Combining green and gray infrastructure for stormwater management and flood risk reduction: Case studies from two urban areas in North Carolina

Urban areas across the country are grappling with the dual challenge of managing stormwater runoff from increasingly severe weather events while maintaining or improving water quality to meet regulatory requirements and community demands. In cities like Raleigh and Fayetteville, North Carolina, this challenge is compounded by rapid growth, aging infrastructure, and complex hydrology. A new wave of resilient stormwater strategies is emerging — where green and gray infrastructure converge to create solutions that are not only technically sound but also environmentally and socially sustainable.

This article, rooted in two real-world case studies, explores how the integration of nature-based features with engineered infrastructure is redefining stormwater management. The goal: reduce flood risk, improve water quality, and enhance community amenities and resilience — principles that are cornerstones of our stormwater profession’s future.

Green meets gray: A resilient approach to urban hydrology

Green infrastructure (GI) leverages natural processes to capture, filter and slow stormwater runoff through techniques like rain gardens, bioswales and green roofs. Gray infrastructure, the bedrock of traditional stormwater systems, includes culverts, concrete conveyance channels, pump stations and underground storm sewer networks. While green practices excel at improving water quality, mitigating frequent, low-intensity storms and enhancing local ecosystems, gray infrastructure remains essential for handling the high flows associated with extreme events.

The convergence of green and gray offers a holistic strategy: one that enhances performance, ensures reliability, and supports multifunctional landscapes. As project goals increasingly call for climate resilience, water quality compliance, and community co-benefits, green-gray hybrid systems are becoming the gold standard.

Richland Creek Watershed, Raleigh: Managing urban growth in a changing landscape

In Raleigh, the Richland Creek Watershed intersects with the rapidly developing Blue Ridge Corridor, a district poised to become a hub of healthcare, education and innovation. However, with densification comes increased impervious cover, larger runoff volumes, and water quality degradation. Recognizing these risks, a collaborative team of researchers and municipal stormwater professionals conducted an in-depth assessment of future land use impacts.

Geospatial and hydrologic analyses revealed the following:

• Up to a 50% increase in impervious surfaces under full corridor build-out.
• Proportional spikes in runoff volume and nutrient loads while rainfall levels remain constant reflects development impacts on watershed water quality and quantity.
• Strain on aging drainage infrastructure not designed for 21st-century development pressures.

The proposed solution: a suite of green-gray practices tailored to the watershed’s evolving character. The GI tools explored included:

• Bioretention cells to intercept and treat runoff from redeveloped commercial lots.
• Green roofs atop institutional and medical facilities.
• Constructed wetlands strategically placed near stream corridors.

A constraint of this case study was that site construction had already begun with design plans for site buildings and underground storage already drafted. The challenge was to find where, within the existing plans, could GI fit while also providing enough functionality and benefits worth construction. Conversations with the city, external stakeholders, and the contractor were held to develop a series of GI concepts that could meet all site constraints and stormwater needs.

Simulations demonstrated that the addition of GI for stormwater management could reduce total runoff volume by up to 30% while capturing significant pollutant loads. When paired with targeted gray infrastructure upgrades, including culvert enlargements and pipe retrofits, the system provides redundancy and resilience, reducing localized flooding while restoring watershed function. This case study documents the key role of collaboration, both between green-gray practices and frequently engaged, multi-jurisdictional stakeholders.

Blounts Creek Watershed, Fayetteville: A legacy system meets modern pressure

Fayetteville’s Blounts Creek Watershed presents a different story. Here, an undersized and aging stormwater network intersects with an undersized and urbanized creek — creating a chronic flood risk for residents, especially those within low- to moderate-income neighborhoods near the city’s downtown core. The challenge: mitigate recurring flooding driven by a combination of riverine surges and stormwater system backups.

Detailed 1D and 2D collection system and riverine modeling was performed for the area as part of the Blounts Creek Watershed Master Plan. It pinpointed the Russell Street and Person Street bridges as critical pinch points in the area’s drainage network. These structures, constrained by outdated spans and inadequate clearance, triggered backwater effects that rippled upstream into the stormwater collection system, causing nuisance flooding during small storms and significant structure flooding and road overtopping during extreme events.

The solution: the Russell-Person Street Bridges and Stream Improvements Project, a multiphase gray-green initiative that exemplifies integrated thinking. It includes:

• Bridge replacements at each stream crossing sized to future hydrologic design standards;
• Channel excavation and regrading to improve conveyance;
• Natural floodplain benches, pools and riparian restoration to reduce peak flows, improve water quality and enhance habitat and;
• Enhanced community green space, tie-in to existing green streets, and improved pedestrian features to foster neighborhood connectivity.

With nearly 200 homes projected to be mitigated from flooding impacts and almost all road overtopping eliminated up to the 100-yr design storm, this solution, which is currently under design, is not only technically feasible but prioritizes social issues like equitable resilience in a historically vulnerable area. Long-term, the improvements unlock system capacity for future upgrades and establish a new benchmark for infrastructure co-benefits.

Takeaways for stormwater professionals

As practitioners in stormwater management, the case studies from Raleigh and Fayetteville underscore several actionable insights:

1. Hybrid systems are the future

Designers must move beyond siloed green or gray solutions and embrace blended strategies. Integrated systems provide flexibility and layered benefits from flood control to improved water quality and urban cooling.

2. Data-driven planning pays dividends

Land use modeling, hydrologic and hydraulic simulations, and infrastructure assessment and prioritization are critical for identifying vulnerabilities and making informed design decisions.

3. Prioritize resilience for all citizens and communities

While infrastructure investments must be cost effective and adequately mitigate drainage impacts, they should also focus on ensuring that all citizens and communities benefit from enhanced drainage, environmental quality and improved livability.

4. Infrastructure can inspire

By weaving enhanced green space, access, and aesthetics into stormwater projects, cities can deliver triple-bottom-line outcomes that transcend compliance and elevate quality of life.

Conclusion: Scaling smart stormwater solutions

North Carolina’s urban watersheds offer a compelling narrative of what is possible when municipalities embrace innovation. The stories of Richland Creek and Blounts Creek show that green-gray infrastructure is not a concept of the future – it is a present-day imperative.

By marrying ecological insight with engineering precision, these communities are demonstrating how stormwater systems can be reimagined to meet the climate, social, and economic demands of the 21st century. For cities across the U.S., the message is clear: the path to resilience runs through integrated infrastructure.

References

1. Fischer, S., Sheldon, J., Jacob, J., & Jing, Y. (2022). Land Use and Stormwater in Richland Creek Watershed. North Carolina State University.
2. City of Raleigh Stormwater Management Division. (2023). Blue Ridge Corridor Planning Documents.
3. City of Fayetteville Engineering and Infrastructure Department. (2023). Blounts Creek Watershed Analysis and Russell-Person Street Project Planning Report.
4. S. Environmental Protection Agency. (2022). Green Infrastructure Design and Implementation Resources.
5. American Society of Civil Engineers (ASCE). (2021). Manual of Practice No. 77: Urban Stormwater Management in the United States.