Just the Facts column: Mega movements of earth reshape the Western landscape
December 9, 2015
Last year, a block of earth 20 times the size of Coors Field slid downhill at upwards of 80 mph, bowling over a forest and killing three Coloradans near Collbran. Two decades prior, and not too far away, a football-field-sized chunk of Interstate 70 heaved upward 14 feet, making the roadbed more suited to monster trucks than Camrys.
As the nation's cradle for steep slopes and high mountains, Colorado experiences more than a thousand mass movements of earth each year.
Should we be concerned?
Most of these are as small as your backyard, but some are the size of towns. Although there are many names to describe different types of downslope earth movements, we can lump them together as "landslides," sensu lato. Landslides are to dirt and rock as avalanches are to snow, except that landslides are a year-round affair.
Fortunately, there is some predictability and patterning to such events. Many are triggered by rainfall — whether from a hovering storm system or an afternoon downpour. And those wildfires we've had in recent years? They just exacerbate the situation. Vegetation-less landscapes slough off curtains of loose sediment, turning rainy runoff into a wall of moving mud.
Water infiltration, which peaks when snowmelt wicks its way down into cracks and pores in the earth, is another big cause of Colorado landslides, generating hillside slumps, rock slides and embankment failures. These types of landslides can lag behind their triggering event, sometimes occurring months after a snowmelt or late-summer monsoon because it can take a long time for the pores and cracks in soils and rock to become filled with slowly-infiltrating meltwater. Not only does the water-saturated dirt become heavier, but water also reduces the frictional characteristics of the soil, inhibiting its ability to "hold" its place on a slope.
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Colorado's large daily temperature variations are another cause of landslides. Think of those days when you walked the dog with morning frost on the ground but later stepped out for lunch on a sunny, 50-degree afternoon. These temperature fluctuations cause expansion and contraction of rock and soil as water in pore spaces and rock fractures repeatedly freezes and thaws. Called ice-jacking, it triggers downslope movement of unstable earth. It's the same thermal process that causes so many potholes to form in our roads.
Not surprisingly, high-elevation communities are in the belly of the landslide-producing beast, much as they are for avalanches. Just pick your favorite pass. Loveland, Red Mountain, Slumgullion — they teem with downward-moving rock and dirt. Ditto for Colorado's high parks and mesas, where slides are catalyzed by flood-and-furrow agricultural irrigation practices, together with our bad habit of lopping off the toes of slopes during development.
And the Front Range?
Western Colorado Springs is such a classic example that it ought to be included in the Federal Emergency Management Agency training handbook. Much of the city was developed over the past few decades, with builders sitting houses on ancient landslides or slide-prone surfaces. It's like a perfect storm. Grade some of these sites, exuberantly irrigate the turf and add a wet winter or two and — kasloosh! — landslides let loose. Ponder it next time you're sipping chai at the Broadmoor.
Surprisingly, Colorado doesn't require all builders, municipalities or agencies to assess risk before commencing development. Moreover, just because a property (home, road, business, farm) was platted back in the day, doesn't mean that it was ever assessed for geo-hazards such as landslides, underground faults, abandoned mines or swelling or collapsible soils.
Fortunately, the lion's share of large land movements move slowly, perhaps creeping an inch or a foot per year. And many landslide-prone areas and regularly moving slides are known and posted on the Colorado Geological Survey website. Go to http://www.coloradogeologicalsurvey.org, mouse over "Geologic Hazards" and click on "Landslides."
There's a saying among landslidologists: "Some landslides you fix, and others you name." Fixing landslides includes buttressing the earth, holding it back using soil- or tie-back anchors or making it less heavy by drying it out or removing weight or earth from atop it. And the landslides that get named? Usually we address them by avoiding them and/or monitoring them with devices that notify authorities when their rate or distance of movement changes markedly.
The good news
Fatalities from these geo-hazards are extremely rare. Also, Colorado has a bunch of dedicated "slide scientists" who're embedded in places that count — like the Colorado Department of Transportation, Colorado Geological Survey and U.S. Geological Survey.
Like the study of earthquakes, the study of landslides has moved from reactive to proactive in the past few decades. With new laser-based scans of ancient and current landslides, Colorado's landslide science could soon be predictive, employing risk-based asset-management principles to protect people and property.
James Hagadorn, Ph.D., is a scientist at the Denver Museum of Nature & Science. Suggestions and comments are welcome at firstname.lastname@example.org.
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