Mailboxes caught in a mudflow following the May 1980 Mount St. Helens volcanic eruption.

A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water.[1] Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second.[2] Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows, allowing them to travel farther and across lower slope angles. Both types of flow are generally mixtures of particles with a wide range of sizes, which typically become sorted by size upon deposition.[3]

Mudflows are often called mudslips, a term applied indiscriminately by the mass media to a variety of mass wasting events.[4] Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called mud failures.[5]

Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).[6]

A statutory definition of "flood-related mudslide" appears in the United States' National Flood Insurance Act of 1968, as amended, codified at 42 USC Sections 4001 and following.

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