Modelling of Tsunamis Generated by Pyroclastic Flows (Ignimbrites)

Authors: de Lange, W.P.; Prasetya, G.S.; Healy, T.R.

Source: Natural Hazards, Volume 24, Number 3, November 2001 , pp. 251-266(16)

Publisher: Springer

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Abstract:

Pyroclastic flows entering the sea played a major role in generating the largest tsunami waves, arising from the 1883 eruption of Krakatau, Indonesia, which caused a considerable death toll, most deaths resulting from the tsunamis. The potential exists for similar events to occur in Indonesia and New Zealand.

Processes leading to tsunami generation by pyroclastic flows, especially those associated with Krakatau-type eruptions, are reviewed. The major processes include:

1. Deposition at the shoreline causing a lateral displacement as the zone of deposition moves offshore.

2. Upward and lateral displacement of water caused by the propagation of a water supported mass-flow.

3. Downward and lateral displacement of water caused by the sinking of debris from a segregated flow travelling over the water surface.

4. Upward displacement of a large volume of water due to the deposition of a caldera-infill ignimbrite or pyroclastic flow deposit.

The pyroclastic flow is modelled as a horizontal piston forcing water displacement. The flow behaves as a wedge of material displacing seawater horizontally and vertically as it moves outwards from the source. Individual pyroclastic flows are treated as linear features that travel along a specific direction from the volcano, exhibiting limited lateral spreading. The event duration for the formation of a large pyroclastic flow and the deposition of the ignimbrite is taken as 200–400 s, with flow velocities dependent on the volume of material erupted.

For simulations it is assumed that the ignimbrite deposit is elliptical with relatively uniform thickness and the principal axis orientated along the flow direction. Therefore the tsunami is generated by defining an elliptical source region and defining an effective displacement behaviour at each node within that region. The effective displacement is defined by a start time, a finish time and a vertical velocity. These three parameters determine when the seafloor starts to rise and how far it travels during a model time step. The result is a seafloor disturbance that propagates away from the source.

The major difficulty with this approach is determination of the appropriate vertical velocity. With a real pyroclastic flow the effective vertical velocity at any point is very high. However the model needs to average the displacement spatially and temporally. Accordingly we apply the model to pyroclastic flows from Mayor Island, New Zealand to examine the influence of model parameters. To further calibrate the numerical model this study is being undertaken in conjunction with physical modelling of the Krakatau 1883 eruption at the Indonesian Tsunami Research Center, BPPT, Jakarta. Historical data will also be used to refine and calibrate the pyroclastic flow model.

Keywords: pyroclastic flows (ignimbrites); water displacement

Document Type: Regular Paper

Affiliations: Coastal Marine Group, Department of Earth Sciences, The University of Waikato, Private Bag 3105, Hamilton, New Zealand

Publication date: November 1, 2001

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