Water Works: Modern Stormwater Management Requires Looking at All Tools

Green stormwater infrastructure (GSI) is increasingly recognized as a more-sustainable approach to stormwater management vs. traditional or even newer flow-through treatment practices. Definitions vary, but GSI practices often include vegetation and soil designed to infiltrate stormwater runoff onsite, preventing it from running off into receiving waters.

Innovative practices—such as proprietary manufactured treatment devices (MTDs)—can be an effective alternative where space is constrained, but they often aren’t considered on par with GSI within prescriptive regulatory frameworks even when they share similar pollutant-removal processes. This oversight reduces compliance flexibility and unnecessarily drives up development costs. Relying exclusively on GSI for stormwater management presents several additional challenges within the site-development context that should be considered when establishing state and local stormwater policy.

Technical and Design Limitations

GSI systems typically are distributed throughout a site to intercept rain closer to where it falls and designed with large surface area (for ponding and treatment) relative to the contributing drainage area. These features can make implementing GSI in dense, mostly impervious urban environments with high land costs challenging. Since GSI frequently relies on vegetation and in-situ soil permeability to perform its function of pollutant capture and runoff reduction, areas with poor soils, high groundwater tables, limited subsoil infiltration rates, and/or steep slopes make poor candidates for GSI design and implementation.

In addition, GSI practices tend to be less effective at mitigating peak flows and flood risks from larger storms on their own, compared to several conventional stormwater practices. For example, in a 2021 paper by Weathers, et al. (iimag.link/oFbjV), researchers from the University of Tennessee at Knoxville and the Ohio State University concluded that although bioretention systems, acting as a proxy for all GSI, “following current design strategies may continue to provide some runoff mitigation, shifting precipitation patterns, including more intense rain events, reveal limitations in their ability to maintain desired performance under future climate conditions.” With extreme rainfall events becoming more frequent due to climate change, GSI alone may not adequately manage large precipitation events moving forward.

Maintenance Concerns

GSI practices, like all post-construction stormwater control measures (SCMs), require maintenance to function correctly. Numerous GSI practices can quickly transition from an effective solution to an ineffective one if the vegetation isn’t available to assist with pollutant removal due to die-off or the media clogs prematurely from lack of effective pretreatment. The sensitivity to pollutant loading and visible accumulation of trash and debris often necessitates more-frequent inspection and maintenance.

Decentralized GSI designs often include multiple practices dispersed throughout a site vs. reliance on a single end-of-pipe solution. This can lead to higher mobilization, inspection and restorative maintenance costs (in some cases) in comparison to maintaining a single SCM treating the entire site.

Material Sourcing and Installation Inconsistencies

Many GSI practices are constructed based upon material specifications that, if followed, should enable pollutant removal performance expectations to be met. This expectation makes consistent material sourcing critical, yet component availability can be highly variable based upon local geography or origin, supplier and supply chain issues. This broad variability can significantly impact the performance of GSI practices.

The correct installation of GSI practices also is critical to meet pollutant-removal performance expectations. Contractor experience plays a major role in determining a successful outcome, with those less familiar with GSI more likely to make a mistake such as using heavy equipment to compact portions of the site designated for infiltration-based SCMs. Timing also plays a role, with GSI practices requiring installation after a site is completely stabilized to protect them from exposure to excess construction sediment and the risk of premature clogging.

Pathway for Acceptance

Creating a pathway for alternative or innovative SCMs is an important program and policy decision point. Stormwater-management compliance today requires more tools, not less, because sites and projects are more challenging to develop. These types of policies are inclusive and economically principled ones that can be accomplished without sacrificing critical water-quality compliance goals. Time is money in the development arena. Reasonable pivots to well-vetted SCMs are needed when site conditions aren’t conducive to constructing GSI practices.

All SCMs should be properly vetted before being allowed for use. Data from the 2020 International BMP Database Summary Report concludes that certain proprietary categories perform equally—or in some cases better—at reducing targeted pollutants than similar-functioning GSI systems. Policy around SCM doesn’t need to be mutually exclusive; these two kids can play nicely together in the sandbox.

Overcoming the challenges of prescriptive GSI-focused policies may require a multi-pronged strategy to foster the acceptance of innovative SCMs.

1. Update regulatory frameworks. Stormwater regulations should be revised to include performance standards that reflect the continued use of GSI where conditions warrant but allow proprietary MTDs and other SCMs proven to meet performance expectations to be utilized when justified by site constraints such as those previously described.

2. Vetting SCM performance. Vet and accept innovative SCMs capable of meeting minimum performance thresholds through approval reciprocity with the nationally recognized field and laboratory monitoring programs.

3. Allow pilot projects and research. Demonstration projects can validate the effectiveness of innovative BMPs, allowing for real-world testing, data collection, and community visibility and buy-in. Partnerships with academic institutions and/or manufacturers often can help ensure minimum performance benchmarks are met or exceeded and facilitate successful technology transfer to the marketplace.

4. Utilize hybrid designs. GSI practices and MTDs are frequently used individually and in treatment trains to meet water-quality goals. However, integrated together they can optimize the best aspects of each type and potentially create more resilient SCMs for water quality and quantity in the future.

5. Ensure maintenance access. All SCMs require maintenance. Ensure access to the system and all internal components are accounted for during the design phase so future maintenance activities are easier. Require maintenance plans that accurately describe the entire process necessary to remove all captured pollutants be included in product submittals and made available to site owners and the design community.

JACOB DORMAN Water Works