Urban centers are more prone to flooding than other areas, because streets, parking lots and buildings are impervious, which means that water cannot penetrate into the ground like in forests or grasslands. Instead, it flows.
Detroit, like many old cities, treats flowing rainwater by combining with sewage. This mixture is then pumped to the treatment plant. During the recent storm, power outages and mechanical problems caused 4 out of 12 pumps in the two main pumping stations to fail.
In the past few years, the agency spent 10 million US dollars on upgrading these two pumping stations alone, and also spent hundreds of millions of US dollars on other improvements. However, a fully modern sewer system would cost more than $17 billion to build a separate rainwater network.
Rainwater infrastructure across the country is aging, and many governments have turned to Band-Aid solutions instead of building more flexible systems, said Mikhail Chester, Infrastructure and Policy Researcher at Arizona State University. Chester added that during major storms, mechanical and electrical systems are bound to occasionally fail.
However, even if the pumping station is operating well, it may not be able to prevent catastrophic floods.
Detroit’s pumping station is similar to many rainwater infrastructure and is designed to deal with a 10-year storm, which means that the chance of rainfall in an hour in any year is about one in ten. According to the National Weather Service, a 10-year storm in the Detroit area will produce about 1.7 inches of rainfall in one hour.
According to the Water Authority, during the June storm, parts of Detroit experienced heavy rainfall, which was more like a once-in-1000-year storm (with rainfall exceeding 3.7 inches in one hour), which far exceeded the pumping station’s ability.
However, the rainfall forecast is based on historical data and may not represent the true probability of a major storm. Anne Jefferson, A hydrologist at Kent State University. Due to climate change, storms that should have a one-tenth probability of occurring in any given year may now occur more frequently. She said that few organizations consider climate change in their infrastructure design.
“We locked ourselves in the past climate,” Jefferson said.
Governments that want to consider climate change when designing their infrastructure face uncertainty—should they plan for the best emission scenarios or the worst? It is also difficult to predict how emissions will affect rainfall.
Planning for larger storms is an admirable goal, but it is also costly. Chester said that larger pumps and pipes are more expensive to build and more difficult to install. He added that the price increase is not linear—in most cases, the price of a pump or pipe that doubles in capacity will double.
Coastal cities face more severe climate threats, and some cities are actively investing to prevent them from appearing. Tampa, Florida, spent $27 million to upgrade pumping stations and other infrastructure after major floods in 2015 and 2016. Tampa bay timesSome upgrades seem to be working—at least this year, the city has avoided flooding during major storms like Hurricane Elsa.
However, rising sea levels along the Tampa coastline may soon cover the pump outlet. If the sea level reaches the point where the water should be drained from the rainwater pipe, the system will not be able to remove the water from the city.
Some cities are seeking to install other features, such as storm ponds and rain gardens, to help manage urban flooding. Jefferson said that grasslands like rain gardens can reduce the volume and speed of excess water. She added that if enough of these facilities are built in the right places, they can help prevent smaller floods, but like other rainwater infrastructure, they are usually not designed to stop floods during larger storms.