Moab just had a 100-year flood. What does that mean and how often does it happen?

For a brief moment, Chuck Williams considered not designating the natural disaster that soused Moab on Saturday a 100-year flood. Maybe the city engineer could get away with calling it a 90-year flood. Or, better yet, a 50-year flood. Something less sensational.

Pinning that 100-year label on the bath the city, and especially its downtown, received after a series of storms dropped almost an inch of rain in 20 minutes wasn’t going to get Williams anything good anyway.

“I was hesitant to call it a 100-year flood because I just was afraid that someone would say, ‘Ah, they’re making that up ‘cause they’re going to get an extra cookie,’” he said. “Well, we don’t.

“My job is to present the data that I believe to be truthful to the best of my ability. And that’s all I’m doing.”

Williams made his designation late Sunday morning, just hours after an unoccupied Jaguar went for a joy ride down Mill Creek and barrels bobbed around in backyards. But how did he know this was a 100-year flood, and what does that mean anyway? And, perhaps more important, with a month left in monsoon season and more rain on the way, what ‘s the likelihood another one will strike?

What is a 100-year flood?

A 100-year flood is not, as the name implies, a flood that only happens once a century. Rather, it has a 1 in 100 chance of occurring in a given year. If you find the term confusing, you’re not alone. Hydrologists at the U.S. Geological Survey have been making a push to change the phrasing and Salt Lake County began phasing out the term from its flood control ordinance in 2011.

The designation arose in 1973 when the National Flood Insurance Program was looking for a standard for determining a city or area’s floodplain. According to a Federal Emergency Management Agency training document, the 100-year flood was a compromise that “fell between what the Corps of Engineers had used as the protection level when they built dams and levees and what most communities used when they designed their stormwater systems.”

The odds of a flood occurring, say, every 10 years vs. every 100 years are set based on historic data about an area’s rainfall and river levels. But they also take into account soil types, vegetation characteristics and land slope, among other factors. So, theoretically, if Moab experienced decades of above-average floods — or of drought and low river levels — that could change the 100-year flood equation.

The recipe for creating a 100-year flood, however, rarely changes. Take one part rain-resistant soil — whether hardened from drought or saturated with water — and add large amounts of rain, often in a short amount of time. Mix turbulently.

“We had a double whammy going against us this time,” Williams said.

Despite its desert locale, Moab actually fell into the trap of having too much moisture in its dirt last weekend. The city had been hit by two smaller flash floods in the weeks prior, and the soil was already soaked through.

The amount of rain that fell wasn’t so alarming, but the compact timing and the location led to issues. Williams noted that Moab actually experienced a 200-year storm last year (storm designations reflect rainfall while flood designations are tied to water levels). Yet because the rain fell almost entirely within the town’s borders, it didn’t raise creek levels enough to be considered a 100-year flood despite overwhelming many streets and basements. Most of Saturday’s rain, meanwhile, fell southeast of town and the runoff swelled Mill Creek.

But Moab also had something else working against it: the burn scar left in the nearby La Sal Mountains after last summer’s Pack Creek Fire. While the scar didn’t play a significant part in Saturday’s flooding, it did in the two previous floods when water and debris flowed unhindered into Pack Creek. Downtown Moab is triangulated by that creek and Mill Creek, which converge just a few blocks to the west.

How is a 100-year-flood determined?

Williams spent last Saturday morning working on a presentation to brief the Moab city council about the damage caused by the flash flood it had weathered on Aug. 11.

“By Saturday night, that was old news,” he said.

Around 7 p.m., rain began to fall just southeast of town. But it wasn’t rain from just one storm cell. Storms were “training” — as in, coming through one after another, like railroad cars — over the area, according to Erin Walter, a meteorologist for the National Weather Service in Grand Junction, Colo., which monitors the Moab area.

The NWS received one report that a total of 1 ¼ inches of rain fell just southeast of town between 7-9 p.m. Nearly an inch of that, however, dropped in a 20-minute sprint early in the deluge.

“It’s a rare amount of water at one time,” Walter said.

NWS data shows Mill Creek swelled to 7.4 feet — five times its normal depth — before entering Moab and was twice that, about 10-feet deeper than usual, after passing through town.

When Williams saw the ominous storm clouds building and checked the radar data, he knew it might be big trouble.

He and his crew went out that night to check on structural damage incurred by the city’s bridges as well as monitor Mill Creek’s water levels. The water rose above two of Moab’s eight roadway bridges and three of its 11 pedestrian bridges. Williams said he believed debris catching under the roadway bridge on 300 South and forcing water up over it was at the root of most of the flooding downtown. One of the pedestrian bridges built in 2018, meanwhile, collapsed under the pressure of another clot of logs, boulders and other debris.

The next morning, Williams returned to the bridges to re-measure the high-water mark. He then took that measurement plus data from gauges positioned at various points along the river and compared them with data in FEMA’s flood insurance study for Grand County, the most recent edition of which is from 2009. That allowed him to guestimate the probability of the occurrence of a flood of that magnitude.

Yes, guestimate.

Williams is the first to admit hydrology is not an exact science. Still, the 66-year-old has been in his position for five years but has been working in flood control for most of his professional life and has now seen three 100-year floods.

When will the next 100-year flood be?

Some studies have begun to show the 100-year flood not only is a misnomer, but that odds of one hitting are far greater than 1% per year. For example, a 2019 Princeton University study modeled the flood probability in four coastal regions of the eastern United States. It found 100-year floods occur about every 30 years and could soon be annual events in those areas.

Michael Mann, a climate scientist at Pennsylvania State University, meanwhile, found that a 500-year flood in New York City before 1800 would now be a 25-year flood. Within 30 years, he wrote in a 2017 paper, it would occur every five years.

In a tweet about the Moab flood, the Utah Democratic Party blamed climate change.

“It seems like we’re experiencing more and more ‘once in a century’ or ‘once in a lifetime’ events lately,” it read. “The climate crisis is real, and it’s here.”

The political party’s not alone in holding that theory. Mann, for one, appears to agree. In discussing his findings with the Washington Post, he explained that “in this case [it’s] because there is a trend toward greater extremes in a warming climate.”

Moab’s 100-year flood happened almost 69 years to the day after its previous flood of record. Flashing through the town on Aug. 21, 1953, that flood moved 5,110 cubic feet per second. A flood of similar size also ripped through the area on Aug. 4, 2010 — a year after the most recent FEMA study for the area. According to NWS records, gauges show it would have had a similar flow to Saturday’s flood. While Williams said he has not done the calculations yet on Saturday’s flood, he believes it to also be more than 5,000 CFS.

More rain is in the forecast for Thursday through Sunday. Will it set off another 100-year flood? Will Williams see another one hit the town in his lifetime?

“No. No,” he said.

Then he paused.

“But, you know, I’m saying that quickly because the odds are against it,” he added “But, I wouldn’t put any guarantees on that.”