability to reduce peak discharge and filter pollutants. Projections in future climate indicate that bioretention cells may be at risk of losing their existing function due to deviations in precipitation frequency and intensity. As such, an ensemble of 10 regional climate models and 17 locations across the contiguous United States were evaluated to provide the widest range of potential future outcomes. Bioretention cells were modeled using USEPA’s Storm Water Management Model (SWMM) to compare observed (1999-2013) and simulated future (2035-2049) performances. To reduce model bias, simulated future climate data was bias-corrected using kernel density distribution mapping (KDDM). Median annual rainfall and 99th percentile rainfall event depths were projected to increase across all 17 locations while median drying period was projected to decrease for 11 locations, indicating fewer events with higher magnitudes of rainfall for a majority of locations. Correspondingly, bioretention cell performance decreased across all 17 locations. Relative percent changes in infiltration loss decreased between 4.0-24.0% across all 17 locations while overflow increased between 0.4-19.6% for 15 locations. Results suggest that bioretention cells in the southern United States are at significant risk of losing their existing function while those in the Midwest and Northeast are at moderate risk. Bioretention cells in the western and northwestern United States performed the best under future climate scenarios but could still lose their existing function if unchanged. This study provides insight on future regional bioretention cell performance trends that can be used to add resiliency to stormwater infrastructure.

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All posts are publicly visible after review by site administrator. Students’ responses to posted questions is factored into scoring for the poster competition. Finalists announced May 25 and awards presented May 26, 2021.

6 thoughts on “Bioretention Cell Performance Under Shifting Precipitation Patterns Across the Contiguous United States

  1. Great job! As we all are well aware, the 1″ storm continues to be the most commonly used storm event for design purposes, but especially in the south, it is far below what has become the normal two year storm event. Your paper underscores why these standards need to change as well as further exploration of retention cell designs.

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    1. Thank you Dennis! You are absolutely right about the change in the frequency of 1″ design storms and the subsequent impact on green infrastructure design. It will be interesting to see how design standards change in the future with so much research indicating a need for updated intensity-duration-frequency curves. And, as you and I both noted, the frequency of 1″ storm events already varies by region, so we may be at a point where the ubiquity of the 1″ design storm does more harm than good, leading to a greater emphasis on site-specific conditions.

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  2. Very nice presentation! I’m curious if you tried to associate what specific changes in the rainfall patterns where most responsible for the decrease in infiltration and increase in overflow. Was it increased intensity, depth, or some combination? Also, will this work be published in the future? I’m interested in digging into it in more detail. Thanks!

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    1. Thank you Rod! That’s an excellent question and one that we are working on fully answering. At this point, we are predicting the decreased infiltration and increased overflow are due a combination of the two factors you mentioned, increased intensity and depth.

      A few main reasons from our study for this prediction:

      1) projected decrease in the # of annual storm events
      2) projected increase in annual rainfall volume
      3) projected increase in drying period between events
      4) projected SIGNIFICANT increase in the frequency of upper percentile (>90th) precipitation events, which includes both rainfall intensity and depth

      In short, our projections show an increase in the frequency of upper percentile storm intensities and depths, but we don’t yet know if one factor (intensity or depth) is more dominant than the other. The simultaneous decrease in infiltration and increase in overflow has been an interesting topic of conversation for us as well. The most likely explanation we came up with is that the surface layer is filling up faster than the soil layer can infiltrate leading to a large volume of initial runoff never making it into the bioretention cell at all.

      This is part of the reason why I mentioned the need for future work to utilize additional bioretention cell configurations. If the surface layer is simply unable to contain enough runoff volume, then increasing the surface layer depth could be a potential solution if allowable within the specific municipality. However, it could also lead to more issues if the drainage time becomes too elongated.

      And yes – we are currently working on publishing this work! We haven’t sent the manuscript off yet though, so if you have any questions in the mean time please feel free to reach out!

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  3. Great presentation and interesting project. I was wondering if/how maintenance considerations were factored into the model? And what do you think explains such a decrease in infiltration across all modeled sites?

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    1. Thank you Natasha! Maintenance considerations were not factored into the model. The primary goal of this study was to enable direct comparison between observed and future bioretention cell performance under climate change, so as many variables as possible were either kept constant or removed. However, maintenance is an excellent question and definitely has the potential to affect the performance if clogging were to occur early on in the life cycle of the bioretention cell. Maintenance strategies would be an excellent addition to future work to enable a realistic comparison of how bioretention cells could perform in the future under both varied climate change scenarios and maintenance strategies.

      Regarding your question on the decrease in infiltration, the reason is most likely due to the SIGNIFICANT increase in the frequency of upper percentile (>90th) precipitation intensities and depths. For example, median 99th and 99.9th percentile event depths increased more than 50% for nearly all locations from the observed (1999-2013) to the future (2035-2049) period. As such, the most realistic explanation for a decrease in infiltration with an increase in upper percentile events is that the surface layer is filling up faster than the soil layer can infiltrate, disallowing surface infiltration and subsequent decrease in native soil infiltration.

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