Mudslides increasingly common in Poudre Canyon burn zone, but greater danger looms

There was a good hour of warning before this life-threatening event — a warning everyone who travels Poudre Canyon should take seriously if it happens again:

“At 2:07 p.m., the National Weather Service advised that they were tracking a strong storm that was stalled over the canyon,” said John Schulz, public information officer for the Larimer County Sheriff’s Office. “At 2:22 there was a flash flood warning issued; at 3:03 there were reports of water crossing the highway. Then it was just a matter of about 15 minutes until the state highway department had closed Highway 14 for mudslides.”

Mudslides have been common in the High Park Fire burn area during July, but what actually occurred on July 12 at mile marker 107, about a mile west of Mishawaka, was definitely something eminently more dangerous: a debris flow. The almost instantaneous release of the soil and loose-rock structure in a canyon and gully sent soil, tree trunks and boulders, some half the size of a medium-sized car, across the highway and into the popular “3-Way Rapid,” almost damming the Cache la Poudre River.

Debris flows are common to burn areas because of the lost vegetative structure, but they are not an event caused only by excess erosion. They are much more like a dam bursting, in terms of speed and mass. Once soils reach the saturation point at which gravity takes over, they travel fast, up to 35 mph.

There are many other areas of Poudre Canyon in immediate danger of much larger debris flows that are capable of taking out houses and larger infrastructure, and could cause death on the highways, according to data from U.S. Geological Survey’s National Landslide Information Center in Golden.

The July 12 event could have been much worse. A debris flow from the 1994 South Canyon fire near Glenwood Springs traveled across four lanes of Interstate 70, and nearly dammed the Colorado River. Debris flows in other naturally prone areas have killed hundreds of people and stranded thousands, including some caught in a slide in the Himalayan area of India this summer.

During the monsoon rain season in July, Poudre Canyon has almost regularly seen mudslides and debris flow, including the July 25 and July 28 storms, which prompted debris flow warnings from the NWS for the Poudre Park area.

“People there really have to be educated about this, though in the end, they are going to make their own decision” about traveling the highway or other prone areas during storm events, said Lynn Highland, a Landslide Information Center geographer. “When there’s a chance of flash flooding in these burn areas, there’s also a chance of debris flows, especially during an intense storm.”

While flash floods and mudslides can also be life-threatening events, debris flows are particularly dangerous because of the initial mass of rock and sediment that is released, which can pick up other loose material, such as tree trunks, along the way.

Debris could block Poudre
Predicted amounts of debris flow in Poudre Canyon are capable of damming the river, though so far, the bigger flows have also moved larger material into the river, such as rocks and trees, through which the water can still flow.

USGS landslide experts are veterans of predicting post-wildfire debris flows, most of which will occur in the three years following a burn. The slope and drainage area of a basin have a lot to do with predicting the possibility of a life-threatening debris flow, but so do soil type, geology and the amount of other loose materials in any given basin.

At mile marker 107, the USGS had given a moderately high likelihood rating for an event to occur at this location, labeled Cedar Gulch, based on a 10-year-storm event producing about 1.7 inches of rainfall in an hour’s time. Under that scenario, Cedar Gulch was given a 30 to 45 percent chance of creating a debris flow of somewhat moderate size, between 25,000 to 50,000 cubic meters.

State Climatologist Nolan Doesken said the storm seemed to fit the criteria, with estimates from the National Weather Service’s Doppler radar indicating between 1 to 2 inches of rain fell on the area and a measured value of 1.45 inches lower in the canyon.

“An inch in an hour is something that might not sound that likely — it’s not an every year occurrence,” Doesken said. “But over the entire burn area, it’s fairly likely there will be an inch-an-hour storm in any given year. And this is the time of year that it’s going to occur — from now to early- or mid-August.”

And there are many more hazardous areas in the Poudre Canyon, according to USGS information. For instance between mile marker 111 and 112, both the Hill Gulch and the Falls Gulch (at Poudre Park) are given a greater than 60 percent probability of producing a debris flow in that 10-year-storm scenario, with estimated flow totals of between 50,000 and 100,000 cubic meters.

Highland said that the motorists stranded between the debris flow and another slide of lesser intensity on July 12 were fairly lucky. Such events are usually heard before they are seen, but that may not have been possible in a moving vehicle.

“Generally the advice is like that of a flash flood — head to higher ground,” she said.

Steep slopes are problematic
Almost all the drainages in the burn area are susceptible to debris flows, though most would produce less debris at the pour point, or the bottom of the gulch into which the flows will empty. The steep slopes in the Poudre Canyon are largely the cause of these larger pour points and the greater hazard; though there are other areas on the eastern and south sides of the burn areas where dangerous flows could occur, including Log and Mill canyons.

The other slide area from July 12, near mile marker 108, was likely another smaller pour point identified in the USGS study. Similarly, an incident on July 18 also produced a mudslide with small rocks, potentially from another small pour point identified in the study, and the lower canyon highway was routinely hit in July with material moving onto the road, though many may not “officially” be categorized as debris flow.

Most of the susceptibility will be gone after next year – the USGS estimates, in fact, are only good for three years – and the ongoing re-vegetation work in the burn area should also decrease the chances of debris flow events. The restoration team, including the cities of Fort Collins and Greeley and Larimer County, received $9.7 million in Emergency Watershed Protection funds from the Natural Resources Conservation Service last month, much of which will be spent on aerial mulching and seeding at a cost of about $3,000 an acre.

Re-vegetation and mulching will help slow flash flooding and, potentially, debris flows and mudslides, but some projects are actually focused on gulches and stream banks with debris removal and debris banks aimed at preventing this material from sliding down into creeks and onto roads. By law, that federal money has to be spent by mid-December, so the team has been working overtime to get the projects in line and under construction.

Larimer County’s fire recovery manager Suzanne Bassinger said the fire recovery team did address whether or not slide material should be removed from the Poudre River in late July, but there were few answers available at that time. Ownership of the riverbed and who would pay for the work are questions that must be addressed, she said, and a Section 404 permit from the U.S. Army Corps of Engineers would be required.

“And these are relatively minor events,” Bassinger noted. “I hope we don’t get a big event.”