A lahar is a flowing mixture of rock debris and water (other than normal streamflow) from a volcano, which encompasses a continuum from debris flows (sediment concentration > or =60% per volume) to hyperconcentrated streamflows (sediment concentration from 20 to 60% per volume). Debris flow deposits are poorly sorted and massive with abundant clasts. Lahars can be either syn-eruptive, post-eruptive or have a non-eruptive origin. Four types of lahars can be generated during an eruption, based on distinct sources of water (i.e. ice, snow, crater lake, river, and rain) that allow the sediments to be removed and incorporated in the lahar (e.g., Mount St.-Helens in 1980, Nevado del Ruiz in 1985). Post-eruptive lahars, which are rain-triggered, occur during several years after an eruption (e.g., still occurring at Pinatubo). Non-eruptive lahars are flows generated on volcanoes without eruptive activity, particularly in the case of a debris avalanche or a lake outburst (e.g., Kelud or Ruapehu). Lahars flow as pulses, whose velocity and discharge are much higher than those of streamflows, including catchments similar in size. Sediment transport capacity of lahars is exceptional, owing to buoyancy, dispersive pressure, and to the amount of cohesive clay and silt. However, the finding of recent experimental works indicates that even clay-rich lahar mixtures have little true cohesion. Therefore, the typical classification of lahars into "cohesive" and "non cohesive" seems to be inappropriate at present. Besides, past work on lahar mechanics used models based on the Bagnold's or the Bingham's theories. Recent advances in experimentation show that a lahar has specific rheological properties: it moves as a surge or series of surges, driven by gravity, by porosity fluctuation, and by pore fluid pressures, in accordance with the Coulomb grain flow model. Grain size distribution and sorting control pore pressure distribution. Lahar mechanics depend on much more than steady-state rheology, because lahars are highly unsteady and typically heterogeneous flows. Lahar can show a succession of debris flow phases, hyperconcentrated flow phases, and sometimes transient streamflow phases. Therefore, some fluids-mechanics concepts and terminology, such as "viscous", "laminar" or "non-Newtonian" are inappropriate to describe the mechanical properties of lahars. Processes of deposition are complex and poorly known. Interpretation of massive and unsorted lahar deposits commonly ascribe the deposition regime to a freezing en masse process. However, recent laboratory experiments highlight that debris-flow deposits may result from incremental deposition processes.

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