The above  picture shows the landslide that happened in La Conchita, CA. Without warning, a wall of mud covered and trapped several residents. While California may not have the horrific snow fall levels seen across much of the country, we have been getting pelted with higher than normal rainfall amounts. Couple this with several major wildfires denuding that hills and mountains and you have potential for major landslide activity.

So, when events present themselves, and you are faced with the potential of landslide activity, what do you tell residents? How can one possibly prepare for this type of event?

The single most important action that should be taken by residents on rainy nights is NOT to sleep in lower-floor bedrooms on the sides of houses that face hazardous slopes.

More than 100 Californias have been killed by debris flows during the past 25 years. Most of these 100 deaths occurred when debris flows buried persons who were sleeping in lower-floor bedrooms that were adjacent to hazardous slopes.

Sudden "mudslides" gushing down rain-sodden slopes and gullies are widely recognized by geologists as a hazard to human life and property. Most "mudslides" are localized in small gullies, threatening only those buildings in their direct path. They can burst out of the soil on almost any rain-saturated hill when rainfall is heavy enough. Often they occur without warning in localities where they have never been seen before.

The ashy slopes left denuded by wildfires in California are especially susceptible to "mudslides" during and immediately after major rainstorms.

Those who live downslope of a wildfire area should be aware of this potential for slope failure that is present until new vegetation rebinds the soil.

This engine discovered an unsafe section of roadway that collapsed because of the weight of the engine. This resulted in no serious injuries because firefighters were wearing their seatbelts. The residents car did not luck out...

What Are Debris Avalanches and Debris Flows?

Debris avalanches and debris flows (both popularly called "mudslides") are shallow landslides, saturated with water, that travel rapidly downslope as muddy slurries. The flowing mud carries rocks, bushes, and other debris as it pours down the slopes.

A debris avalanche (Figure 1) is a fast-moving debris flow that travels faster than about 10 mph or approximately 25 yards in about 5 seconds. Speeds in excess of 20 mph are not uncommon, and speeds in excess of 100 mph, although rare, do occur locally.

What Dangers Are Posed by Debris Avalanches?

Debris avalanches pose hazards that are often overlooked. Houses in the path of debris avalanches can be severely damaged or demolished. Persons in these structures can be severely injured or killed.

Most rainstorms are of such low intensity that they do not trigger debris avalanches. Some intense storms may trigger only a few debris avalanches. However, when the ground is already saturated from previous rain, even relatively short high-intensity rainstorms may trigger debris avalanches. For example, in January 1982, an intense rainstorm triggered literally tens of thousands of debris avalanches in the San Francisco Bay Area. These 1982 debris avalanches caught people unaware and caused 14 deaths and many injuries and destroyed or damaged several hundred homes and other structures.

What Causes Debris Avalanches and Debris Flows?

The most common cause of debris avalanches and debris flows is the combination of heavy rainfall, steep slopes, and loose soil. Most fairly steep slopes have enough soil and loose rock for potential landslides. Although "stable" when dry, such slopes can produce local debris flows, often without warning.

Normally the source of the excess water is intense rainfall, although broken water pipes or misdirected runoff concentrated by roads, roofs, or large paved areas may trigger, or help to trigger, debris avalanches and debris flows. In California, most debris flows occur during wet winters.

Where Do Debris Flows and Debris Avalanches Occur?

Debris avalanches occur all over the world. They are particularly common in mountainous areas underlain by rocks that produce sandy soils. Debris avalanches have been noted in southern California during at least nine rainy seasons since 1915. They have occurred in northern California during at least 14 rainy seasons since 1905.

Debris flows are known to start on slopes as low as 15 degrees, but the more dangerous, faster moving flows (debris avalanches) are more likely to develop on steeper slopes. About two-thirds of all debris avalanches start in hollows or troughs at the heads of small drainage courses. Typically, a debris avalanche bursts out of a hillside and flows quickly downslope, inundating anything in its path. Because the path of a debris flow is controlled by the local topography just like flowing water, debris avalanches and debris flows generally follow stream courses.

Slopes burned by range and forest fires are especially susceptible to debris avalanches and debris flows because of the absence of vegetation and roots to bind the soil. The areas directly downslope are especially subject to damage from debris flows.

What Can Be Done to Avoid or Reduce the Hazard Posed by  Debris Avalanches?

To be safe, assume that all drainages in steep, hilly, or mountainous areas are capable of carrying debris flows, especially if relatively loose, sandy soils are present in the watershed. Areas that have been burned by regional fires are especially vulnerable.

Avoid building sites at the bottoms and mouths of steep ravines and drainage courses. These areas are the most likely to be inundated by debris flows. The outer "banks" of bends along such ravines also should be avoided because swiftly flowing debris avalanches can "ride up" out of the bottom of the stream channel where it bends.

Avoid building on or below steep slopes. In general, the steeper the slope the greater the risk. If these areas must be used, consult with a soils engineer and an engineering geologist. These specialists will be able to evaluate the potential for mudslide problems and give advice on the best way to minimize the risk to life and property.

The hazard from debris flows that occurs in modified slope cuts can be decreased by: 

1) limiting the height and slope of cuts and fills, 

2) properly compacting fills and keying them into bedrock, and 

3) properly controlling the flow of water onto slopes.

If steep cuts or fills occur below the discharge points of runoff water from streets, downspouts, or similar drainage facilities onto a slope, it may be wise to obtain advice from an engineering geologist or erosion control specialist.

In some cases, walls can be built to deflect potential mudflows away from or around structures. To be effective, diversion walls must be properly designed and regularly maintained.

"Mud Floods" and "Debris Floods" Pose Hazards, Too

Residents living directly downslope of mountainous wildfire areas should be aware that, in addition to life-threatening potential debris flows and other forms of mass movement, there is another, perhaps deadlier hazard-- debris flooding or mud flooding at and near the mouths of channels that drain burned-over, ashy slopes. Studies have shown that, in the first year following a wildfire, sediment yields and peak discharges or such streams can increase up to 35-fold. Thus occupants of dwellings near such drainage channels could be endangered by floods that incorporate enormous amounts of debris and mud washed off the burned hillsides.

Tips and Clues That May Save Your Life...

  • Mitigation of hazards from debris flows and debris avalanches through construction of permanent engineering measures takes considerable time and money. In the meantime, preparation for rapid evacuations should be made.

  • Before and during rains, frequent inspection of the slopes (above vulnerable sites) for extension cracks and other symptoms of downslope movements of slope materials can be a guide to impending failure and a warning to evacuate. In particular, watch for new springs or seeps on slopes; cracks in snow, ice, soil, or rock; bulges at the base of slopes; the appearance of holes or bare spots on hillsides; tilting trees; or increased muddiness of streams. Any sudden increase in runoff is cause for concern.

  • Listen for unusual rumbling sounds or noises that may indicate shifting bedrock or breaking vegetation or structures.

  • Stay alert to the amount of rain falling locally during intense rainstorms. Buy a rain gauge (an inexpensive plastic one will suffice) and install it where it can be checked frequently.

  • Whenever rainfall has exceeded 3 or 4 inches per day or ¼ inch per hour, the soil may be waterlogged and more rain can trigger mudflows.

  • Again, the single most important action that should be taken by residents on rainy nights is NOT to sleep in lower-floor bedrooms on the sides of houses that face hazardous slopes. More than 100 Californias have been killed by debris flows during the past 25 years. Most of these 100 deaths occurred when debris flows buried persons who were sleeping in lower-floor bedrooms that were adjacent to hazardous slopes.

    In the interest of fire and life safety,


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Mike, what's the story behind the red Battallion SUV stuck- did they drive into it unknowingly or were they caught by more movement?

Good article BTW! ;-)
Those who live downslope of a wildfire area should be aware of this potential for slope failure that is present until new vegetation rebinds the soil.
A few years ago we had one or two (can't remember) DSE firefighters killed after being caught in a slide situation from this exact situation. There'd been a fire rip through the area previously, then huge rainfall and they were caught in their vehicle.
Sad but ultamate problem is development and building that ignores or allows homes to be built in these areas in the first place. IE the homes along the cliffs of California. Geologic areas that are ignored. Seems that dealing with the mud flows or carnage afterwords is about lack of development standards and oversight of development. Money talks and codes walk. All our public education for safety preparedness an being self prepared only are adequate when the development standards are adhered to and the developers feet are held to the fire. Sad fact is our coastle communities are effected by geologic activity that is a time bomb waiting to happen. Throw in way to much rain, hillsides denuded by fire, inproper storm drainage an worst of all earchquake activity an you have a story for disaster. However, you see them build right back in the same places after the dust settles. The Darwin Awards are supported again.
YUP! People ignore the facts and believe it will never happen here. Then they critisize us after the fact as if we had anything to do with their own ignorance or lack of self protection. However! as I said the Darwin Awards are always looking for postings. And much of our work is due to stupid people duing stupid things.
I've been teaching emergency management for a number of years and we've always said that communities have no memory.

By this we mean, people either die or move on and new people move into the community. They haven't gone through the same events as those before them, so they have no reference or nothing to remind them of what could or has happened. Thus they make bad decisions about where to live, how to protect themselves, etc.

This makes our job even harder as they have little or no resillience.
As I said, our resolve is ever vigilant to plan, prepare, educate and respond to mans and mother natures emergencies. Without them we would all be out of work.
Sorry BZY, went off on a tangent there. Last winter we dealt with a major mudslide here. One of our main roads going up river was totally blocked with a 20' high mud slide. Originally I was called as a passerby saw mud running off the hillside onto the roadway. When I arrived I saw water, rocks and mud coming down over the roadway. After about 5 minutes of sitting there watching this and wondering if I should shut down the road, a wall of mud and debris about 20' high came down and accross the road. This went on for about 45 minutes. By this time we had emergency units above and below the slide shutting off traffic. This ended up being blocked for two days because the material kept moving so it wasnt safe to get equipment in or on it to clear the road. At one point we had to have the main power shut down as power poles were being pushed over to the point where we were cocerned that the power lines were going to break. This is a main road to get to homes up river an unfortunatly there is only one way in or out. We arrainged to put people up in Motels in town who could not get home. It ended up being caused because a developer had worked on a hillside above the road and had not provided adequate drainage for stormwater. A year latter he continues to work the same hillsides and soon there will be homes up there. This Winter we has some mudd come down at that location however, nothing like that day a year ago. We are still working to try to develope alternate means of getting people in and out of the are.
This is the only mud slide we have to think about around here.
But you still give a lot of great information as always.

This is what I saw when I arrived on scene of our last mud incident in NC...

But with the amount of rain we have had this year it's good information, thanks!
These are the only Mudslides we deal with here in the Low Country. I prefer them over the other two kinds.

I thought you NC guys were more into coleslaw wrestling.
I don't have specifics on this but was under the impression that they were assisting with evacuation and were caught in a mudflow. This is a LAFD Battalion Chief and they, generally speaking,don't make foolish mistakes. Everyone was caught off guard with the volume of mud and debris that were kicked loose by prior wildland fires in the area. Additional photos below show the results of too much rain too fast... Overwhelming local resources and the ability for the hillsides to remain intact.


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