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Review of contemporary terminology for damaging surficial processes: stream flow, hyperconcentrated sediment flow, debris flow, mud flow, mud flood, mudslide
Keaton, Jeffrey R.
Keaton, Jeffrey R.
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2019
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Abstract
The term "mudslide" seems to be favored by news media for all localized processes in which damage is caused by moving earth materials, regardless of whether the processes involved mud or sliding. The term came into prominence sixty years ago when an atmospheric river moved over southern California with disastrous landslides, debris flows, and floods. Very costly floodlike damage resulting from "mudslides" was added to the National Flood Insurance Program coverage as a result of the 1969 disaster. Landslides were, and still are, excluded from the flood insurance program and other insurance instruments. Because of lack of clarity in what range of phenomena was intended to be covered by the flood insurance program, a panel of experts assembled by the National Research Council (NRC) categorized the continuum of moving water to moving earth into (1) clear-water floods, (2) mud floods, (3) mud flows, and (4) other landslides. These categories recognize the lack of clear distinction between slope-movement classifications and floodlike damage caused by more fluid "landslides." Earth materials in the most recent landslide classification are subdivided into rock and soil; soil is further subdivided into debris and earth. Debris is composed of mineral fragments with 20 to 80 percent coarser than sand size, whereas earth is 80 percent sand and finer fragments. Mud was used in older geology-based classifications in a way similar to earth (e.g., mudstone). Mud is not a technical term in engineering usage. Sediment-water mixtures with sufficient water to behave hydraulically were called mud floods by the NRC. Mud flows differ from mud floods by having viscoplastic behavior, which allow mud flows to support fragments with densities greater than water during transportation and when the mass comes to rest. Mud flows and debris flows have a velocity-dependent strength (matrix viscosity) and a velocity-independent strength (shearing resistance of the mass). As a mud flow or debris flow slows to a stop, the velocity-dependent strength goes to zero; however, dense fragments do not sink or settle into the mass because of its static shearing resistance. The deposits of this spectrum of processes have distinctive sedimentary structures: clear-water flood deposits are stratified, graded, and fining upward; debris-flow deposits are unsorted, unstratified, and fully matrix supported; hyperconcentrated-flow deposits are fully clast supported. Debris-flow deposits may contain megaclasts, if they are available in the source area. Geoscientists and engineers need to understand contemporary terms used in the media to communicate with emergency managers and a variety of non-specialists, but also should recognize characteristics that are associated with specialized technical terms.
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