Implications of Critical Flow Phenomena for Estimating Lava Flux During Recent Activity at Kīlauea Volcano
Abstract
A first order challenge for rapid assessment of lava flow hazards is measuring the flux of lava from an effusive vent. Critically, lava flux controls the rate of flow front advance and the ultimate distance of flow, and offers insight into magma source dynamics. Lava flux also affects the rate and extent of crust formation on the flow surface, and thus modulates the style of flow (open channel or through enclosed lava tubes). Estimating lava flux requires measurement of flow velocity, width and depth. The flow width and surface velocity through an open channel can be measured with calibrated video, and average flow velocity approximated as 2/3 of the (maximum) surface velocity. Determining flow depth, however, is more difficult. Minimum flow depths are commonly estimated from the height of floating lava "boats" coupled with an assumption of neutral buoyancy. Here we suggest another method, which uses observations of persistent standing waves and diagonal shocks within proximal lava channels. In fluvial systems, these phenomena provide evidence of critical flow. At critical flow in a semi-circular channel, flow depth d is related to the wavelength L as L=2πd, and flow velocity U calculated as U = √(gd), where g is gravity. In contrast to fluvial systems, however, bubbly lava is non-Newtonian and critical flow in lava channels occurs under laminar (low Reynolds number) rather than turbulent (high Reynolds number) flow conditions because of the high lava viscosity.
We test the application of critical flow theory to open lava channels using data from the ongoing eruption of Kīlauea volcano. Standing waves are captured in lidar data in the near-vent lava channel issuing from fissure 8 during July 8-11. Measured values of L 24 m suggest a channel depth of 3.8 m, a mean velocity of 6.1 m/s and a flux of 850 m3/s for a spillway width 37m. The equivalent DRE flux is 110 m3/s using a near-vent vesicularity measurement of 85%. This flux is within the range of estimates from nadir UAS videos using the same methods, as well as surface velocity analysis integrated with depth estimates from critical flow and the Jeffreys equation. Values also fall within the bounds of lava flux estimated from both preliminary satellite-based time-averaged discharge rates and the rate of magma withdrawal from the Kīlauea summit area.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43J0277C
- Keywords:
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- 8499 General or miscellaneous;
- VOLCANOLOGY