Urban centers are more prone to flooding than other areas because streets, parking lots and buildings are impermeable, which means water cannot seep into the ground like it would in a forest or a meadow. Instead, it sinks.
Detroit, like many older cities, manages stormwater by combining it with sewage. This mixture is then pumped to processing plants. During the recent storm, power outages and mechanical problems knocked out four of the 12 pumps at two large pumping stations.
The agency has spent $ 10 million in recent years to upgrade just these two pumping stations, and hundreds of millions more for other upgrades. But a complete modernization of the sewer system would require the construction of a separate stormwater system at a cost of more than $ 17 billion.
Stormwater infrastructure across the country is aging and many governments have resorted to dressing solutions instead of building more resilient systems, according to Mikhail Chester, infrastructure and policy researcher at Arizona State University. And mechanical and electrical systems are doomed to fail occasionally during major storms, adds Chester.
However, even if the pumping stations had worked flawlessly, they might not have prevented catastrophic flooding.
Detroit’s pumping stations, similar to many stormwater infrastructure, were designed to cope with a 10-year storm, which means an amount of precipitation in an hour that has about a 1 in 10 chance of occurring. in a given year. A 10-year storm in the Detroit area would equate to about 1.7 inches of precipitation in an hour, according to data from the National Weather Service.
During the June storm, parts of Detroit experienced intense precipitation levels that would be more characteristic of a 1,000-year-old storm (over 3.7 inches of rain in an hour), well beyond the capacity of pumping stations, according to the water authority.
But the precipitation forecast is based on historical data that may not represent the true probabilities of major storms, according to Anne Jefferson, hydrologist at Kent State University. Storms that would have a 1 in 10 chance of occurring in any given year are likely to occur more often now due to climate change. And she says few agencies factor climate change into their infrastructure designs.
“We lock ourselves in a climate of the past,” says Jefferson.
Governments hoping to take climate change into account when designing infrastructure face uncertainty: should they plan for the best emissions scenarios or the worst? And how exactly the emissions will affect precipitation is difficult to predict.
Forecasting bigger storms is an admirable goal, but it is also costly. Bigger pumps and hoses are more expensive to build and more difficult to install, says Chester. And the price increases aren’t linear, he adds – a pump or hose with double the capacity will be more than double the price in most cases.
Coastal cities face even more serious climate threats, and some are investing aggressively to avoid them. Tampa, Florida spent $ 27 million to upgrade pump stations and other infrastructure after the major floods of 2015 and 2016, according to the Tampa Bay weather. Some of the improvements appear to be working – at least this year the city has avoided flooding during major storms like Hurricane Elsa.
However, rising waters along the Tampa coastline may soon cover the pump outlets. If the sea level reaches where the water is supposed to come out of the storm pipes, the system will not be able to extract the water from the city.
Some cities are looking to install other features, such as storm ponds and rain gardens, to help manage urban flooding. Grassy areas like rain gardens can reduce the volume and speed of excess water, Jefferson explains. If enough of these facilities are built in the right places, they can help prevent small floods, she adds, but like other stormwater management infrastructure, they are usually not designed to stop flooding during storms. more important.