An experimental study of the surface thermal signature of hot subaerial isoviscous gravity currents: Implications for thermal monitoring of lava flows and domes

Abstract

[1] Management of eruptions requires a knowledge of lava effusion rates, for which a safe thermal proxy is often used. However, this thermal proxy does not take into account the flow dynamics and is basically time-independent. In order to establish a more robust framework that can link eruption rates and surface thermal signals of lavas measured remotely, we investigate the spreading of a hot, isoviscous, axisymmetric subaerial gravity current injected at constant rate from a point source onto a horizontal substrate. We performed laboratory experiments and found that the surface thermal structure became steady after an initial transient. We develop a theoretical model for a spreading fluid cooled by radiation and convection at its surface that also predicts a steady thermal regime. We show that, despite the model's simplicity relative to lava flows, it yields the correct order of magnitude for the effusion rate required to produce the radiant flux measured on natural lava flows. For typical thermal lava properties and an effusion rate between 0.1 and 10 m 3 s À1 , the model predicts a steady radiated heat flux ranging from 10 8 to 10 10 W. The assessed effusion rate varies quasi-linearly with the steady heat flux, with much weaker dependence on the flow viscosity. This relationship is valid only after a transient time which scales as the diffusive time, ranging from a few days for small basaltic flows to several years for lava domes. The thermal proxy appears thus less reliable to follow sharp variations of the effusion rate during an eruption.

Benchmarking computational fluid dynamics models of lava flow simulation for hazard assessment, forecasting, and risk management

“…Both theory and experiments have also been used to examine non-Newtonian viscous flows that simulate the behavior of flows with crystals (Hulme, 1974;Osmond and Griffiths, 2001;Balmforth et al, 2006;Lyman, et al, 2005;Castruccio et al, 2010). Introducing cooling in theory and analogue fluids has captured the thermal signature of flows and the impacts of temperature-dependent rheology and a surface crust on flow emplacement (Griffiths and Fink, 1993;Griffiths et al, 2003;Kerr et al, 2006;Garel et al, 2012). Most recently, the use of molten basalt in experiments has extended this work, although still at the laboratory scale (Lev et al, 2012;Edwards et al, 2013;Dietterich et al, 2015).…”

“…Axisymmetric dome extrusion has an isoviscous analytical solution (Huppert, 1982), which we plot alongside the experimental measurements (Garel et al, 2012) and results from all models that support temperature calculation in Fig. 4a.…”

“…While the scale of these experiments is small compared with natural flows, they share fundamental physics, such as low Reynolds numbers, large Bond numbers, and high Peclet numbers (Garel et al, 2012;Lev et al, 2012;Dietterich et al, 2015), which indicate laminar flow, negligible surface tension, and a very thin thermal boundary. However, natural lava flows contain much more complexity than is captured in these benchmarks, including multiphase rheology (bubbles, melt and crystals), growth and strength of a surface crust, and changing effusion rates (e.g., Cashman and Mangan, 2014).…”

“…In this benchmark, previously defined as BM3 by Cordonnier et al (2015) and based on experiments by Garel et al (2012), a hot Newtonian fluid is extruded from a point source onto a horizontal plane and allowed to cool to the ambient air temperature. The propagation of isoviscous axisymmetric flow has an analytical solution (Huppert, 1982), and data capturing the radial propagation and flow surface temperatures are provided by experiments with hot silicone oil (Garel et al, 2012). Surface temperature is presented as normalized temperature, ranging between the ambient temperature (0) and the eruption temperature (1).…”

Risk evaluation, detection and simulation during effusive eruption disasters

“…Uses TM-derived heat fluxes to derive lava discharge rates at Kilauea, as well as mass fluxes for lava lakes at Erebus, Erta Ale & Nyiragongo Wright et al (2001): Clarification of the of the discharge rate conversion technique as applied to satellite IR data Dragoni & Tallarico (2009) Harris & Baloga (2009) Harris et al (2010): Clarification and testing of the relation used to convert lava area to discharge rate Garel et al (2012):…”