A new study finds that a high density of green stormwater infrastructure can provide enhanced mitigation of peak flows and runoff volumes compared to large, detention-based stormwater control practices.
Green Stormwater Infrastructure to Reduce Suburban Runoff
The Issue with Runoff
Across the United States, suburban development is replacing agricultural and forested lands. In urban and suburban areas, large amounts of stormwater runoff are generated from rooftops and roadways during rain events. Runoff is quickly piped to streams and rivers, leading to flash flooding, stream bank erosion, and damages to stream health. Reducing nutrients, sediment, and contaminants in stormwater is a restoration priority for large estuaries such as the Chesapeake Bay.
Stormwater control practices are installed on the landscape to delay, capture, and filter runoff before it reaches streams and rivers. The types of practices installed to manage runoff have shifted from large practices that focus on storing and delaying runoff (for example, ponds) to a wide variety of smaller, nature-based practices that use soil and vegetation to replicate natural functions such as infiltration and evapotranspiration. Bioretention gardens that soak up and filter runoff are one example of nature-based, green stormwater infrastructure practices.
The U.S. Geological Survey worked with Montgomery County, MD to better understand the effectiveness of selected stormwater practices. Fourteen years of streamflow monitoring in Clarksburg, Maryland, a suburb of Washington, DC, within the Chesapeake Bay watershed, provided an opportunity to investigate how the amount and timing of runoff changed as the area transitioned from agriculture to suburban development with a high density of green stormwater infrastructure practices. The study included a forested control site, an urban control site with large, detention-based stormwater control practices, and two treatment sites that transitioned from agriculture to suburban development with green stormwater infrastructure.
This study asked two main questions:
- How does the amount of runoff change when an agricultural area is developed into a suburban neighborhood with a high density of green stormwater infrastructure?
- Can suburban development with a high density of green stormwater infrastructure store and infiltrate enough runoff to replicate forested conditions?
Key Study Findings
The study found that a high density of green stormwater infrastructure can provide enhanced mitigation of peak flows and runoff volumes compared to large, detention-based stormwater control practices (urban control site). However, green stormwater infrastructure was not able to completely replicate forested conditions across a range of precipitation events.
- The amount of precipitation needed to trigger a streamflow response at the green stormwater infrastructure sites was similar to that at the forested control site and was two to three times greater (0.5–0.6 inch) than that at the urban control site (0.2 inch).
- Runoff volume and peak streamflow at the green stormwater infrastructure sites were typically lower than those at the urban control site for precipitation events less than 0.8 inch.
- Streamflow changes were more severe at the green stormwater infrastructure site with more impermeable surfaces (44 percent of the area), even with twice the density of green stormwater infrastructure, as compared to the green stormwater infrastructure site with fewer impermeable surfaces (33 percent).
Green stormwater infrastructure can reduce the effects of suburban development on streams by storing and infiltrating runoff on the landscape before it reaches streams. Green stormwater infrastructure in the study area was particularly effective at mitigrating runoff for precipitation events less than 0.8 inch. This amount is considerably lower than the stormwater pond design criteria (1-year, 24-hour event equivalent to 2.6 inches of precipitation) but similar to the 1-inch design criteria for all other stormwater practices. The amount on impervious surface in a watershed was an important influence the on the effectiveness of stormwater controls. Additional green stormwater infrastructure would be needed to maintain pre-development hydrologic conditions for precipitation events larger than 1 inch.
Hopkins, K.G., Bhaskar, A.S., Woznicki, S.A., and Fanelli, R.M., 2019, Changes in event-based streamflow magnitude and timing after suburban development with infiltration-based stormwater management: Hydrological Processes, https://doi.org/10.1002/hyp.13593.