Flooding caused by severe rains has motivated planners to devise a variety of novel solutions, ranging from planting rain gardens to erecting green roofs.
The Civic Garden Center of Greater Cincinnati features a green roof that helps collect rainwater. Image Credit: Andrew Higley/UC Marketing + Brand.
However, in many circumstances, nothing works better than a basic hole in the ground—a detention basin. This is the conclusion of a study conducted by University of Cincinnati geography students in partnership with the Hamilton County Conservation District.
Man Qi, the lead author and a Doctoral Student at the UC College of Arts and Sciences, stated that cities are inventing new techniques to capture rainwater and direct it where it is most needed, especially during droughts. Low-impact development approaches include innovations such as permeable pavement, which allows rainfall to sink into the ground rather than channeling it elsewhere.
A bioretention cell made of ornamental or landscaping plants over soil particularly designed to drain quickly spread over a thick layer of gravel that does the same is another innovation. This absorbs a lot of rain without causing standing pools of open water.
Rainwater must be kept from leaking onto neighboring properties or roads in new commercial or residential developments. Because hard surfaces such as buildings and parking lots cannot absorb heavy rainfall, water must be collected or diverted to avoid property damage.
A detention pond is a common practice. It temporarily stores the water and releases it into the air or the groundwater or nearby streams at a low rate to reduce the risk of flooding. It also provides some ecological benefits.
Man Qi, Study Lead Author and Doctoral Student, College of Arts and Sciences, University of Cincinnati
Qi collaborated with the Hamilton County Conservation District to assess the efficacy of flood-prevention strategies like detention basins and bioretention cells in five different scenarios.
Qi presented their findings at the annual meeting of the American Association of Geographers in Denver.
In residential areas where the impervious area is less than 40%, low-impact development practices are better. But if 70% or more of the ground surface is impervious, it’s best to put in detention basins. The flood risk can be greatly reduced.
Man Qi, Study Lead Author and Doctoral Student, College of Arts and Sciences, University of Cincinnati
Climate change is predicted to increase the frequency and severity of rainstorms, making purposeful drainage planning even more necessary in cities, according to Professor Lin Liu, co-author of the paper and Co-Director of UC’s Joint Center of GIS and Spatial Analysis.
Cities trap heat with hard reflective surfaces, elevating the ambient temperature by up to 7 ºC during the day. According to Liu, the urban heat island effect can offer additional energy for devastating storms.
He adds, “Global warming and urban sprawl have contributed to extreme weather. Coupled with the urban heat island effect many metropolitan cities have experienced more extreme precipitation events. As a result, urban flooding has become an increasing threat to the loss of human life and property damage in many cities around the globe.”
You can see the extreme rainfalls happen more frequently. Intense, heavy rains could inundate networks designed to prevent flooding and the capacity of drainage networks could fail because they simply can’t hold that much stormwater.
Man Qi, Study Lead Author and Doctoral Student, College of Arts and Sciences, University of Cincinnati
When her sister’s home in China’s Zhengzhou City was submerged by floodwaters in 2021, the subject of urban flooding got personal for Qi. The calamity, one of the worst floods in the city’s history, affected about 10 million people. It rained as much in a single day as the city sometimes gets in an entire year.
Qi adds, “My sister’s family was badly affected by the flooding. The water flooded their garage. Their cars were underwater. It was very scary.”
The flood killed nearly 400 people and caused $10 billion in property damage.
Flooding, according to Qi, disproportionately impacts lower-income residents who lack the capacity to recover as rapidly after a calamity.
Flood insurers design maps around these once-in-a-century calamities. However, as a result of climate change, they are becoming more common, according to Qi.
Qi concludes, “Floods affect roads and infrastructure and interrupt society. Schools are closed. Businesses are closed. The impacts are widespread.”