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Researchers gauge green infrastructure’s capacity for high runoff

University of Maryland, College Park, researchers are approaching urban and suburban stormwater management with the ultimate goal of increasing resiliency to major weather events. With models predicting more rain than historical levels, plus an increased frequency of particularly intense and destructive storms, they note, flooding is a major concern in communities that are becoming more settled.

In a new Journal of Water Resources Planning and Management case study, researchers examine two distinct watersheds and demonstrate that even small, decentralized stormwater management practices like rain gardens can make a big cumulative difference to watershed resiliency.

“What we design now is in place for 20 or 30 years, so we should design it with future climate conditions in mind as opposed to what the past rain has looked like,” says Environmental Science & Technology Assistant Professor Mitchell Pavao-Zuckerman. “This work puts emphasis on what’s happening in local upland spaces that has immediate implications for the people who are living in these watersheds for future flood mitigation, but connects this to the broader issues of how increased runoff links to the health of the Chesapeake Bay.”

He and graduate student Emma Giese take a practical look at what suburban areas are currently doing to manage stormwater, and provide evidence on how and why to implement green infrastructure based on how management systems will hold up in the future. They leveraged United States Geological Survey (USGS) data for two watersheds in Clarksburg, Md., a growing suburban town in Montgomery County. Each watershed has a distinct development history: One has several larger-scale detention ponds or stormwater basins for a more traditional approach to stormwater management, the other a heavy presence of smaller-scale green infrastructure like rain gardens, dry detention ponds, and sand filters.

Both watersheds were monitored before and after development to see the impacts of green infrastructure, and are near a weather monitoring station with readily accessible climate data. “Green infrastructure consists of things with a much smaller footprint than a stormwater basin, but there are more of them in the watershed, so it comes down to measuring the aggregated effect of a lot of small things in one watershed rather than one or two large things in another watershed,” says Pavao-Zuckerman. “Partnering with the USGS to have a good data source at the watershed scale and finding the right model for the question was key.”

To model future scenarios for the two watersheds, Pavao-Zuckerman and Giese enlisted College of Agriculture & Natural Resources Professor Adel Shirmohammadi. “Together, we were able to use the USGS data to train the Soil and Water Assessment Tool or SWAT model, taking into account the geography of the watersheds, slope, soil type, impervious surface, built versus open space, and other parameters to determine how much rainfall actually becomes runoff or flooding risk,” Pavao-Zuckerman observes.

The U.S. Department of Agriculture Agricultural Research Service and Texas A&M University AgriLife Research program developed SWAT as a small watershed to river basin-scale model. It simulates the quality and quantity of surface and ground water and predict the environmental impact of land use and land management practices. SWAT is widely used in assessing soil erosion prevention and control, non-point source pollution control and regional management in watersheds.

Using this model, Professor Shirmohammadi and Giese were then able to take climate change projection data for increased storm frequency and rainfall to run a variety of future scenarios and see how these different watersheds would manage. Ultimately, they found that the watershed with more green infrastructure was able to buffer and absorb more of the increased rainfall than the more traditionally designed watershed with larger stormwater basins. However, with larger or more intense rain events, both systems failed to handle the amount of precipitation successfully.

“We are seeing more large storm events so either the systems are overwhelmed or are still saturated by the time the next storm event comes,” says Pavao-Zuckerman. “This points to the need to plan for these more intense weather events in stormwater management infrastructure.”

To combat this issue, he and Giese found that increasing the capacity for some of the existing systems or increasing the presence of green infrastructure in the watersheds made them more resilient to future extreme rain events. With that in mind, they worked with Amanda Rockler, watershed restoration specialist and senior agent with UMD Extension, to provide insight into what was feasible to implement. “Our work allows us to see what the added return on investment in these different climate and stormwater management scenarios might be,” says Pavao-Zuckerman.

The full “Assessing Watershed-Scale Stormwater Green Infrastructure Response to Climate Change in Clarksburg, Maryland” article is posted at pavaozuckerman.wordpress.com/publications.