Laboratory tests conducted by Masaki Tominaga of the Tsukuba Center in Ibaraki Prefecture, Japan, and Rand E. Eads of the Redwood Sciences Laboratory of the U.S. Forest Service in Arcata, California, found that if rainfall is steady, the surface layer of the ground remains saturated with water. This impedes the escape of air from below, which blocks further saturation.
But if there’s a pause in the rain, the water soaks into the ground below the surface layer, which in turn allows the trapped air to escape. When the rain resumes, it rapidly soaks downward and overtakes the descending ‘wetting front’ from the first rain. This creates a zone saturated with water, which in turn creates sufficient pressure to facilitate sliding.
Abundant water runoff and prolonged, intense precipitation are not the only conditions that contribute to mudslides. Earthquakes, volcanic eruptions, changes in groundwater levels, alternating freeze/thaw cycles, and the steepening of slopes by erosion can also cause mud flows. Both extremely dry and very humid areas can have them.
“The West Coast is especially susceptible to mudslides because of the earthquakes, rainfall and wildfires that happen in that region,” said Highland. “In California, there is a ‘mudslide season,’ lasting from December to April.”
“Construction and the reckless modification of land, such as improper draining of an area prior to building on or near it, can also create conditions ripe for a mudslide,” Highland continues.
How do you stop these unpredictable, rolling disasters? Everett “Buzz” Waid had an idea. He’s the inventor and design engineer of the Trap Bag Barrier, a mudslide detainment and redirection system. The bags, produced by his Fort Meyers, Florida-based company, are used around the country, but even he claims they’re not “silver bullets.”
“A trap bag is a pentagonalshaped, geotextile cellular barrier that works like an accordion and can be rapidly deployed,” said Waid.
“Its primary function is to redirect mud and materials so they don’t flow down slopes or mountains.”
Simply put, t o impede mud flows you have to combat the driving force elements with resisting forces. This can mean adding buttress material or retaining walls to a slope, diverting excess water, and/or improving stormwater drainage. The goal is to slow down and deflect the mud, steering it to where it can do the least harm.
The company doesn’t claim that trap bags can completely stop mudslides, but that they will divert them, as well as protect against flooding, erosion and slope failures. “We’ve used them to stop up dams, build up reservoirs, as retaining walls, and up slopes to make them impervious to water,” says Waid.
“Recently, north of Manitou Springs, in Cascade, Colorado, we used the bags to protect an elementary school,” Waid said. “Mud had slid down across Highway 24, up against the school building and into the playground.”
“When we were brought in, the school had eight feet of mud lying against its structure,” Waid said. “After the county came in and cleaned it all out, we put a barrier in, vegetated it and put rocks in front of it.” It worked. Even with the recent heavy rains that have caused flooding in other parts of Colorado, they haven’t had another issue at the school.
Waid is presently working on Manitou Springs Creek, a 25-footwide, 15-foot-deep ravine that runs through the city. “The entire ravine is filled with mud and debris,” he says. And with every rainfall, more runoff fills the creek.
Soon, it won’t be able to handle extra volume of any kind.
Waid’s concern is that “if they experience even a quarter- or a half-inch of rain in a relatively short time, the runoff will have nowhere to go. The precipitation will loosen the debris, causing it to overflow and spread out into the community like it was sitting on an alluvial plain.”
Worse yet, above the city sits a burn area. “Where there’s been fire, there will be slides,” explains Highland. “Fire leads to mudslides, because it burns and kills plant roots. Roots hold the soil together and stabilize the land, making it less likely to be swept away.”
When soil and ash mixes with runoff, it creates mud flow. The mud flow can grab rocks and broken tree limbs. It then becomes a debris flow, sort of like a big sludge of unstoppable cement roaring down any stream channels it can find.
“All the revegetation that was in place came down with the first mudslide,” Waid continues. “Not only do they have to start from scratch in terms of revegetation, but now there’s nothing holding the mountain in place.” If another slide occurs, where is it going to go?
“This is why we’re currently working on plans that can hopefully save the city the way we saved the school,” said Waid. “ We want to increase the height of the creek’s sides until it can be cleaned out. This way, in case of future weather events, the mud and water will be contained.”
Minimizing or preventing damage
In a situation where it’s not possible to prevent a future mudslide, there are still several ways to confine or divert its flow and slow down—or even stop—its force. The easiest method is to construct a retaining wall from gravel, brick, stone, cement or steel. Constructing a wall or berm to reinforce the bottom of a slope will help prevent large chunks of land from sliding during precipitation.
Sandbags or barriers, such as trap bags, can also be used to create walls or canals to redirect or divert flow. Diversion of water away from a slope, or clearing the debris out of its path helps, too. You can also improve the drainage at a site or a slope, reduce a slope’s steepness or reduce its weight by excavating the top.
Waid adds, “Another way to prevent damage from mudslides is to build artificial reservoirs or divert the mud to natural ones. You can also create containment dams or confinement basins to settle out or slow the flow.”
Vegetating slopes is an excellent way to prevent future slides. This seems obvious, yet too often, many wellintentioned erosion control and slope stabilization efforts do not incorporate plant material, whose roots hang onto the earth.
Maintaining existing vegetation, such as trees, shrubs, and groundcovers, both on and above steep slopes, anchors soil and absorbs excess water. Vegetation, properly installed and maintained, protects slopes by reducing erosion, strengthening soil and increasing general slope stability.
“A combination of a stable wall with vegetation works well for preventing future mudslides,” says Waid. “You can vegetate the wall created by the bags. Let’s say, worst case, another mudslide comes through. It may take off the vegetation, but the hill or slope will still stand.”
There’s no substitute for common sense. People living in hilly or mountainous areas, especially ones that are prone to wildfires, earthquakes or heavy rainfall, need to be aware of the threat. Residents who reside in communities such as these should be educated about the dangers as well as prevention.
“With regard to mudslides, instead of being a solutions guy, I’d prefer to be a preventive guy,” says Waid. “I’d like to stop or mitigate a problem before it occurs.” In his opinion, it’s all about being more proactive than reactive.
Singh says there’s one thing we can predict for sure about mudslides, and that’s that there’ll be more of them. “With the cycle of weather and how much rain we’re getting (in Washington State) versus ten years ago, it’s probably going to get worse.”
But he’s also seeing a new approach to treating areas after slides. “Now, contractors are trying to use the least amount of disturbance when stabilizing an area.
Once the mud has been cleaned up, crews are using attachments that allow the equipment to sit further away from the unstable, volatile land.”
The mudslide in Manitou Springs, Colorado, stranded vehicles, closed a highway, decimated homes and businesses, and worst of all, took lives. According to a local resident, she was swept away by the flood, but was able to grab a tree, fling herself onto a ridge and crawl to higher ground. “I lost everything, but I survived,” she said.
While we can’t stop mudslides completely, proper deterrence and redirection techniques can prevent tragedies like this from ever happening again.