Landslide and Mudflow, What's the Difference?

Oso mudslide from above
Sawaya Photography/Getty Images.

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 reports referred to the event as a "mudslide". Others called it a "landslide," while still others used the term "mudflow."  These three terms are sometimes used interchangeably, but they do not have the same meaning.

This article will explain what these terms usually mean and why their meanings are important.

Landslide

Many commercial property policies use the word "landslide" but few actually define the term. How do you know what type of event constitutes a landslide?

When policyholders and insurers disagree about the meanings of terms not defined in insurance contracts, courts often use definitions established in previous court rulings. If no prior decisions have been made, courts may rely on common 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 Excavation, mining, logging, irrigation and other human activities can cause landslides.

Many landslides occur after a slope has become saturated by water from rain or melting snow (structural causes).

Excessive rain was a major factor in the Oso slide.

Mudflow

Like "landslide", the term "mudflow" appears in many commercial property policies but is rarely defined. However, this term is defined in the Standard Flood Insurance Policy used in 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.

The definition of "mudflow" in the NFIP policy specifically excludes landslide, slope failure, and other types of earth movement. The policy also states that "mudflow" does not include a saturated soil mass moving by liquidity down a slope.

What's the difference between "a river of liquid mud" and "a saturated soil mass?" The answer has to do with the amount of water the mixture contains. To qualify as mudflow, a mixture of earth and water must be liquid. A thick mass of mud that slides down a hill is not "a river of liquid mud." Thus, it does not qualify as mudflow.

Mudslide

Like "mudflow", the word "mudslide" is used in many commercial property policies but the term is not defined. The policy form used in the federal flood program makes no mention of mudslide.

According to the USGS, "mudslide" is an imprecise term. It is used by laymen and the news media to refer to a variety of events including landslides and floods laden with debris. 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.

Note that an event dubbed a "mudslide" may qualify for coverage under the flood policy if it meets the definition of "mudflow". That is, if a policyholder submits a claim for damage caused by liquefied mud, the damage may be covered 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. In some policies, all three perils are listed under an exclusion entitled "earth movement." In other policies mudflow and mudslide are excluded in conjunction with flood (or water).

The definition of "flood" in the federal flood policy includes "mudflow." As mentioned above, "mudflow" excludes both landslide and mud that is not in a liquefied state.