The interaction of ash flows with ridges |
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| Authors: | Andrew W Woods Marcus I Bursik Andrei V Kurbatov |
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| Institution: | (1) School of Mathematics, University of Bristol, Bristol, BS8 1TW,;(2) Department of Geology, SUNY, Buffalo, New York 14260 USA, US |
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| Abstract: | Using both laboratory experiments and theoretical models, we examine the different flow regimes that may develop when an
ash flow encounters a ridge. For very small ridges, all the flow may pass over the ridge. For intermediate-size ridges, the
flow may be partially blocked, with a fraction of the flow reflected upstream as a travelling bore. In this case, the remainder
of the flow, which does pass over the ridge, is hydraulically controlled at the ridge crest. Finally, if the ridge is sufficiently
high, then the flow will be totally blocked. New laboratory experiments show that the sedimentation patterns associated with
these flow regimes may be very different. Most importantly, flows that involve partial blocking and the formation of upstream
propagating bores display enhanced sedimentation upstream of the ridge, analogous to valley-ponded and caldera-fill deposits.
In contrast, under some circumstances, if the flow is able to scale a ridge, the deposit may be relatively unaffected by the
presence of the ridge. The minimum ridge height that leads to total blocking of the flow increases with mass eruption rate
and has a complex variation with distance from the source. In a one-dimensional channel, the minimum ridge height that causes
blocking increases with distance downstream. This is because the flow becomes less dense through sedimentation of particles
and entrainment of air and so requires less energy to scale a ridge of a particular height. In axisymmetric flow, the minimum
ridge height initially decreases with distance downstream as the flow spreads radially, but subsequently increases as the
flow becomes less dense through sedimentation and entrainment. A new quantitative model of dilute ash flows propagating over
ridges indicates that flows with mass fluxes in excess of 108–109 kg/s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source, whereas smaller flows
are likely to be totally blocked by such an obstacle. Our results shed new insight on the possible long-range transport mechanism
of several large flows including the Ata, Fisher and Aniakchak pyroclastic flows.
Received: 20 December 1996 / Accepted: 15 March 1998 |
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