When record-breaking rains turned parts of the Front Range into a flooded disaster area last fall, an overlooked, major geological phenomenon was also causing damage in three foothills counties.

A federal government geologist — who recently put his findings, and his warnings, into a published study — tallied 1,138 debris flows from Sept. 9-13 in Boulder, Larimer and Jefferson counties.

“This was historically unprecedented,” said research geologist Jeff Coe with the U.S. Geological Survey.

Coe said widespread rains and earth movements in such high numbers normally occur on the coasts of North America and are often associated with hurricanes. They haven’t been seen before in this magnitude in the country’s interior.

The 2013 rains that triggered so many of those Front Range debris flows — movements of loose mud, sand, soil, rock and water — pounded an area equal in size to Rhode Island.

Debris flows are differentiated from mud slides by containing more than half materials that are larger than grains of sand. They are often slower than mudslides but can reach speeds of up to 100 mph.

While headlines focused on flooding, those debris flows, ranging from small driveway-sized flows to slides covering acres, were responsible for three of nine storm-related fatalities. Two teens died when they were caught in one of hundreds around Boulder. A Jamestown man was killed when a slope let loose and came down on his home.

Coe said the number of incidents and where they occurred on mostly south-facing slopes should serve as a warning to emergency coordinators in Colorado.

“The localized nature of debris flows and their infrequent occurrence compared to other natural hazards create a situation in which residents and government officials are generally unaware of the threats they pose,” Coe wrote in his co-authored paper in the October issue of GSA Today.

Government officials are now more aware as a result of the study.

“These slides weren’t on my office’s radar,” said Sgt. Dan Barber of the Boulder Office of Emergency Management. “We were pretty much shocked at the number of them.”

The affected counties already have some new geological maps and will have more to help pinpoint areas of concern. The Colorado Geological Survey is at work on maps that senior research geologist Matt Morgan said will show detailed susceptibility. The data is complete for the Boulder County map, which is going to be released to the public this fall. Maps for Larimer and Jefferson counties will be completed later.

“The concern has been focused on floodplains, and there is a tendency to forget about debris flows. But debris flows make flooding worse,” Morgan said.

Two other unusual and deadly incidents occurred in other areas this year. A slope slid and buried three men near Collbran on the Western Slope in April, and a slide near Oso, Wash., killed 43 people in March.

“These were all historically unprecedented,” Coe said. “With each one of these we said that. … You can only say that so much before there is a suspicion that something is changing.”

Nancy Lofholm: 970-256-1957, nlofholm@denverpost.com or twitter.com/nlofholm