Abstract
Abstract The relationship between valley morphology and ash-cloud surge development for 12 pyroclastic density currents (PDCs) at Soufrière Hills Volcano (SHV), Montserrat is investigated. Channel slope, sinuosity and cross-sectional area were measured from high-resolution digital elevation models (DEMs) using geographical information system (GIS) software; and were compared to geometric parameters of the deposits. The data illustrate three surge-generation regimes: a proximal area of rapid expansion; a medial deflation zone; and a steadier distal surge ‘fringe'. The extent to which these regimes develop varies with flow volume. For larger flows, within the proximal and medial regimes, a strong inverse correlation exists between surge detachment and valley cross-sectional area. Surge detachment is also correlated with observed and modelled flow velocities. Areas of topography-induced increases in velocity are interpreted to result in more pervasive fragmentation and fluidization, and thus enhanced surge generation. Distally, surge deposits appear as fringes with decaying extents, indicative of more passive expansion and decreasing velocity. The results indicate that surge mobility and detachment are a complex product of flow mass flux and topography, and that future efforts to model dense–dilute coupled flows will need to account for and integrate several mechanisms acting on different parts of the flow.
Highlights
The development and subsequent decoupling of dilute, turbulent ash-cloud surges from dense basal pyroclastic flows is poorly understood, unpredictable and often results in deadly consequences
Decoupled pyroclastic surges were responsible for fatalities at Mt Pelee, Martinique in 1902 (Fisher & Heiken 1982); Unzen, Japan on 3 June 1991 (Yamamoto et al 1993); at Soufriere Hills Volcano (SHV), Montserrat on 25 June 1997 (Loughlin et al 2002a, b); and at Merapi Volcano, Indonesia on 18–19 December 1930, 22
Some simulation tools exist for modelling coupled dense –dilute flows using Lagrangian methods (e.g. Takahashi & Tsujimoto 2000; Todesco et al 2002; Dartevelle et al 2004; Valentine et al 2011), but these models are complex and computationally expensive
Summary
The development and subsequent decoupling of dilute, turbulent ash-cloud surges from dense basal pyroclastic flows is poorly understood, unpredictable and often results in deadly consequences. Decoupled pyroclastic surges were responsible for fatalities at Mt Pelee, Martinique in 1902 (Fisher & Heiken 1982); Unzen, Japan on 3 June 1991 (Yamamoto et al 1993); at Soufriere Hills Volcano (SHV), Montserrat on 25 June 1997 (Loughlin et al 2002a, b); and at Merapi Volcano, Indonesia on 18–19 December 1930, 22 November 1994 and October –November 2010 (Bourdier & Abdurachman 2001; Smithsonian Institution 2011; Jenkins et al 2013) Several of these events, and their resultant deposits, have been well described and their dynamics analysed in detail retrospectively. The model considers the sedimentation of clasts and entrainment of air as the surge moves laterally away from the dense basal avalanche; the limits of surge inundation are drawn where the bulk density of the surge falls below that of ambient air They do not, capture the complex physics associated with flow emplacement, and, in cases where they have been applied, the need for improved reliability in delineating ash-cloud surge inundation has been highlighted (SAC 2007). Complex field relationships that result from the spatial and temporal variability of PDCs need to be reliably distilled in order to understand key physical processes which apply generically and which can be incorporated into dense– dilute coupled
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