Lahars: deposits, origins, and behaviour

A lahar is a flowing mixture of rock debris and water (other than normal st reamflow) from a volcano, which encompasses a continuum from debris flows ( sediment concentration greater than or equal to 60 % per volume) to hyperco ncentrated 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 ori gin. Four types of lahars can be generated during an eruption, based on dis tinct 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., Moun t 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., st ill occurring at Pinatubo). Non-eruptive lahars are flows generated on volc anoes without eruptive activity, Particularly in the case of a debris avala nche or a lake outburst (e.g., Kelud or Ruapehu). Lahars flow as pulses, whose velocity and discharge are much higher than th ose of streamflows, including catchments similar in size. Sediment transpor t capacity of lahars is exceptional, owing to buoyancy, dispersive pressure , and to the amount of cohesive clay and silt. However, the finding of rece nt experimental works indicates that even clay-rich lahar mixtures have lit tle true cohesion. Therefore, the typical classification of lahars into muc h less than cohesive much greater than and much less than non cohesive much greater than 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 lah ar has specific rheological properties: it moves as a surge or series of su rges, driven by gravity, by porosity fluctuation, and by pore fluid pressur es. in accordance with the Coulomb grain flow model. Grain size distributio n and sorting control pore pressure distribution. Lahar mechanics depend on much more than steady-state rheology, because lah ars are highly unsteady and typically heterogeneous flows. Lahar can show a succession of debris flow phases, hyperconcentrated flow phases, and somet imes transient streamflow phases. Therefore, some fluids-mechanics concepts and terminology, such as much less than viscous much greater than, much le ss than laminar much greater than or much less than non-Newtonian much grea ter than are inappropriate to describe the mechanical properties of lahars. Processes of deposition are complex and poorly known. interpretation of mas sive and unsorted lahar deposits commonly ascribe the deposition regime to a freezing en masse process. However, recent laboratory experiments highlig ht that debris-flow deposits may result from incremental deposition process es.