What causes spring floods? » Yale Climate Connections

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Spring is flood season for much of the United States. From the Pacific Northwest to New England, snowmelt combines with spring rains to raise water levels in rivers, lakes and streams. Sometimes a perfect storm of factors creates a major flood.

Everything from soil to snowpack to nighttime temperature plays a role in the severity of a spring flood. Due to the large number of different elements involved, it can be difficult to predict the risk of flooding in any given year. As climate change influences many of these variables, forecasting can become even more difficult.

“If you’re missing a certain ingredient, you go from just a nuisance flood to maybe a disaster. And so it’s very difficult to know going forward where the risks are,” said Melissa Widhalm, associate director from Midwest Regional Climate Center.

Rapid warming causes snowmelt floods

Rapid warming and warm nighttime temperatures can dramatically increase the rate of snowmelt and lead to flooding. According to research by Xubin Zeng, a professor at the University of Arizona, very hot temperatures are the main drivers of extreme flooding caused by snowmelt in the United States. Zeng and his co-authors mapped and studied extreme snowmelt events in the Americas between 1988 and 2017.

Todd Shea, lead meteorologist at National Weather Service office in La Crosse, Wisconsinsaid you can have almost every other factor indicating a spring flood, and a flood may not occur if the temperature warms slowly.

“I remember one year when everything looked really dark, like we were going to a pretty nasty spring melt, but we had the perfect, very slow, gradual melt,” Shea said.

Shea also mentioned that rapid warm-ups are often accompanied by nighttime temperatures above freezing, allowing the snow to continue to melt overnight. The ideal scenario, he said, is when daytime temperatures warm enough during the day to gradually melt the snow, then drop back below freezing at night. The slower the snow melts, the lower the risk of flooding.

Climate change is expected to make warm days and warm nights more likely in early spring and late winter. “Unfortunately, when we talk about climate change, we’re talking about extreme temperature swings that pose a real threat,” Widhalm said. “We’re talking about having more of these erratic, really cool, really warm-ups in kind of these quick snaps, and that’s when we start to see issues.”

Precipitation on snow melts snow faster

According to Zeng’s research, the second most likely factor leading to rapid snowmelt is rain, which is expected to be a growing problem, especially in California’s Sierra Nevada mountains and the Pacific Northwest. .

Eunsang Cho, hydrologist working at NASA Goddard Space Flight Center, said this type of rapid, rain-induced snowmelt occurred in northern California in 2017 during the Oroville Dam failure. A lot of snow accumulated in the Sierra Nevada Mountains that year, and when a river-driven atmospheric rain event fell on that snow, it accelerated the melt and quickly filled Lake Oroville. The high rate of discharge needed to keep up with rainfall and melt damaged the dam’s main spillway, causing evacuations for communities downstream.

This problem is not limited to the western United States. Zeng’s research found that the northeast is also particularly at risk from flooding caused by rapid snowmelt due to rain. Rain in late spring and winter is also becoming more common in the Midwest, according to Widhalm. And the overall rainfall also increases during these months.

“Here in Indiana, we’re talking about a 20 percent increase in winter precipitation,” she said. “We are talking about an increase of 13 to 16% in the spring. So that’s a lot of water at a time when we are already vulnerable to flooding.

Soil’s ability to store water has an impact on flooding

Shea said when the National Weather Service creates spring flood outlook reports, an important factor they consider is the moisture content of the soil and how much water it will be able to store when the snow melts. .

If the soil is already saturated with precipitation in summer and fall, it will not be able to store as much runoff. The composition of the soil – such as whether it is mostly clay or sandy – also affects how much it will be able to absorb. Cho said the Red River in the northern basin of Minnesota and North Dakota is particularly prone to flooding due to soil composition.

“It’s a really flat area and the ground is very impermeable, which means it’s very clayey ground,” he said. “Even though small amounts of snow melted, it easily caused flooding.”

This type of impermeability also occurs when the ground is frozen, as frozen ground is also highly impermeable. This may become more common in the Midwest due to climate change where snow accumulation may be reduced and leave the ground uninsulated. This allows the ground to freeze deeper.

“If I get a big storm on top of that, what’s going to happen to that water?” Well, it’s going to hit the ground like a rock, and it’s going to flow into our waterways and rapidly increase our flows,” Widhalm said.

Cho also mentioned that in urban areas along the Atlantic coast, this problem can be compounded by rising sea levels and impermeable paved surfaces. Sea level rise can raise groundwater levels, making the soil more saturated and less able to store water from snowmelt. And paved surfaces prevent runoff from entering the ground.

Water levels and ice cover in rivers

As with soil, the ability of watersheds to store more water is important during the spring freshet season. If rivers and other bodies of water are already full before the onset of spring warming, the risk of flooding is higher. Also, if the ice on the rivers has frozen heavily, it can cause problems. Shea said when the ice is thick, it often breaks up later in the spring, which can cause ice jams and flooding.

With climate change increasing extreme precipitation everywhere and overall precipitation in many places east of the Rockies, the risk that there will already be plenty of water in the watershed before the snow melts increases.

Flooding due to snowmelt is a compound event.

All of the experts interviewed for this story pointed out that nothing causes a spring flood. Snowmelt flooding is a compound event that can make it unpredictable.

Widhalm cited extreme flooding in Nebraska in the spring of 2019 as an example. “In 2019 when they had these terrible, terrible floods in March, well, let’s look at what happened,” she said. “In February they had record snowfall, they had record cold. Well, then you had March and you had this intense warm-up. The combination of those three things, plus precipitation during this warming, has caused flooding.

Because there are so many variables in a snowmelt flood, it’s hard for experts to predict whether climate change will make them worse or not. But some regions — Cho’s research flagged the Pacific Northwest, Alaska and Canada as particularly vulnerable — can expect a higher risk of extreme spring flooding in the future. And climate change may increase the frequency, if not the magnitude, of these types of events in other parts of the northern United States as well.

“We’re talking about weather superimposed on climate,” Widhalm said. “Fortunately, it is quite difficult to get this perfect storm of events to line up to create disaster. But if you strengthen the system, if you create an environment more conducive to rapid development or intensification of storms, we’re now talking about increasing the potential to have greater snow dumps, we’re creating the potential for more environments with rapid temperature changes.

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