Colorado's 1000-Year Rain

The overflowing St. Vrain River hit the ponds just upstream from this neighborhood in Longmont. The water then followed the train tracks next to the neighborhood, resulting in significant flooding in the area.

A 1000-year event refers to an event that, in any given year, has a 1-in-1,000 chance of occuring. By extension, a 100-year event has, in any given year, a 1-in-100 chance of occuring. In September of 2013, parts of Colorado experienced a 1,000-year rain that led to a 100-year flood. A total of 4,500 acres were affected and hundreds of homes and farms were damaged or destroyed. Thousands of people had to evacuate and more were left stranded.

It began raining on the afternoon of Monday, September 9th, when a slow-moving cold front met warm-monsoonal air from the South. In a state that had been experiencing a drought, this came as a welcome relief. The rain continued on Tuesday. Between Monday and Tuesday, there was a combined total of 1.27 inches of rainfall in Boulder County. By Wednesday morning, the ground was saturated.

It kept raining.

Wednesday evening, reports began coming in across the Front Range that there was simply too much water and it was beginning to cause problems. Roads were closed because of flooding, or predicted flooding, in Boulder, Longmont, and Erie. Towns in the mountains like Lyons, Estes Park, and Jamestown became isolated as roads in and out of the towns were covered in water or destroyed by rising river levels.

A road damaged by raging flood waters outside of Jamestown. Damaged roads left residents stranded.

The situation on Thursday went from bad to worse. Heavy rainfall and saturated ground made rivers swell to enormous sizes and dangerous speeds, destroying roads and homes along their paths. Little James Creek, for example, grew to 5 times its normal size and Boulder Creek grew from flowing at 54 cubic feet per second to 4500 cfs. The rainfall also created a landslide in Fourmile Canyon along Highway 119, closing the road out of Gold Hill and other mountain towns.

The Big Thompson river flows much higher than usual, wreaking havoc on the area near Loveland.

The chart shows the hourly rainfall with the flow of Boulder Creek. Chart produced by Cooperative Institute for Research in Environmental Sciences.

Rain slowed and stopped by Saturday, allowing helicopters to reach those who were stranded, beginning the largest U.S. airlift evacuation since Hurricane Katrina. The rescue efforts brought people who had been stranded in Lyons to Longmont. In total, over 1,200 people were evacuated.

On Sunday, the rain began again, grounding helicopters and creating new flash-floods. By Monday morning, however, the rain was over for good. It was then that people could see the devastating effects of so much water, and could begin the long process of recovery.

Overall, the storm dumped 17.5 inches of rain on the city of Boulder over eight days. For context, Boulder gets an average of 20.66 inches of rain over an entire year. While the exact amount of rainfall differs between towns, in all cases, there was far more water than the ground could absorb, causing major flooding and costing a total of 2.9 billion dollars in damages. Rebuilding roads and buildings was a lengthy and expensive process. Recovery efforts are ongoing in some areas.

A map of North-Central Colorado with the total rainfall in affected areas.

Sources:

Brennan, Charlie. "Eight Days, 1,000-year Rain, 100-year Flood." Boulder Daily Camera. September 22, 2013. Accessed November 08, 2018. http://www.dailycamera.com/news/boulder-flood/ci_24148258/boulder-county-colorado-flood-2013-survival-100-rain-100-year-flood.

"Floods in Colorado." NASA. September 21, 2013. Accessed November 08, 2018. https://earthobservatory.nasa.gov/images/82090/floods-in-colorado.

Taylor, Alan. "Historic Flooding Across Colorado." The Atlantic. September 16, 2013. Accessed November 05, 2018. https://www.theatlantic.com/photo/2013/09/historic-flooding-across-colorado/100591/.