Nov. 18, 2013 03:43

...and the mud kept sliding down

“The thick, rushing mud battered my car as if it were a toy, tossing it wherever it wanted,” said John Schroyer, a videographer for The Colorado Springs Gazette. “I couldn’t believe what was happening.” He was driving to work in Manitou Springs when he found himself trapped in a moving mudslide.

“It was unreal. I was carried by the surging mud for a couple of hundred feet down the road. My car smashed against the adjacent cliff, then hit a light pole, as we travelled,” recalls Schroyer. With no time to spare, he climbed out of his car onto its roof and jumped across the flowing mud to semihard ground. He was very lucky to have gotten out alive.

“Mudslides occur in all 50 U.S. states and can happen at any time, with or without rainfall,” said Lynn Highland, a geographer at the U.S. Geological Survey’s (USGS) National Landslide Center in Denver, Colorado. Mudslides don’t just cause dirty messes; they are extremely destructive, even deadly, as the one in Manitou Springs was. Every year, mudslides kill more people than hurricanes, typhoons and cyclones combined.

These torrents of wet, sticky earth can travel at speeds up to 50 mph. They can move houses off their foundations or bury structures within seconds. Composed of at least 50 percent silt and clay particles and up to 30 percent water, they grow in momentum and size as they move, picking up trees, boulders, cars or whatever else is in their path. They can strike without warning, making the job of staying out of their way almost impossible.

“The first thing we do, when dealing with a mudslide,” says Jaswinder Singh, manager of A & J Retaining Wall Company in Brier, Washington, “is to secure the safety of the people. Then we move immediately to clean up the mess, so we can access the site.”

“Mostly, we find the slopes in our state slide because the construction workers who’ve come before us didn’t properly reclaim the land,” says Singh, “meaning that the soil was just dumped there, not reinforced or equipped with proper drainage.”

The company has stabilized sliding slopes in many different ways, but has found that what works best is “nailing” the soil in place. This involves drilling into slopes and driving in anchors.

“Once the anchors are in place, you drive large nails into them to stabilize the hill,” said Singh. The nail-and-anchor placement is determined by the degree angle of the slope. “This technique seems to work well in mitigating slides.”

They also utilize retaining walls. “This is usually the easiest solution,” says Singh, “but whether we pour concrete or lay brick will depend on the site and its accessibility, as these factors can limit your choices.”

He cites this example. “Rainfall in Seattle, Washington, can be extremely heavy. One of our customers had a 30-foot slope in his backyard. He went to bed as it started to rain. The next morning, he discovered the 30-foot slope was now an eight-foot one. Most of it had turned to mud, which had flowed into the street, blocking it.”

“After the city cleared away the debris,” continues Singh, “we went in and removed some of the more unstable soil. But the area was still too unstable for us to bring any kind of equipment in. We finally had to bring in a crane and lift it up 100 feet in the air to place it on a flat area at the top of the slide.” To hold the hill, they installed a 15- foot-high, 8-foot-wide retaining wall. The wall was then backfilled with clean rock for proper drainage.

Because different areas of land have different soil compositions, as well as varying slopes and geographic characteristics, it can be tough to determine how prone to mudslides an area might be. As mentioned previously, they can hit any time of the year and without any warning. “The only thing we do know is that they’re known to recur in areas previously hit by mudslides,” said Highland.

Not surprisingly, past history also shows that mudslides are more likely to occur when hillsides or slopes are heavily saturated with runoff from rainwater or snow melt. Interestingly, though, it has recently been discovered that mudslides occur more frequently when precipitation resumes after a pause than they do during steady rainfalls.

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.

Future hopes

“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.

Also in Soil Erosion News

In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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