Landslide and Mudflow: What's the Difference?

Terms That Can Be Used Interchangeably but Mean Different Things

House collapsing on a cliff

In March of 2014 part of a hillside in Oso, Washington, collapsed after weeks of heavy rain. The slide destroyed dozens of homes and killed 43 people. The hillside collapse was covered extensively by the news media. Some writers labeled the event a "mudslide" while others called it a "landslide" or a "mudflow." These terms appear in many commercial property policies. They are sometimes used interchangeably but they have different meanings.

Homes collapse in rainy season, Laguna Niguel, CA
Image courtesy of [Spencer Grant] / Getty Images.

What Is a Landslide?

Few property policies define the word "landslide." When words aren't defined in a policy, insurers and courts often rely on dictionary definitions. The Merriam-Webster dictionary defines landslide as a large mass of rocks and earth that suddenly and quickly moves down the side of a mountain or hill.

Landslides occur in all 50 states according to the U.S. Geological Survey (USGS). The USGS cites three primary causes:

  • Geological. Slides can be triggered by characteristics of the slide materials themselves. For instance, rocks may slide because they are weak or cracked.
  • Structural. Slides can occur if the structure of rocks or earth is changed by ice, rain, melting snow, fire (that removes vegetation), volcanoes, earthquakes or other factors.
  • Human. Human activities like excavation, mining, logging, and irrigation can cause landslides.

One of the most common causes of landslides is slope saturation due to a rainstorm. Excessive rain was a major factor in the Oso slide.

What Is Mudflow?

Like "landslide", the term "mudflow" is rarely defined in commercial property policies. This term is defined in the General Property Form, the policy form used to provide flood insurance to businesses under the National Flood Insurance Program (NFIP). The flood policy defines "mudflow" as:

A river of liquid and flowing mud on the surfaces of normally dry land areas, as when earth is carried by a current of water. Other earth movements, such as landslide, slope failure, or a saturated soil mass moving by liquidity down a slope, are not mudflows.

A key phrase in the definition of mudflow is "a river of liquid and flowing mud." Mudflow means mud in liquid form. The definition excludes "a saturated soil mass moving by liquidity down a slope" because "a saturated soil mass" is not a liquid.

FEMA (the NFIP administrator) distinguishes between mudflow and landslide in an article on emergency preparedness. The agency defines mudflow as rapid movements of rock, earth, and other debris saturated with water. It states that mudflows occur when the ground becomes saturated with rainwater or snowmelt and forms a slurry. FEMA defines landslide as a mass of rock, earth or debris that moves down a slope.

Note that both mudflows and landslides are composed of rock, earth, and debris. They differ in the amount of water they contain. Mudflows contain enough water to form a flowing liquid while landslides do not.

What's a Mudslide?

While many property policies refer to "mudslides," they rarely define the term. The Merriam-Webster dictionary defines "mudslide" as a large mass of wet earth that suddenly and quickly moves down the side of a mountain or hill. Essentially, a mudslide is a landslide that contains mud.

FEMA uses the word "mudslide" in some news releases but doesn't define it. This term does not appear in the NFIP property form. An event dubbed a "mudslide" by the news media (or anyone else) might qualify for coverage under the flood policy if it meets the definition of "mudflow". In other words, if a policyholder submits a claim for damage to covered property caused by liquefied mud, the damage should be covered. Coverage should apply even if the policyholder refers to the cause as a "mudslide."

Commercial Property Insurance

Mudflow, mudslide, and landslide are excluded perils under most commercial property policies. Many policies address these perils in the same manner as the ISO property policy. The latter excludes landslide under the earth movement exclusion. It excludes mudslide and mudflow under the flood exclusion, which is entitled "water."

You can insure your business against damage caused by mudflow by purchasing a flood policy. You can't buy flood insurance directly from the federal government. Instead, coverage is available from insurers that have contracted with FEMA to sell policies on the agency's behalf. Your agent or broker can help you obtain coverage.

Most standard property insurers don't offer coverage for losses caused by landslides or mudslides. However, coverage may be available under a Difference in Conditions (DIC) policy. Most DIC policies cover damage caused by floods, earthquakes, or both. Some may include coverage for landslides. Presumably, a policy that covers landslides will also cover mudslides. The policy wording will determine the scope of coverage.