Gray, no, green! Green, no, gray! No, this is not the latest dress color controversy from nearly a decade ago. Instead, it is a common refrain heard among stormwater management practitioners when discussing water quality and water quantity infrastructure needs.
Green is green infrastructure (GI) which includes bioswales, bioretention cells, such as rain gardens, and other practices that mimic the natural hydrologic cycle through their pollutant removal processes. Gray infrastructure is more traditional storage and conveyance systems like pipe networks and hard channels that don’t allow runoff to infiltrate into the ground. As communities learn to adapt to future infrastructure needs related to climate change, particularly increases in storm volumes and frequency, both infrastructure types are necessary because existing research indicates that neither alone will fully address impacts.
Stormwater practitioners in areas already impacted by climate change are attempting to design stormwater systems that strike a balance between current and future stormwater needs, regulatory requirements and costs. Designing with these variables in mind is like forecasting the future. The challenge is evaluating whether a system designed today can meet tomorrow’s needs.
Green infrastructure is a commonly used compliance tool promoted as a best practice for addressing climate change impacts. However, GI may not be the sole answer to the problem. Emerging studies indicate that climate change poses significant risk to GI practices, specifically that GI will not be able to continue to effectively manage urban stormwater runoff under current design standards.1
If bioretention, which is considered the gold standard for GI practices, will be challenged to store and treat future runoff amounts, what about other structural best management practices (BMPs)? Can anything be done to bring more certainty to this very uncertain scenario? Optimizing the use of the full suite of currently available tools to provide treatment and volume capacity as well as reduce costs may help stem the coming tide.
Hybrid infrastructure offers a potential solution. Hybrid infrastructure is a compliance strategy that blends natural systems with conventional ones to produce adaptable, optimized BMP designs capable of achieving desired co-benefits.
A 2018 study published by the United States Environmental Protection Agency (EPA) concluded that “for overall post-treatment site-scale performance, simulations using both conventional and green infrastructure BMP scenarios generally remove more runoff volume and pollutant mass under future climate conditions (increased precipitation and runoff) compared to current conditions.”2 The study expands on that, stating: “GI practices that rely on treatment without volume storage will be at a disadvantage for climate change adaptation, but approaches that rely only on adaptation of conventional practices may not have the flexibility to address multiple performance objectives.”
In the EPA study, the hybrid infrastructure approaches varied between the modeled sites. A few combinations studied included infiltration practices and permeable pavement with a dry detention basin; distributed bioretention with a dry detention basin; and green roofs, permeable pavement and bioretention with an underground dry detention basin. While these are primarily examples of land-based, structural BMPs, the study’s conclusions also apply to project designs that utilize proprietary treatment practices, also known as manufactured treatment devices.
Mitigating increased runoff from new and redevelopment projects under future climate projections is a major obstacle. A critical design consideration involves addressing the creation of additional volume capacity while still effectively integrating it into the overall site design.
Biofilters Plus Storage as Solution
There is no one size fits all approach. In a surface-based application, a project designer may elect to increase the filter surface area of a bioretention practice or retention pond if land is available. However, on a space constrained site, such as a limited right of way (ROW) within a residential subdivision or other highly impervious sites, including redevelopment parcels, alternatives to conventional surface-based solutions are needed (Figure 1). To maximize co-benefits of GI in this example, an engineered high flow rate biofilter could be paired with an underground BMP infiltration gallery or detention system in a more compact footprint to meet the desired quantity and quality goals.
The upstream high flow rate biofilter provides flow-through treatment capturing pollutants such as sediment, nutrients, metals and trash at the source while the downstream system provides runoff reduction or peak flow mitigation (Figure 2). Since the high flow rate biofilter treats stormwater at higher media flow rates than traditional bioretention, it uses a smaller surface area to manage larger drainage areas and increased runoff volume thereby creating more usable space on-site.
Pollutants filtered out by the high flow rate biofilter are prevented from migrating downstream and are easily accessed for maintenance purposes. This sequestration has the added benefit of protecting the downstream infiltration gallery from premature clogging which extends the system’s useful life span. With system longevity increasingly gaining in importance as a design element, failing to consider the need to preserve in-situ infiltration rates could result in additional runoff storage volumes being regulated in the future if performance expectations go unmet.
Several areas of the United States already impacted by climate change have regulatory frameworks that support the used of hybrid infrastructure as a solution now and into the future. The State of Maryland’s Department of the Environment allows proprietary biofiltration to be designed in conjunction with infiltration components to meet required environmental site design compliance standards. Additionally, in Virginia Beach, Virginia, USA, new stormwater regulations passed in 2020 require design storm depths to use NOAA Atlas 14 plus 20% for BMP design and the city includes a mix of green and gray infrastructure within their own flood mitigation strategies to address that increase. These two examples are not all-inclusive, yet they establish a realistic baseline for what other communities can consider incentivizing hybrid infrastructure as a stormwater management tool.
It is reasonable to think that addressing climate change may automatically increase overall project costs. Installing BMPs with larger surface areas or using larger diameter pipe sizes sounds expensive. However, that may not always be the case. The basis of hybrid infrastructure is rooted in flexible designs.
In the earlier limited residential subdivision ROW example, the high flow rate media uses significantly less surface area footprint at a cost savings over a conventional, surface-based bioretention system. Different pipe materials can also be selected to lower costs further.
Amenities like new green space or additional parking stalls can be installed where a surface-based BMP would otherwise be located. Each value-added amenity may offset a portion of the required stormwater management costs. This concept is particularly prudent when considering linear retrofit projects with limited rights-of-way (Figure 3). High flow rate biofiltration systems can be installed in existing curb lines to treat the new impervious area while the additional runoff volume can either be accepted via the existing storm sewer network if capacity exists or be moved offline into a secondary pipe network to infiltrate or be slowly released back to the existing storm network over an extended period. Utilizing a hybrid infrastructure solution in this case would eliminate costs associated with purchasing land for the installation of a large surface-based stormwater detention facility.
Hybrid infrastructure is not a panacea for all future stormwater problems. However, it can address many concerns related to future water quantity projections and associated water quality issues. Incorporating pathways for hybrid infrastructure utilization today through flexible regulatory frameworks has the potential to mitigate tomorrow’s problems with a little planning now. Future communities will look back and thank us for proactively addressing this concern.
References
- Tirpak RA, Hathaway JM, Khojandi A, Weathers M, Epps TH. 2021. Building resiliency to climate change uncertainty through bioretention design modifications. Journal of Environmental Management, Volume 287, 2021, 112300. Available online at https://doi.org/10.1016/j.jenvman.2021.112300 (https://www.sciencedirect.com/science/article/pii/S0301479721003625).
- U.S. EPA (Environmental Protection Agency). 2018. Improving the resilience of BMPs in a changing environment: urban stormwater modeling studies. Office of Research and Development, Washington, DC; EPA/600/R-17/469F. Available online at http://www.epa.gov/research.
About the Expert
Jacob Dorman is the regional regulatory manager for Contech Engineered
Solutions LLC.
