Since the devastating landslide hit the town of Oso, Wash. last month, people who live near hill slopes or mountainsides have been asking if something similar could happen to them. Though Boise has not seen the tragic loss of life the Oso slide brought, the city is no stranger to floods and mudslides near its foothills. Boise State University geology professor Jen Pierce knows landslides in Boise are still possible.
Pierce walks along Cottonwood Creek, a popular Boise recreation spot. It’s abandoned on this day due to a steady spring rain.
“When you look at the stratigraphy (that's geologist-speak for layers) in Cottonwood Creek, which I’ve done with my students, you can actually see evidence of large debris flows in this creak in the past,” She says.
That’s been going on for centuries. Notably, in 1959 tons of mud poured into Boise along this creek and others after a summer storm. It covered much of northeast Boise in 10 inches of mud.
But even though it’s raining we’re not in any danger.
“If we got this same amount of rain following a fire, this same amount of rain potentially could trigger a flood or debris flow,” Pierce says.
That’s what happened in 1959. The rain came after a big fire burned in the foothills. Landslides after fires are fairly common in Idaho. For example, Blaine County saw some after last year’s Beaver Creek Fire. Fire can lead to floods for a few reasons.
Most importantly, fire gets rid of the plants on a hillside that hold the soil together.
“Also just the effects of fire itself,” Pierce says. “Fire limits the amount of water that can infiltrate through a hillside. So rain falls on those hill slopes, it doesn’t infiltrate, instead it runs off. As it runs off it gathers more and more sediment.”
Since 1959, a lot has been built in the areas most inundated with mud. That includes St. Luke’s Hospital, and further east, the Harris Ranch subdivision.
David Hayes lives just two ridges over from Cottonwood Creek at the base of the foothills. The 1959 fire that triggered Boise’s big mudslide started not far from his neighborhood. Standing in his front door, Hayes says he’s not concerned about fire. And even though he had to build a retaining wall on his property to keep the hillside in place, he never really considered the possibility of landslides.
“When we built a retaining wall we had a lot of interest from the city. They wanted the wall built to certain specs because of the instability of the soils around here,” Hayes says. “But I didn’t think about landslides when we bought.”
Hayes says he’s still not concerned about landslides even after the recent slide in Washington. He thinks the geologic features of the two areas are just too different. And he’s right about the differences. The conditions that caused Oso slide last month aren’t really found around Boise. The city is probably not in danger without a significant event like heavy rain after a fire. But Boise State geologist Jen Pierce says that’s a very real possibility.
“I am concerned,” Pierce says. “Fire and fire-related debris-flows present a significant hazard to residents in Boise.”
Pierce wants people who live in and near Boise’s foothills to be more aware of the risks of fire. And she wants officials to think more about Boise’s history of fire and landslides when planning new communities.
She thinks what happened in 1959 could happen again. That’s despite a monumental effort to make sure it wouldn’t. It was called The Boise Front Watershed Restoration Project.
The U.S. Forest Service made this film about it in the 60s.
Forest Service, BLM and other land management workers dug miles of horizontal trenches across the hill slopes. The trenching began less than two months after the fire and flood. It took 13 bulldozers 3,000 hours over the next few years to complete the project. In some places, you can still see the striped pattern those trenches created.
“Their purpose is to intercept the flow, to prevent it from accumulating, to prevent it from being larger,” says Terry Hardy the watershed program manager and burned area emergency response coordinator for the Boise National Forest.
Hardy says digging trenches across hill slopes is only done in the most extreme cases where there’s a high risk of future slides and loss of life. The post-1959 trenching project and others that followed it changed the look and ecology of Boise’s foothills. Five decades ago, the practice of contour trenching was viewed as a definitive solution. But since then, scientists have debated whether or not it actually works. Pierce says it doesn’t help much, and in some cases, may do more harm than good.
“There’s some pretty good evidence that by digging those trenches you can actually potentially increase the risk of future landslides because that creates areas for water to pool up,” Pierce says.
Others defend the use of trenching. As recently as 1996, the Forest Service dug trenches above Boise after a foothills fire increased the risk of floods. Terry Hardy says Boise’s foothill trenches have kept serious floods and slides from happening.
“Contour trenches, they’re designed for a certain level of storm event, so it should be effective up to that certain precipitation,” Hardy says. “You can’t protect against everything.”
In other words, Hardy believes the trenches will prevent floods and landslides if a big storm follows a fire, but maybe not a really really big storm.
Hardy and Pierce do agree that even after years of trenching and many other less visible prevention techniques, Boise could see another mudflow like the big one in 1959. Controlling how much rain falls, of course, isn’t an option. But here’s one more thing Pierce and Hardy agree on; the only sure way to prevent landslides in this area is to keep fires from starting and minimize their severity when they do.
Copyright 2014 Boise State Public Radio