Taylor identified an inner zone of total destruction, where the kinetic force and temperature of the nueée destroyed nearly everything in the region, surrounded by a region of partial destruction, where the velocity of the nueée had reduced and the damage was primarily due to temperature [1958, p. 39]. The figure below indicates these regions.
|A Map of the regions surround Mt. Lamington, indicating the zones of complete and partial devastation. [Taylor 1958].|
Like the pyroclastic surges of the 1902 eruption of Mt. Pelée, the zone of devastation extended approximately 10-12 km from the crater, and was strongly influenced by topography, which directed flows more strongly toward the west and south at Pelée, and toward the northeast at Lamington, due to the geometry of the crater. Within the zone of devastation at Mt. Lamington, nearly everything was flattened. There were occasional sturdy tree trunks, or some groups of tree trunks which were shielded from the flow by a topographical feature. At the settlement of Higaturu, 10 km (6 mi) to the north of the crater, only one house remained reasonably intact, and it was moved 4.5 m (15 ft) northward by the force of the flow [Taylor 1958, p. 40].
Taylor made several important observations concerning the direction, velocity, temperature, and distribution of the nuée ardente at Mt. Lamington. The following listing summarizes his observations. Unless otherwise noted, the references in this listing are to page numbers from Taylor .
|A jeep impaled on broken tree trunks. Based on the direction of fallen trees in area near the vehicle, Taylor concluded that this was the result of locally high velocities due to turbulence rather than high general velocities [pp. 43-44]. [Taylor 1958].|
Taylor concluded that this effect was due to locally high velocities resulting from turbulence rather that high general velocities based on two observations: general velocities sufficient to place the jeep in the trees would have carried away building fragments that had remained in place; and the direction of fallen trees indicated local turbulence [pp. 43-44].
A more specific indicator of the kinetic energy was the flagpole at the Higaturu hospital. The flagpole was severely deformed by the force of the flow; it appears in the upper part of the image below, nearly parallel with the ground. Note the severely damaged building in the background.
|In the background is a hospital building destroyed by the nuée ardente. The upper part of the image shows a portion of a flagpole that was bent over by the force of the flow. [Taylor 1958].|
An engineering analysis of the flagpole was initiated in order to estimate the velocity of the nuée ardente. The study included contributions from three authors: R. Dunning, L.B. Bogan, and A.K. Johnston [pp. 101-109]. This study is one of the few examples of a detailed structural analysis of the effects of pyroclastic flow. The diagram below shows the deformed shape of the flagpole.
|A diagram showing the deformed dimensions of the flagpole at Higaturu. [Taylor 1958].|
Applying classical fluid dynamics and using material properties obtained from testing samples of the flagpole itself, Dunning concluded that the velocity was 98 m/s (219 MPH), however as Johnston points out, Dunning assumed a constant flow of pure air, neglecting dynamic effects, stratification of velocity, and the additional mass of wind-born volcanic material. The effects of such material were clear however, as Bogan's close examination of the steel tubing revealed the paint was removed from the upstream side of the pole, but remained on the downstream side, a sandblasting effect that was commonly observed on trees. Microscopic analysis of the surface revealed pitting and localized cold work of the metal, very likely the result of impact from pebbles and gravel travelling at high velocity. Taking into account the factors of dynamics, stratification, and additional mass, Bogan roughly estimated the actual velocity to be "considerably less than 200 m.p.h. [89 m/s], and maybe no more than 100 m.p.h.[45 m/s]" [p. 109].
Regardless of the density of the flow, Dunning's analysis and assumptions lead to the conclusion that the kinetic energy corresponded to a steady flow with a stagnation pressure of approximately 560 kg/m2 (114 lb/ft2); this is an extremely high load, much larger than would be induced by any wind event, except for a tornado.
Observing patterns of charring and other temperature effects, Taylor concluded that the temperatures were much lower than those prevailing at Pelée in 1902. At Higaturu, approximately 10 km (6 miles) from the crater (approximately the same distance as St. Pierre from Mt. Pelée), temperatures were not high enough to ignite or char. These effects and other effects led to the conclusion that temperatures were in the range of 200°C lasting one to two minutes [p. 46].
|Next: Assessing the Kinetic Energy Potential of Pyroclastic Flow|