A landslide is the downward movement of large masses of soil, mud, rocks, and other organic debris. Areas with steep slopes, such as mountainous regions, are more susceptible to landslides. If you’re out in the wilderness during a landslide, your life could be in danger. All hikers, backpackers, and outdoor enthusiasts need to know what to do in case of a landslide so they can protect themselves and the people they’re with.
What We'll Cover
- What Causes Landslides?
- What Areas Are at the Most Risk of Landslides?
- Landslide Facts
- Types of Landslides
- Rotational (Slump) Landslides
- Translational Landslides
- Block Slides
- Debris Flows
- Debris Avalanches
- Lateral Spread
- Side Effects of Landslides
- How to Prepare in Advance for a Landslide
- Early Warning Signs of a Landslide
- How to Survive a Landslide
- Listen to Local News Alerts on the Radio
- Heed All Warnings & Evacuation Notices
- Stay Awake during High Risk Storms
- Move Out of the Landslide Path
- Move to Your Home’s Second Story
- Don’t Cross Bridges with Incoming Flow
- Evacuate Uphill
- Curl into a Ball if You Can’t Escape
- What to Do After a Landslide
- Stay Away from the Slide Area
- Watch for the Secondary Effects of Landslides
- Search for Survivors
- Report Broken Utility Lines
- Replant Damaged Ground
- Where Can You Find Landslide Information for Your Area?
What Causes Landslides?
The main cause of a landslide is the influence of gravity on weakened ground. Some landslides will happen over time, while more destructive ones happen suddenly after a triggering event, such as an earthquake or heavy rainfall.
Water can be a big trigger of landslides because it changes the pressure within the slope, which leads to instability. The water-laden slope materials become heavier and succumb to gravity, and this is most commonly referred to as a mudslide. Too much water is one of the most common triggers of a landslide. Another factor that contributes to landslides is the weakened slope materials, which includes both human-related activity and natural geological weathering. The destruction of natural vegetation by logging, fires, and droughts has also been associated with an increased risk of landslides.
What Areas Are at the Most Risk of Landslides?
Landslides can occur anywhere, but the most at-risk areas in America include the Appalachian Mountains, the Rocky Mountains, the Pacific Coast Ranges, and parts of Hawaii and Alaska. The most landslide-prone areas are usually mountainous, have lack of vegetation to anchor the soil in place, or have coarse soil. For example, a defrosted mountainside would be at a big risk for a landslide.
A landslide is one natural disaster that can occur at any time in almost every location. Landslides cause a lot of damage, and they cost $1-2 billion dollars each year, only in the United States. Fortunately, casualties for landslides are typically fairly low in the US at just 25-50 each year. However, the casualty rate is much higher worldwide. The largest landslide on record in the US was during the eruption of Mt. St. Helens in 1980, which travelled 14 miles and reached a velocity of 75-150 mph.
Types of Landslides
Different landslides can be grouped by scale, where they occur, the cause, and the dominant type of movement. An individual landslide is often formed by more than one type of movement, so classifying landslides into distinct groups can be difficult. No matter the type of landslide, one can be a potential hazard.
Rotational (Slump) Landslides
Rotational landslides happen where more resistant rocks go over underlying weaker rocks. The pressure causes the more durable rocks to be rotated along a natural curve in the earth so they travel down and out. The slip surface of this type of landslide tends to be very deep.
Translational landslides are the type of landslide that most people are familiar with. In this type of landslide, a portion of landmass slides downward on top of the ground. This type of landslide is most often caused by vibration, undercutting, excavation, or differential weathering.
A block side is a translational landslide where the moving mass is a single unit or closely related units that move downslope as one mass. The simplest type of block slide is just large blocks of rock that move downhill and leave a slide surface behind them. Block slides can be quite large, which often leads to a lot of damage to the environment where they occur.
A rockfall is a sudden collapse or fall of a large mass of material. Rockfalls occur along cliffs of very steep slopes where masses of earth or rocks can begin to detach and start free falling. Rockfalls are fast, and because they occur suddenly, they are also very dangerous. They most commonly occur in the spring because the natural freezing and thawing loosens the jointed rocks.
A topple can most easily be described by comparing it to a row of dominoes. When there are multiple vertical layers of earth and one falls over, it crashes into the layer in front of it, which crashes into the layer in front of that until they spill down a cliff. There are three types of topples: rock topples, debris topples, and earth topples.
A debris flow can be a common type of landslide. These landslides range from sloppy, wet mudflow to rock debris that is similar in texture to wet concrete. These begin on a steep slope of more than 20 degrees. However, they can continue to travel over gentler ground that only slopes about 10 degrees. How far the debris travels will depend on how much debris it carries compared to the water volume. Debris flows usually start as a translational slide, but the rubble and wet mix slide downslope and form a slurry. Debris flows usually leave a trail of rubble and form distinctive levees or ridges.
A debris avalanche a large landslide that moves incredibly quickly. The debris avalanche is formed after an unstable slope collapses and the debris is transported away from the slope. These large-scale avalanches usually happen on very steep volcanoes. Debris avalanches can be either hot or cold, and hot avalanches are typically the result of volcanic activity.
Earthflows occur when earth that is composed of fine-grain materials becomes saturated with water. Areas most susceptible to earthflows are where the earth is made up of sand, clay, silt, and volcanic rock. This type of landslide can be extremely slow to a point where it’s difficult to notice a landslide is occurring or extremely fast. The higher the water content in the earth, the faster the speed.
Also known as downhill creeps and soil creeps, creeps are the slow downward movement of earth down a low-grade slope. Creeps can be caused by soil expanding and contracting as it’s exposed to water. It can be difficult to tell when a creep is occurring, but if you look at things planted in the ground that are supposed to be upright, such as telephone poles, headstones, or trees, they will be leaning forward.
There are two kinds of lateral slides: earthflows and mudflows. A mudflow, often called a mudslide, occurs when earth comprised of at least 50% fine-grain material becomes saturated with water. Mudflows typically occur on gently sloping land. Earthflows also occur when fine-grain material becomes saturated with water, but the difference between the two lies in the speed of the flow. Mudflows are extremely fast, and earthflows are significantly slower. In fact, earthflows can be as slow as creeps in some cases.
Side Effects of Landslides
Landslides can cause disastrous side effects. Injury, property damage, and death are just the beginning if someone or something is in the way of a landslide, and the effects of a landslide can be seen for years after it has occurred.
Tsunamis can be created on impact as a fast-moving landslide mass enters the water, causing the water to displace and create a giant wave. Research in the Canary Islands shows that giant landslides can generate large tsunami waves that can potentially reach coastline as far away as North America.
Wildfire and landslides go hand-in-hand, so as the risk of wildfires rises, so does the risk of landslides. Wildfires help cause landslides by destabilizing the earth and creating conditions where it can become dislodged more easily. Debris flows are the most common type of landslide created by wildfires, and more wildland firefighters have died from falling debris than from actual fire.
How to Prepare in Advance for a Landslide
Landslides can happen quickly and suddenly, so it’s important to prepare for one in advance. Landslides occur in every state and are caused by many factor, as discussed previously. No matter where you live, you need to be ready for an evacuation warning.
Add Landslide-Averse Features to Your Home
There are plenty of things you can do to your home to help prepare it in advance for a landslide.
Follow Proper Land Use Procedures
Some of these proper land-use procedures include avoiding building anything near mountain edges, near drainage, along natural erosion areas, or near slopes. If you purchase a property that has not abided by these procedures, seek a professional opinion for how to proceed with it.
Increase Vegetation on Your Property
Get a ground assessment of your property and consult with a professional so you can plant the right ground cover to protect slopes. Some plants that help prevent soil erosion on slopes include creeping Myrtle, interrupted fern, spotted dead nettle, border grass, and creeping phlox.
Build Retaining Walls, Channels, and Deflection Walls
You can protect your property by building retaining walls, especially in mudflow areas. You want to direct the flow around buildings, but you need to be careful. If you build walls that divert debris flow and the flow goes to your neighbor’s property, you could be held liable for the damages.
What Not to Do
When adding vegetation and building retaining walls, you don’t want to put yard waste on a slope or install an irrigation feature on a hillside. Be sure you don’t place swimming pools near a slope or direct runoff from downspouts to a slope.
Create an Emergency Kit
A basic emergency kit has a gallon of water per person per day for at least three days, a three-day supply of food, a battery-powered weather radio, a first aid kit, flashlights, extra batteries, a whistle, and more. What you put in your emergency kit is going to be up to you, but these are the most basic things that you should absolutely have.
Other things that can be helpful include solar lanterns, which are convenient if you don’t want to rely on batteries, dust masks, duct tape, plastic sheeting, and some sort of disinfectant like moist towelettes or hand sanitizer. Everyone in your household should know where the emergency kit is, and you should check it regularly to ensure that anything perishable is still good.
Create an Emergency Plan
When creating an emergency plan with your family, consider the specific needs in your household. You need to consider the ages of the people in the home, responsibilities, frequented locations, medical and dietary needs, accessibility needs, and pets. Once you have your plan in order, make sure everyone knows what their responsibility is and where they need to go. Practicing the plan can also be helpful.
Early Warning Signs of a Landslide
If you live in a landslide-prone area, it helps to know the landslide warning signs. Saturated ground seeps or springs in an area that were not typically wet before can be a sign, along with unusual bulges or new cracks in the street pavement, sidewalks, or ground. Take note if you see soil moving away from foundations. Be on the lookout for leaning telephone poles, retaining walls, fences, or trees, along with offset fence lines. While these can be a result of creep, they can also be signs that a large landslide is coming.
If there are things moving relative to the main house, such as patios tilting or decks moving, pay close attention. Not only should you pay attention to certain things moving, but you also want to pay attention to unusual sounds such as boulders knocking together or trees cracking. This can indicate moving debris. A quiet rumbling sound that gets louder in volume is more noticeable as the landslide is coming near if one is currently occurring.
If there are creeks nearby, you should be paying attention to the water level in them. If there is a rapid increase in creek water levels or a sudden decrease in creek water levels, even though rain is still falling, these are signs to pay attention to.
If you are in a wildfire burn area, it’s good to sign up for emergency alerts and pay attention to the weather forecast for the burn area. Note that the weather in the burn area can be different from your current location. Listen for rushing water, mud, or other unusual sounds if there is bad weather in a burn area.
How to Survive a Landslide
If you are caught in the path of a landslide, there are a number of things you can do to protect your life. Landslides can strike without warning and move at extremely high speeds, so you may not have a lot of time to prepare.
Listen to Local News Alerts on the Radio
Be sure to have a radio on to get warnings if the conditions are ripe for a landslide. Since landslides are fast-moving, you may only get one warning, so pay attention. Battery-operated or crank radios are best to have in the event of an emergency because you never know when the power could go out.
Heed All Warnings & Evacuation Notices
An evacuation notice is for your own safety. Move quickly and pay attention to any warnings and notices to get out of the path. It’s much easier to heed the warnings if you are familiar with the area to know available routes out of the path. If you don’t know the area, get to know it before an emergency.
Stay Awake during High Risk Storms
If you think you are going to doze off during an impending storm, you need to do what you can to stay awake. A large percentage of landslide deaths happen when the victim is asleep. Wait for the storm to pass and for the weather alerts to declare the area safe before you turn in for the night. You could be tired and groggy in the morning, but your life can be saved. If you’re with another person, you can also sleep in shifts so one person can wake up the other in the event of a new alert.
Move Out of the Landslide Path
If you are stuck outside with no shelter, you have no option but to try and move out of the landslide’s path. This will only work if the landslide is slow-moving. If you try doing this with a fast one, it will likely overtake you. If it’s slow, you can move far enough sideways to get out of the way before it hits you. Be sure to not run in the same direction it’s moving.
Move to Your Home’s Second Story
If you are remaining at home during a landslide, move to the second story if it has one. The second story can prevent you from taking the brunt of the debris. Take cover under a strong table and hold on firmly in order to protect yourself from injury.
Don’t Cross Bridges with Incoming Flow
If you are driving, don’t cross a bridge with an incoming flow. The bridges may be washed out, so you want to turn around. Embankments along roadsides are more susceptible to landslides, so watch the road for any collapsed mud, fallen rocks, or damaged pavement.
Since landslides flow down a slope, evacuating uphill can give you some of the best chances for survival. You want to avoid any low-lying and river valleys during this time since this is where a lot of the debris can end up. Remember gravity moves downhill.
Curl into a Ball if You Can’t Escape
Since most landslides are too large and too fast to run away from, if you aren’t able to find reliable shelter then curl up in a ball in order to protect your head. If you have a tarp or blanket with you, use it to protect any exposed limbs. If the landslide does overtake you, there is a chance it won’t cover you completely, so you could be able to dig your way up. Don’t hide behind a tree since the landslide could be strong enough to knock it on top of you.
What to Do After a Landslide
After a landslide, you still need to remain safe and follow landslide protocol. Here is what you can do once the landslide has ended.
Stay Away from the Slide Area
Stay away from the area, because additional landslides and flooding can be common after an initial landslide. Don’t enter the area to rescue someone without any backup. You may get trapped and hurt yourself.
Watch for the Secondary Effects of Landslides
Floods can sometimes follow landslides and debris flow, so you need to be on the lookout for any flooding. These two events often occur together because both can be started by the same weather event.
Search for Survivors
If you are able to, search for injured or trapped people near the area, but be sure it’s safe to do so. You may have to help those who require special assistance, such as elderly people, those with disabilities, and infants.
Report Broken Utility Lines
Don’t go near any broken utility lines, especially if there are downed power lines by water. Reporting these lines can get the utilities turned off and prevent further injury or hazard. You may also need to boil any tap water you are using in your because landslides can contaminate or break pipelines.
Replant Damaged Ground
Replant any damaged ground as soon as you can, because erosion caused by loss of ground cover can lead to flash flooding during the next storm. This can help strengthen the soil to prevent flash flooding and any future landslides. If the area still seems unstable, there may be more work you have to do.
Where Can You Find Landslide Information for Your Area?
The U.S. Geological Survey Landslide Program has publications and information on its website. If you are looking for landslide information in your area, contact your city or county geology or planning office. All 50 states also have state geological surveys that can be accessed through a link on the USGS website.
Landslides can be dangerous natural disasters because they can happen anywhere and while there are some warning signs, they tend to happen without much notice. Even though there are many different types of landslides, there are also plenty of ways to prepare and survive a landslide if you are caught in one.
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