When it rains, it pours. That’s the new reality in a changing climate, and it’s changing how we think about designing our communities.
Scientists refer to the size of rainfalls by the statistical likeliness of a storm happening in a given year. Perhaps you’ve heard the term “10 year storm” or “100 year storm” in the news after big rains. A “10 year” storm is a term that describes a 24-hour period when an area receives about 4.27 inches of rain. A “10 year” storm has about a 10 percent chance of happening in a given year.
When we are designing our communities, we use these probabilities to decide how large the pipes and other features need to be to handle most rain events. Larger, less common rains might only happen a few times in a lifetime and it’s hardly possible to build infrastructure that can handle any amount of rain.
The problem is, to borrow a well-worn phrase, storms today “just ain’t what they used to be.” The type of storm that has a 10 percent chance of happening is occurring more often. Even moderate climate models suggest that by the middle of the century, what today is a “100 year storm” will only be a 10 year storm. Bigger storms are more common, and what we think of as a “big storm” is getting, well, bigger.
We’ve seen this issue play out in living color the past few years. Last year the Twin Cities saw the rainiest spring since record keeping began in the late 1800’s. Lake Minnetonka was higher than it had even been for 45 straight days and Minnehaha Creek surged at a record 889 cubic feet per second (flow considered safe for canoeing is between 75 and 150 CFS). The Hiawatha and Meadowbrook Golf Courses spent weeks underwater and suffered millions of dollars in damage. Sanitary sewer systems backed up near Lake Minnetonka.
In 2012 Duluth’s normally pleasant springtime quickly turned sour when the city received more than 10 inches of rain in a 24 hour span in some areas. The city’s infrastructure was quickly overwhelmed and the effects were devastating: washed out bridges, buckled roads, massive sinkholes, flooded homes. An 8-year-old boy was swept into a storm drain for a six-block ride he miraculously survived. Seals swam out of the zoo.
Ten inches in 24 hours exceeds what is even now considered a “1,000 year rain,” something you’d think might skip an entire civilization. But in the wake of the flooding, Minnesota Public Radio meteorologist Paul Huttner wrote, “Many have asked me if events like the Duluth Flood can happen again. The answer is not if, but when. With the increase in extreme rainfall in Minnesota, it’s only a matter of time until the next 5″ or 10″+ rainfall event in a major Minnesota city.”
As Duluth rebuilds its infrastructure after that devastating rainfall, it and other communities can prevent future damage from these crushing storms.
There is not enough space here to fully cover the actions available to make our communities less susceptible to flooding and other issues caused by more intense and frequent storms, but there are many options out there. A recent study by the Minnehaha Creek Watershed District and a host of partners looked at ways that fully built and developing communities can adapt how they manage water in a changing climate. Find a summary and the full results at www.minnehahacreek.org/wet.
Sherry White is the Minnehaha Creek Watershed District Board of Managers president.