Stormwater runoff bioretention system designed to combat urban flooding

Technology captures and treats runoff before reaching watercourses so that it infiltrates the soil at sustainable flow rates.

By Elton Alisson  |  Agência FAPESP – Researchers at the University of São Paulo’s São Carlos Engineering School (EESC-USP) in Brazil are working on a sustainable urban rainwater drainage system designed to mitigate flooding in cities.

Some of the results from their research, performed as part of a Thematic Project supported by FAPESP, were presented at a conference on “Water, Megacities and Global Change” held in early December at UNESCO in Paris as a side event to the United Nations Climate Change Conference (COP21).

“The system captures stormwater runoff before it’s discharged directly into a river or stream so that it can be treated and then allowed to infiltrate the soil at a sustainable flow rate, mitigating the risk of flooding,” said Altair Rosa, an engineer who is participating in the project, in an interview given to Agência FAPESP.

The drainage solution entails the construction of permeable underground filters in flood-prone areas, with alternating layers of filter media consisting of grass, sand, gravel and geotextile membranes. The filters take out pollutants and temporarily detain excess runoff.

Technically speaking, this is a bioretention system. It functions as a detention pond to attenuate flow, allow sediment to settle, and process stormwater runoff biologically. Then, as inflows diminish, the water is released and naturally absorbed into the soil by infiltration.

According to Rosa, who is participating in the project via a scholarship awarded by FAPESP, the system mitigates the risk of urban flooding by controlling runoff and lowering peak flow rates into basins or microbasins, such as rivers and streams, in urbanized areas.

“The system also helps sustain the water table, and piping won’t be needed for that purpose,” Rosa said. “It retains and treats stormwater that would otherwise be wasted.”

The top layer consists of vegetation, designed to detain runoff without causing erosion.

Like the layers of sand, gravel and geotextile, the vegetation layer stops pollutants from entering the water table, subsoil or watercourses.

“The runoff is filtered and treated as it passes through all these layers, so it’s much cleaner when it reaches the water table,” Rosa said.

Performance

To assess the system’s performance, the researchers mapped eight critically flood-prone areas on the University of São Paulo’s campus in São Carlos, São Paulo State, and selected one of them for implementation of the system.

To try to forecast whether the system would have the capacity to store the amount of rainwater expected to fall in the area where it was installed, they used a new method of drainage system design developed under the aegis of the project in collaboration with groups at other universities and research institutions.

Among the distinctive features of the method is the use of simulations based on future climate scenarios and georeferenced data for plant cover, degree of urbanization, and new buildings planned, among other considerations.

In addition to rainwater quality and quantity indicators, the design method also works with estimates of contamination risk, which is not considered in traditional methods, according to the researchers.

The resulting construction process is modular and staggered, ensuring that the drainage system is scalable and cost-effective and that it operates efficiently.

“This method of designing modular scalable drainage systems is crucial for countries like Brazil, where urbanization is proceeding at a rapid pace and heavy rainfall is frequent,” said Eduardo Mario Mendiondo, a professor at EESC-USP and principal investigator for the project.

Preliminary results demonstrated the new design model’s accuracy in predicting the amount of stormwater runoff the bioretention system can store with a satisfactory safety margin, even at peak flow rates.

The system implemented on USP’s São Carlos campus also proved capable of retaining all the stormwater runoff captured in recent weeks, including in late December, when daily rainfall in the area exceeded 60 mm, more than the average for the last 80 years.

“We found that even with unexpectedly high levels of runoff, the system worked well and was able to retain all the water it received,” Rosa said.

The researchers installed sensors to monitor water quality and quantity at the bioretention system’s entrance and exit, as well as a real-time data transmission system for command and control of its operation.

“The idea of using these sensors is that in the near future the advent of smart city technology will enable citizens themselves, or people who live in an apartment building, for example, to control the treatment of water pollution using a bioretention system like this one,” said Mendiondo, who is also head of research and development at the Natural Disaster Surveillance & Early Warning Center (CEMADEN), subordinated to the Ministry of Science, Technology & Innovation (MCTI).

Preliminary analysis showed that the monitoring sensors accurately measured levels of metals in the water received by the system, as well as substances such as nitrite, nitrate and phosphorus.

“The system is relatively new, so water quality and soil infiltration will steadily improve as time goes on,” Rosa said.

Advantages

According to the researchers, the advantages of their sustainable drainage system include the possibility of modification and expansion depending on the degree of urbanization in a particular area, the low cost, the lack of interference with the landscape, and the ability to control pollution in areas with heavy vehicle traffic because pollutants are swept away with stormwater runoff into the system.

It can be used in conjunction with existing urban drainage systems based on canalization of watercourses, and it can also be used for effluent treatment.

“Instead of just putting conventional drains in a street, for example, you can link them to this sustainable drainage system, so that in addition to bearing away the stormwater runoff you make sure it’s absorbed by the soil while filtering out part of the pollution created by constant urbanization,” Mendiondo said.

Sustainable drainage systems using a combination of engineering, architecture, landscaping and chemistry techniques, among others, are well-known in France, Australia and the US, where they have been in use since the 1990s, he added.

In Brazil, they are still new, although they have been studied by his group at EESC-USP for the past ten years on the basis of pilot experiments such as the ongoing trial on USP’s campus in São Carlos.

“These compensatory techniques can contribute viable elements to the adaptation plans that are increasingly being implemented in countries already being obliged to manage potential risks of disasters arising from excessive urbanization and the impact of flooding on cities,” Mendiondo said.

According to Mendiondo, these techniques offer inputs for further research while also promoting long-term change in the urban drainage systems used in Brazil. The latter have generated conflicts due to lack of maintenance, obsolescence, and failure to avoid disasters and flooding.

“Flooding causes economic and human losses,” he said. “The National Civil Defense Policy and the global disaster risk reduction framework, which helped frame discussions at COP21, prioritize action to mitigate the impact of flooding.”