Lahars (volcanic debris flows) that originate on the slopes of stratovolcanoes represent one of the most dangerous hazards during volcanic unrest, volcanic eruptions, and post-eruptive times. Lahars can originate in a variety of ways, including augmented melting associated with geothermal heating, pyroclastic flows and dome collapse on snow and ice-clad volcano slopes, liquefaction of debris avalanches, and rainfall-generated flows on the slopes of tephra-covered volcanoes (Pierson, 1989). Worldwide, lahars have claimed tens of thousands of lives (Tilling, 1989). The most dramatic example of this situation occurred in 1985 when a relatively small eruption at Nevado del Ruiz volcano in Colombia melted snow and ice and released lahars down three of the four major drainage systems, killing more than 23,000 people (Pierson and others, 1990). Lahars represent an especially insidious volcanic hazard because they can flow for tens to hundreds of kilometers down valleys and reach places far removed, and often spared, from other volcano hazards. The movement of a flood or lahar down a valley, and the magnitude, timing, and attenuation of these flows constitute an essential piece of hazards information needed to adequately portray volcanic risks in downstream locations.
Over the past 25 years, stratigraphic interpretations, dynamic reconstructions, and some direct monitoring have been the cornerstones for the assessment of lahar hazards at active, or potentially active volcanoes (e.g., Crandell, 1980; Scott and others, 1995; Cummans, 1981). These studies are important but limited because results depend on the presence and identification of appropriate flow deposits, which may not occur in the most important locations, and the assumption that lahar hydrograph characteristics measured or reconstructed at one site, can be extrapolated to another location.
Surface-water flow models have the advantage of providing dynamic information about flow waves of different size and characteristics at any location along any valley. Stratigraphic studies and flow models are complementary in that one provides real field data of past events that can be used to calibrate a flow model, while a calibrated flow model can extend lahar and flood-wave results to locations where deposits or high-water marks are not preserved. A reliable lahar-flow model can also aid in the evaluation of risk in places where large flows may not have yet occurred.
Return to:
[Report Menu] ...
