Rim Communities Face Serious Debris Flow Risk

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The steep canyons above Pine and Strawberry can generate devastating debris flows, especially after wildfires have denuded the slopes above, a study by the Arizona Geological Survey has concluded.

Geologists discovered deposits in Pine, Strawberry, Webber, Poison, Cow and Bray canyons left by repeated slurry-like flows of water, mud and rocks. Such liquefied flows of debris can carry even sizeable boulders and logs in a chaotic rush, inflicting damage more like a landslide than a flood.

The study was intended to test computer models the Arizona Geological Service hopes can identify areas at risk from such debris flows.

As it turned out, researchers decided the computer models won’t work well without better data about soil types and rainfall. However, the on-the-ground survey of the canyons revealed a potential threat to the two communities perched at the base of the Mogollon Rim, which already rank as among the most fire-threatened places in the country.

The study comes after a unprecedented plague of wildfires had burned in the Rim Country, which raise the risk of debris flows by stripping away the trees and brush that usually hold soil in place.

“Arizona has experienced a dramatic increase in the area burned by wildfires during the drought of the past decade,” concluded geologist Ann Youberg.

“Debris flow hazard in Arizona is increasing due to larger and more frequent wildfires denuding hillsides of protective vegetation. The steep terrain associated with the Rim is conducive to the generation of debris flows and debris flows have occurred after several recent fires in the area.”

The report concluded that the drainages above Pine and Strawberry pose a moderate to severe threat of massive debris flows, but noted that the leveling of the terrain between the mouths of the canyons and the developed areas make it unlikely such a debris flow would actually reach the houses and businesses near the highway.

“Although the potential for post-fire debris flows in these areas is high, it is unlikely debris flows will have the momentum to move down the low-gradient main channel into the developed areas — although floods and hyperconcentrated flows along the main channels may affect developed areas,” the report concluded.

However, the region should undertake changes to cope with the threat, including thinning the forest on the Rim and installing much bigger culverts under roads to accommodate the much-higher than expected rush of debris.

An intense storm dumping about two inches in an hour on a recently burned area could send a tremendous flow crashing down the short, steep canyons cutting into the Rim above the communities, the report said. Rocks, trees, mud and gravel can comprise 60 percent of such flows, which can rush down canyons with the speed of water carrying battering ram boulders and trees.

Many of the canyons on the front face of the Rim pose a similar risk, but most have not been surveyed for the layering of ancient and recent debris flows documented in the canyons above Pine and Strawberry, the report concluded. The report recommends a risk assessment for a 450-square-mile swath of northern Gila County.

“Gila County has been affected by three of the five very large recent fires in Arizona,” the report concluded.

An “extreme” wildfire risk affects about 70 percent of Gila County, with another 20 percent facing “high or medium” risk. That means at least 75 percent of the county’s residents are living in a high-risk area.

The debris flow study demonstrates that a wildfire doesn’t have to actually burn through town to have a big impact.

More than 1.8 million acres of Arizona wildlands burned between 2002 and 2005. Reconstructions of fire patterns based on more than 1,000 years of tree-ring data show that the region has always had certain periods when the number and extent of wildfires increased dramatically for years or even decades.

Such fluctuations in the extent of wildfires probably represent periods of extended drought and short-term climate shifts. However, the most recent rise in fires has created much more dangerous conditions, due to the overgrown condition of the forest. Before the Forest Service presided over a century of clear-cutting, fire-suppression and grazing, ponderosa pine forests averaged 50 to 100 trees per acre. Now, the forests harbor more than 1,000 trees per acre across millions of acres.

Fires in such thickets burn much hotter. Instead of leaving a patchwork of unburned trees and brush, such fires consume almost every twig. In addition, they sterilize and sometimes fuse the soil, creating “hydrophobic” conditions. Such scorched soil can hold much less moisture, causing a dramatic increase in runoff.

The researchers had hoped several computer models would provide an easy way to predict the risk of downslope debris flow after such a major fire. The slopes above Pine and Strawberry looked like a good place to test the predictive powers of the computer model, since the 2007 Promontory Fire, the 2006 February Fire, the 2004 Webber Fire, the 1990 Bray Fire, the 2002 Pack Rat Fire, the 1990 Dude Fire all directly affected the watershed.

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