Rain Barrels and Other Household Stormwater Strategies Are Working — For Now.

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Science & Technology

Rain Barrels and Other Household Stormwater Strategies Are Working - For Now.

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In the last two decades coastal urban areas have taken steps to better minimize flooding and runoff by creating more permeable surfaces and encouraging residents to participate in water retention and use-reduction programs. These efforts, ranging from adding rain barrels and cisterns, to installing water-efficient fixtures, are making a difference, according to new research from Drexel University. And they may play an even more important role as sea level rise and the extreme weather effects of climate change increase the threat of flooding in these communities.

In their paper, recently published in the journal Urban Climate, a team of researchers from Drexel's College of Engineering, measured the efficacy of decentralized stormwater management strategies and, for the first time, established a baseline for including their impact in stormwater models used by municipalities and urban planners.

Their findings suggest that these household-level efforts can combine to reduce the volume of stormwater flooding and sewer overflows by 11-13%. It also indicates that these reductions can be sustained in the face of more frequent flooding due to climate change and sea level rise.

"This marks one of the first pieces of research to extensively model combinations of decentralized stormwater management strategies," said Amanda Carneiro Marques, PhD, an assistant professor in the College of Engineering, who led the study. "In order to address current and future water management challenges in coastal urban areas, it is essential to combine this data with modeling that accounts for more extreme weather events and sea level rise - which our research strives to provide."

These models, and the management strategies they generate, have become increasingly important for older coastal cities in the aftermath of severe weather events, like Hurricane Irene, Superstorm Sandy and Hurricane Ida. Cities like Boston, New York and Philadelphia are susceptible to a particular challenge due to their combined sewer systems in which stormwater and sewage flow into the same treatment system. This means that during extreme precipitation events, stormwater can overwhelm the system causing it to discharge raw sewage into a nearby body of water.

In accordance with the Clean Water Act's mitigation goals, municipalities are taking steps to prevent combined sewer overflows, including employing "circular" strategies that encourage water retention and percolation - rather than runoff - and help to minimize the overall amount of water flowing into the sewer system.

"When we talk about circular economy, most people think about recycling waste and materials. But the concept of circular economy also applies to water. Here we're talking about installing rainwater cisterns, reusing water from the sink to flush toilets and using water-efficient appliances. While those may seem like small steps at the individual level, we found that if they are widely adopted, they can have a significant collective impact," said Fernanda Cruz Rios, PhD, an assistant professor in the College of Engineering, who was a co-author of the research. "They help reduce demand on the municipal supply, and the energy needed to treat and distribute that water. But what we found was that these strategies offer a real double benefit. In areas like Philadelphia and Camden, that are particularly vulnerable due to their proximity to large bodies of water, we were able to see a meaningful reduction in flooding volumes when circular water strategies were adopted."

Despite the ongoing compliance efforts, the cities of Philadelphia and Camden have around 16 billion gallons of combined sewer overflow, according to reports from their respective water utilities.

Over the last two and a half decades, Camden has established a stormwater management plan, including decentralized strategies, an upgrade to stormwater infrastructure and storm water management modeling. To understand the overall impact of these efforts, in hopes of applying findings to other vulnerable urban areas, the Drexel team created a computer model of stormwater movement using data from Cramer Hill, a neighborhood of Camden in the state's "coastal hazard zone" that is highly susceptible to tidal and storm-surge flooding.

The researchers structured the model so that it could quantify the distinct effects of outdoor strategies (cisterns and rain barrels) and indoor strategies - efficient fixtures and reusing sink wash water (greywater) for toilet flushing. It simulated and measured the effect of 16 different combinations of strategies, ranging from rain barrels of different sizes, to varying levels of adoption of water-efficient fixtures and greywater reuse.

Each scenario was simulated using precipitation and tidal data from 2014 as a baseline and flooding measurements projected at locations across the neighborhood and at five stormwater outfall areas that contribute to combined sewer overflow discharges into the Delaware River.

The results show that the combination of retrofitting homes with water-efficient fixtures as well as installing rain barrels and practicing greywater reuse, when adopted by 75% of households, could reduce the volume of combined sewer overflows by as much as 11% and the volume of flood waters by up to 13%.

"This finding is an important and encouraging piece of information for municipalities that are facing similar challenges," Marques said. "Although broad adoption of these measures can be challenging to achieve, confirmation of its effectiveness may help to make the case."

To test the durability of decentralized strategies against future climate conditions, the team adjusted its model to show the effects of 10%, 20% and 30% increases in precipitation intensity and rises in sea level of 30 centimeters, 90 centimeters and 1.8 meters.

Their results indicated that both conditions expectedly exacerbate combined sewer overflows and flooding. Increased precipitation commensurately increased sewer overflow and flood volume, while higher sea levels also create a backpressure on the sewer system that prevents regular drainage and increases surface flooding. However, the decentralized strategies remained robust in the face of these challenges, reducing the increased volumes by 11-13% - indicating to the researchers that they could play a crucial role in future mitigation planning.

"Taken together, these findings show the need for integrated planning that complements the existing centralized combined sewer system, such as combining decentralized circular water intervention with centralized, system-wide infrastructure upgrades," Marques said. "No single intervention can fully address the intertwined challenges of combined sewer overflow and flooding, that is expected to further deteriorate due to climate change."

According to the team, future research should examine public perception and barriers to adoption of decentralized measures. It could also expand flooding models to study the water quality impact of combined sewer overflows and flooding originating from adjacent bodies of water. Refining the model as it is used in more areas will also improve its ability to project effective mitigation strategies that are a fit for each municipality.

"Our findings are a first step toward filling a significant knowledge gap -these strategies have rarely been evaluated in combination and stress tested against climate change and sea level rise projections," Marques. "With hydrologic and hydraulic modeling at block-by-block resolution, we believe this information could play an important role in resilience planning for many coastal urban areas."

This research was supported by the Consortium for Climate Risks in the Urban Northeast.

In addition to Marques and Cruz Rios, Drexel researchers Meghna Rajbhandari; Katelyn Singh, Franco Montalto, PhD; and Ahmad Haseeb Payab, PhD, contributed to this research.

Read the full paper here: https://www.sciencedirect.com/science/article/pii/S2212095526001185This is an RTE component

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