Debris Flow/Lahars/Floods

Primary reference(s)

Gudmundsson, M.T., 2015. Hazards from lahars and Jökulhlaups. In: Sigurdsson, H., B. Houghton, S. McNutt et al. (eds.). The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 971-984.

Vallance, J.W. and R.M. Iverson, 2015. Lahars and their deposits. In: Sigurdsson, H., B. Houghton, S. McNutt et al. (eds.). The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 649-664.

Additional scientific description

Lahars are sometimes referred to as debris flows and colloquially as volcanic mudflows. The word ‘lahar’ is a generic term for a complex flow phenomenon encompassing a wide range of flow types with different physical parameters. Sub-glacial eruptions can produce floods and lahars, known as ‘Jökulhlaups’ in Iceland (Gudmundsson, 2015).

Lahars can be extremely mobile, flowing at high speeds on steep volcanic terrains and for long distances (tens of kilometres) along valleys. A single lahar can consist of multiple alternating phases of flow with differing characteristics (Vallance and Iverson, 2015).

Lahars are typically topographically confined flows, so existing channel networks often control the dominant flow routing. However, lahars can be much larger than typical streamflows (both in the depth of the flow and the flow rate) so that overbanking is possible for lahars. Lahars are generally categorised as primary (syn-eruption) and secondary (post-eruption) (Vallance and Iverson, 2015).

Primary lahars are caused directly by volcanic eruptions through a range of processes including the disruption of crater lakes, the melting/erosion of glacial ice and snow by volcanic flows (e.g., pyroclastic density currents), the mixing of tephra with rain and ground water, and the incorporation of ground water into debris avalanches. Primary lahars may be hot for an extended time during their motion (Pierson and Major, 2014).

Secondary lahars occur due to the remobilisation of erupted pyroclastic deposits, often during intense and/or long-lasting rainfall, as a volcano’s drainage system responds to the surface deposits added during eruptions and can continue for many years after an eruption with a decreasing frequency over time (Pierson and Major, 2014).

However, eruptive activity and secondary lahars can occur contemporaneously during long-lived eruptions at persistently active volcanoes.

Measurable and modellable parameters include flow speed, flow density, temperature, dynamic pressure, flow and deposit thickness, maximum runout, area of invasion, triggering factors (e.g., rainfall), solids volume concentration, eroded depth, friction coefficients.

There is little correlation between the magnitude of an eruption and the volume of primary lahars. An example is the 1985 eruption of Nevado del Ruiz, Colombia, which was a relatively small eruption in terms of erupted volume, but pyroclastic density currents flowing over an extensive summit ice and snow cap resulted in substantial glacial and snow melting (2×107 m3), initiating large (peak discharge <48,000 m3/s), fast (<17 m/s) lahars simultaneously in several drainages (Pierson et al., 1990). The devastating consequences included the loss of more than 24,000 lives (Brown et al., 2017). The magnitude of secondary lahars is dependent on rainfall intensity and duration, as well as sediment availability, so the largest lahar can occur a long time (possibly years) after an eruption (Pierson and Major, 2014).

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

The impact of lahars varies greatly depending on the flow type and magnitude, the weather conditions, the geomorphology, and the characteristics of the exposed assets. Fatalities are caused by burying, impact injury or drowning. There were 72 fatal incidents as a result of lahars between 1500 AD and 2017, with a total of 49,938 fatalities that occurred between 1 and 100 km from the source volcano (Brown et al., 2017). Infrastructure (including critical facilities), personal property, agricultural lands and livestock can be destroyed, buried or damaged. Lahars can erode and remove top-soils from farmlands.

Emergency response and clean-up can be difficult due to the material left behind by lahars. Lahar hazard mitigation has included evacuation before eruptions or storms, channel and dam engineering, land management and early warning systems (Pierson et al., 2014). Mapping the possible paths and dynamics of lahars can help to identify exposed communities and assets. The strong topographic control means that simple flow routing models (e.g., Iverson et al., 1998) can be effective, although models that incorporate flow dynamics provide additional useful information such as arrival time and dynamic pressure (Manville et al., 2013).