Rain, hail will pack more punch, raise risk of flooding


When rain falls, if it does, Colorado faces a rising risk of flooding, mudslides and rockfalls — like those that closed Interstate 70 last year — due to an “expanding bullseye” of barren, scorched and less-stable terrain.

And Colorado officials say everybody will need flood insurance and other protection — not just residents living in federally-designated hazard zones.

The “bullseye” concept describes the growing area in Colorado where hard rain and hail pack more punch — caused by a shift to aridity amid a 22-year severe drought and wildfires burning more land, state climatologist Russ Schumacher said. Some 36 burn scars from fires cover more than 700 square miles.

“If burn scars are larger, the rainstorms that happen every summer are going to be more likely to hit a burn scar,” Schumacher said.

The state’s increased vulnerability to floods and slides as terrain changes “is a risk that people need to be aware of,” Schumacher said. “There’s been so much focus on drought in recent years, for good reason. People may not be thinking as much about the risk of flooding.”

This week and next, National Weather Service forecasters are anticipating rain and scattered severe thunderstorms. The first couple of weeks of June typically bring the craziest storms, though meteorologists say severe weather and flash flooding can happen throughout the summer.  This year, precipitation from winter through late May remained relatively paltry.

Major floods scouring Colorado’s semi-arid landscapes — triggered by heavy rain and hail — historically ranked among the worst disasters. The biggest floods hit Pueblo in 1921, Denver in 1965 and the northern Front Range (Big Thompson River) in 1976. Most recently, floods in 2013 swept down from mountains and across plains along the South Platte River into Nebraska, destroying 1,800 structures and causing damages estimated by the insurance industry at $4 billion.

And accelerating erosion led to havoc last year when heavy rain on July 29 triggered the destruction of I-70 in Colorado’s Glenwood Canyon — where destabilization after the Grizzly Creek Fire in 2020 prompted a visit from the chief of the U.S. Geological Survey who warned of “very high potential” for catastrophic slides — costing an estimated $116 million and closing I-70 for two weeks.

Traditionally, trees, shrubs and other vegetation reduced erosion and stabilized slopes so that Colorado could withstand heavy rain and hail.

“But as you have less vegetation cover and soil — as it gets drier — you expect to see greater amounts of erosion,” said Karen Berry, director of the Colorado Geological survey. “And, we find that the types of rainfall we get have been changing due to climate change,” tending to be “more intense” when storms hit, Berry said.

Federal hazard maps traditionally focused on flood plains and often weren’t updated for decades.

“But what if the flood plains migrate?  There needs to be some sort of buffer area,” Berry said.

“Now, you are going to have a greater risk of these things happening,” she said, urging efforts to boost resilience.

“We could do better to predict where we are most vulnerable,” she said, pointing to new construction “at the bottom of steep drainages” and other high-risk terrains. “If you can avoid living in those areas, that’s great.”

Colorado officials began revising flood risk maps after 2013 — when a year’s worth of rain fell in three days, setting off the floods. The latest maps of the Front Range show streams and rivers moved significantly, typically after extreme calamities, rendering Federal Emergency Management Agency maps inapplicable.

Residents embrace near a washed-out home ...

Helen H. Richardson, The Denver Post

Residents embrace near a washed-out home in Jamestown, Colo. on Sept. 14, 2013. Flooding hit the mountain community hard, and residents were trapped for days with no road into or out of town.

As the impacts of rain and hail storms increase, the number of National Weather Service flood alerts in Colorado has been increasing. Forecasters issued a handful of alerts each year in the 1990s. In 2005, they issued 44. In 2013, they issued 176. Last year, they issued a record 285, according to NWS data.

Climate warming creates conditions conducive to extreme storms. Colorado’s average temperatures increased over the past three decades by 2 degrees — exceeding the global average rate. State officials project temps will increase by an additional 2.5 degrees to 5 degrees before 2050 — favoring an intensification of changes.

So far this year during extreme drought, only three alerts have been issued, NWS meteorologist Russell Danielson said. “The strongest storms don’t really start until June.”

Year-by-year fluctuations in numbers of flash flood alerts are “mostly due to the weather” and “if you’re in a drought year you typically have less,” Danielson said.

“However, if you have more burn scars — we had large wildfires in 2020 — that soil will be hydrophobic for three to five years and those soils really cannot absorb water,” he said, citing recent Glenwood Canyon flooding, mudslides and rockfalls.

“In a drier, warming climate, you’re likely going to have more wildfires. They’re going to be larger, more devastating. And, then, those create the hydrophobic soil that then makes those areas more susceptible to flash flooding,” Danielson said.

“It is a plethora of events, coming together, making Colorado more vulnerable to some of these natural disasters.”

In 2014, the largest landslide in Colorado's history covered nearly a square mile six miles southeast of Collbran in Mesa County.

AAron Ontiveroz, The Denver Post

In 2014, the largest landslide in Colorado’s history covered nearly a square mile six miles southeast of Collbran in Mesa County.