As the climate warms, more and more communities across the United States are facing an increase in 100-year storm events.
And that’s a fact, according to Daniel Wright, an assistant professor of civil and environmental engineering at the University of Wisconsin-Madison. Wright published results of his latest study in July 2019 in the American Geophysical Union’s Geophysical Research Letters. His research paper is a part of a five-year, $507,000 National Science Foundation CAREER Award grant.
The growing trend of more rain falling more frequently across the United States could pose serious challenges to the country’s hydrologic infrastructure. In July, heavy rains slammed into Washington D.C. and dumped a month’s worth of rain on the city in an hour. The downpour overwhelmed the city’s infrastructure, leading to widespread flooding and storm water breaking into the subway system. In New York, heavy rains inundated streets and forced subways to stop. In March, the Spencer Dam in Nebraska failed as floodwaters engorged the Niobrara River. Wright’s study could help inform future decisions about how to shore up existing structures and plan for new, sturdier ones.
“The study was really driven by a motivation to express trends that we’re seeing in increasing rainfall extremes in terms that both engineers and the public can relate to more easily, rather than just a vague statement that rain storms are getting worse,” he says. “I think most people accept that.”
In part of the study, Wright focused on two categories of storm events: 10-year storms and 100-year storms. An “x-year” storm event refers to the probability of a certain amount of rainfall falling at a specific location during a year, and 100-year storms are more severe but less likely than 10-year ones.
Both categories have become more common in recent years. There was an average of 45 10-year storms and eight 100-year storms each year across the U.S. during the 1950s. Nowadays, there are, on average, 62 10-year storms, and 12 100-year storms per year.
These changes vary regionally, however. Wright found, for example, that 100-year storms have become 85% more common in the eastern United States over the 1950-2017 period, but only 51% more common in the western United States.
For his research, Wright used data 911 weather stations from across the country with records dating back to at least 1950 that are part of the Global Historical Climatology network.
The amount of rainfall that defines these events varies by location and by event duration. For example, in Madison, Wisconsin, the National Oceanic and Atmospheric Administration (NOAA) defines a 100-year event for one day (24 hours) as 6.58 inches of rain. Under the same guidelines, NOAA defines a one-hour 100-year event as 3.04 inches of rain.
Wright says it’s no surprise to see that rainfall totals have increased; climate scientists have warned of that phenomenon for years. However, he says his research helps bring into focus the effect the change is having on America’s infrastructure, which is often decades old.
In many instances, Wright says, infrastructure has been designed to Weather Bureau storm standards laid out in 1961. That data, and the flood maps that go with it, is also important to the National Flood Insurance program, which is managed by the Federal Emergency Management Agency, Wright says.
NOAA does update its metrics for rainfall events, and Wright says the agency has nearly completed a process of rolling out new standards across the country. For instance, the agency last year updated its metrics for Texas, which upped the expected 24-hour rainfall for a 100-year event in Houston, Texas, from 13 inches to 18 inches.
However, Wright says that while NOAA has been updating its standards over the last 15 years, in many locations, those newer metrics are already out of date.
“We wanted to quantify the extent to how these storms are changing, and that translates to how often infrastructure design standards are being exceeded,” Wright says. “If the rainfall is changing, a piece of infrastructure designed to meet those standards might not be performing to the rate that it needs to.”