Bull Volcanol (2005) 67:292–313 DOI 10.1007/s00445-004-0365-7
RESEARCH ARTICLE
Peter J. Baxter · Robin Boyle · Paul Cole · Augusto Neri · Robin Spence · Giulio Zuccaro
The impacts of pyroclastic surges on buildings at the eruption of the Soufrire Hills volcano, Montserrat Received: 8 October 2003 / Accepted: 22 April 2004 / Published online: 2 February 2005 Springer-Verlag 2005
Abstract We investigated the impacts on buildings of three pyroclastic surges that struck three separate villages on 25 June, 21 September and 26 December, 1997, during the course of the andesitic dome building eruption of the Soufrire Hills Volcano, Montserrat, which began on 18 July, 1995. A detailed analysis of the building damage of the 26 December event was used to compare the findings on the flow and behaviour of dilute pyroclastic density currents (PDCs) with the classical reports of PDCs from historical eruptions of similar size. The main characteristics of the PDC, as inferred from the building damage, Editorial responsibility: T. Druitt P. J. Baxter ()) University of Cambridge Clinical School, Addenbrooke’s Hospital, Cambridge, CB2 2QQ, United Kingdom e-mail:
[email protected] Tel.: +44-1223-336590 Fax: +44-1223-336584 R. Boyle Department of Geography, University of Edinburgh, Edinburgh, EH8 9XP, United Kingdom P. Cole Department of Geography, University of Coventry, Coventry CV1 5FB, United Kingdom A. Neri Centro per la Modellistica Fisica e Pericolosita dei Processi Vulcanici, Istituto Nazionale di Geofisica e Vulcanologia, I-56126 Pisa, Italy R. Spence Department of Architecture, University of Cambridge, Cambridge, CB2 2EB, United Kingdom G. Zuccaro Department Scienza delle Costruzione, University of Naples “Federico II”, Naples, Italy
were the lateral loading and directionality of the current; the impacts corresponded to the dynamic pressure of the PDC, with a relatively slow rate of rise and without the peak overpressure or a shock front associated with explosive blast; and the entrainment of missiles and ground materials which greatly added to the destructiveness of the PDC. The high temperature of the ash, causing the rapid ignition of furniture and other combustibles, was a major cause of damage even where the dynamic pressure was low at the periphery of the current. The vulnerability of buildings lay in the openings, mainly windows, which allowed the current to enter the building envelope, and in the flammable contents, as well as the lack of resistance to the intense heat and dynamic pressure of some types of vernacular building construction, such as wooden chattel houses, rubble masonry walls and galvanised steel-sheet roofs. Marked variability in the level of damage due to dynamic pressure (in a range 1–5 kPa, or more) was evident throughout most of the impact area, except for the zone of total loss, and this was attributable to the effects of topography and sheltering, and projectiles, and probably localised variations in current velocity and density. A marked velocity gradient existed from the outer part to the central axis of the PDC, where buildings and vegetation were razed to the ground. The gradient correlated with the impacts due to lateral loading and heat transfer, as well as the size of the projectiles, whilst the temperature of the ash in the undiluted PDC was probably uniform across the impact area. The main hazard characteristics of the PDCs were very consistent with those described by other authors in the classic eruptions of Pele (1902), Lamington (1951) and St Helens (1980), despite differences in the eruptive styles and scales. We devised for the first time a building damage scale for dynamic pressure which can be used in research and in future volcanic emergencies for modelling PDCs and making informed judgements on their potential impacts. Keywords Pyroclastic density current · Buildings · Dynamic pressure scale
293
Introduction In explosive volcanic eruptions, pyroclastic flows and surges (pyroclastic density currents), ash falls and lahars are the most hazardous phenomena for human settlements. Pyroclastic density currents (PDCs) are potentially the most destructive and the least predictable of these phenomena in their physics and movement, and their behaviour is also determined by their velocity and temperature. Surges form more dilute and turbulent suspension clouds than pyroclastic flows, which can also move as concentrated granular avalanches, though the two types overlap and can co-exist in the same event (Druitt, 1998). PDCs are conventionally studied from their deposits, but valuable information on their dynamics and hazard characteristics can be obtained by observations of their environmental impacts (Clarke and Voight, 2000; Waitt, 1981). We describe here a trans-disciplinary approach to the study of PDCs which, for the first time since Alfred Lacroix’s classical study at the ruined city of St Pierre in 1902 (Lacroix, 1904), analysed their impact on the built environment. We examined the effects of the three major surges, or dilute PDCs, that struck three separate villages in 1997 during the lava dome eruption of the Soufrire Hills volcano, Montserrat, which began on 18 July 1995. As violent PDCs of small volume (they covered areas less than 10 km2) they resembled the most frequent type in the world historical record and the one most likely to be encountered by volcanologists and emergency planners. Comparable events in recent times include the destruction of St Pierre, Martinique, 1902, by Montagne Pele (Lacroix, 1904), and the cataclysmic eruption of Mount St Helens on May 18, 1980 which has been widely studied by many authors (e.g., Lipman and Mullineaux, 1981). Classical descriptions of two other seminal eruptions, at Soufrire, St Vincent, 1902 and Mount Lamington, Papua, 1951, provide further evidence of the near certainty of death to people caught out in the open in such events, whilst survival is possible inside resistant buildings (Anderson and Flett, 1903; Taylor, 1983). All of these eruptions, including the June 25, 1997 eruptive event on Montserrat studied here, involved the loss of human life and this investigation forms part of continuing research by the authors into the mitigation of the impacts of explosive eruptions in urban areas (Baxter et al., 1998; Baxter, 2000; Esposti Ongaro et al., 2002; Spence et al., 2004a; Spence et al., 2004b; Todesco et al., 2002).
Methods The three PDC-forming events occurred on June 25 (Streatham), September 21 (Tuitt’s and White’s Ghauts) and December 26, 1997 (St Patrick’s) and the areas affected are shown in Fig. 1. Owing to the volcanic hazard the surviving buildings were photographed from the air in a helicopter on several different occasions, but more detailed ground observations eventually became possible for
the study of buildings in the last event, though only safely for a few days. A detailed study of the June 25 eruption was made which included obtaining eyewitness accounts and recording the fate of human victims, as reported elsewhere (Loughlin et al., 2002a). Most of the main ground survey of the buildings in the 26 December event took place over three days in August 1998 before renewed pyroclastic flow activity caused us to prematurely cease working. The damaged buildings had remained in a good state of preservation during this period. Where applicable, we incorporated certain details of the eruptions and the ground deposits obtained by volcanologists working at the Montserrat Volcano Observatory. For a full set of papers on the Soufrire Hills Volcano and its activity in 1997, the reader is referred to Druitt and Kokelaar (2002). In 1989 Montserrat was struck by Hurricane Hugo, one of the severest Atlantic hurricanes of the 20th century. Nearly every building on the island was badly damaged, with the result that most new houses were built to hurricane-resistant standards. Many were therefore also able to resist the impacts of the PDCs and make this rare study possible. However, no census of the building stock before the eruptions was done, so a statistical analysis of the numbers and types of houses destroyed was not feasible. Nevertheless, we classified the main building types in the damage areas based on the housing found on the rest of the island as follows: – Concrete block walls with timber board roof – Concrete block walls with reinforced concrete roof – Concrete block walls with galvanised corrugated steel roof on wooden rafters – Rubble masonry/concrete with galvanised corrugated steel roof on wooden rafters – Timber walls and roof (e.g., chattel houses) In the study area of the December 26 event, we found that the sturdiest houses were those with concrete block walls with either wooden board roofs or reinforced concrete roofs. The most resistant buildings to damage were single storey and reinforced concrete construction. The boarded roofs were usually covered by bitumen shingles, but sometimes galvanised steel sheets were used instead. The vast majority of windows were aluminium framed with metal, wood or PVC louvers; glazed windows were uncommon. From the aerial photographs we identified 216 buildings which survived the December 26 surge and were suitable for study. Print enlargements were used by one of us (RB), who had not been to Montserrat, to give the buildings grid references and to classify them according to structural type and their degree of damage, using a preliminary form of the damage scale which we later modified. This provisional damage scale was devised following the ground survey of the houses and adopted the features that could be most readily distinguished on the enlargements, as follows: Level 0. Minimal or no observable damage. Windows and roof intact.
294 Fig. 1 Map of South-east Montserrat, showing the Soufrire Hills volcano and the areas impacted by pyroclastic density currents; the boxed areas are those in Figs. 3, 5 and 6
Level 1. One or two windows imploded on the side facing the volcano, exit windows blown outwards, frames intact, part of the roof burnt out from internal fire, or partially burnt through in places from the external heat of flow. Level 2. All windows on the side of the volcano imploded, frames missing, and windows on the opposite sides blown out, including frames, with the roof lifted off (especially galvanised steel sheets) Level 3. As for Level 2, but with loss of parts of the internal and/or external walls.
Level 4. Walls removed, only parts or none of the structure remaining. A digital elevation model of the study area based on 1:2500 scale topographic maps with a vertical resolution of 3 m was created specifically for this study by the British Geological Survey. An analysis was also performed using a data base built in ArcView GIS and for each house the damage class, house type, distance from crater, distance from the axial line and topographical variable (such as slope) were included (Boyle, 1999). We report only the most applicable findings here.
295
Fig. 2 a June 25, 1997, Streatham village and Windy Hill. The house in Fig. 2b is at the top left hand corner. The houses in the foreground are burnt out, with less damaged houses towards the periphery of the PDC zone. The presence of upright steel fences, trees and poles reveals the low dynamic pressure here as the PDC was slowing as it flowed up the hill. Six people were killed outside their houses in this area, one other person survived with severe burns inside his house which burnt down. For orientation, the sugar mill tower in the top right hand corner is marked on the map in Fig. 3. Arrow shows direction of PDC. b June 25, 1997, Streatham village. A house with glazed windows (see Fig. 2a), which was unfurnished. Ash has imploded the bedroom windows facing the
direction of the PDC, but the roof damage was caused directly by the heat of the PDC burning away the wooden boards. The boundary fence (steel poles) is intact. The inside rooms of the house showed no evidence of infiltration by the PDC. The house in front (Fig. 2a) was fully protected by hurricane boards over the windows (see text). c June 25, 1997, Streatham village. The sugar mill tower in Fig. 2c is seen in the top right hand corner of Fig. 2a. d The wooden roof house is burnt out, and the water tank (marked on the map in Fig. 3) and car have been propelled along in the PDC, possibly from the flank of the volcano down which the PDC has flowed before sweeping up the hill at Streatham
To devise a more detailed and generalisable damage scale we used data from an analytical study of the established strengths of materials and known structural forms of typical buildings, and experimental in situ work on representative buildings, in the area around Vesuvius, Italy, as part of an EU funded study on pyroclastic flow modelling at this volcano (Baxter, 2000; Spence et al., 2004a, 2004b). We also compared our findings with probabilistic ranges for the destruction of building elements for nuclear weapon blasts as provided by Valentine (1998) and Glasstone and Dolan (1977).
buildings damaged or destroyed in the December 26 (Boxing Day) event.
Results The general description and interpretation of the impacts of the three PDC events is followed by the main study of
Dome collapses on June 25 & September 21, 1997 At 12.55 on June 25, a large and sustained collapse of the lava dome began, with three large pyroclastic flows inundating an area of 4 km2 on the north and north-east flanks of the volcano. The most extensive ash cloud surge detached from the third pyroclastic flow on the northern flank of the volcano and swept through Streatham village, climbing 70 m on Windy Hill (Fig. 1) with an average velocity of about 35 m/s (Loughlin et al., 2002b). Nineteen people were killed and seven injured, with six bodies being retrieved from inside the surge area where the hot cloud caught up with them as they fled towards their houses. The temperature of the surge was in the region of
296 Fig. 3 Map of Streatham village area showing the level of heat and fire damage at the edge of the 25 June surge which detached from the main PDC. Houses are burnt out up to the edge of the PDC deposit, but a few wooden buildings have remained largely intact, probably because they were more resistant to the entry of the hot ash (see text)
400C (Loughlin et al., 2002b). Many houses were destroyed in the pyroclastic flow and the remaining badly damaged houses were surrounded by thick deposits, but in the surge area there were thin deposits and little evidence of raised dynamic pressure (lateral loading), whilst the fences around properties mostly remained intact and utility poles and their wires were undamaged by the blast (Fig. 2a-d). Instead, the high temperature caused most damage. The houses that remained standing had their windows and roofs burnt out or the roofs facing the volcano were only partially burned away. Details of the eruption with some photographs of buildings in this event
can be found elsewhere (Loughlin et al. 2002a), but we document in this paper a map of the surge area showing the extent of the building damage due to the intense heat of the surge (Fig. 3). This map shows how even wooden buildings engulfed at the periphery of a surge can remain intact and how rapidly the high temperature attenuated in the area where the PDC was slowing as it flowed up Windy Hill. Nevertheless, six badly burned bodies were found at three locations within the mapped zone, indicating that humans were not able to survive in the open here (Loughlin et al., 2002a)
297 Fig. 4 a–c September 21, 1997. Tuitt’s. Houses burnt out by low momentum PDC with the property fences intact. The furniture has mostly combusted, but kitchen fittings visible in c(same house as top house in b). The PDC direction is right to left
On September 21 at 03.54, another large dome collapse occurred down Tuitt’s Ghaut and White’s Ghaut towards the sea (Fig. 1). Ash cloud surges were generated by the flows which inundated Tuitt’s village leaving only a thin deposit of fine ash but every house was gutted with fire. The temperature of the surge was also around 400C. (MVO, 1999). The houses showed similar appearances to those in the June 25 event (Fig. 4a–c). Lateral blast eruptive event, December 26, 1997 Eruption and PDC deposit description The eruptive event began at 0300 on December 26, 1997, when a debris avalanche formed filling much of the White River to within 100 m of the sea, followed by a high energy PDC which devastated 10 km2 of southern Montserrat as far as the ocean (Figs. 1 and 5). The lava dome, under high internal pressure, explosively disintegrated and the formed material collapsed under gravity to flow down the south-west flank towards St Patrick’s village (Fig. 1). According to seismic data, the PDC may have occurred in three pulses over a period of about 12 minutes, the middle one being the strongest (Sparks et al., 2002). In the central zone, the PDC had depositional as well as erosional features. A full volcanological description of this event is given by Sparks et al. (2002), and an account of the dynamics of the collapse and decompression of the dome to form the dilute suspension flow (PDC) may be found in Woods (2000) and Woods et al. (2002).
The area devastated by this high energy PDC lateral blast is shown in Figs. 5 and 6. The PDC impacted a 70 sector centred on the dome between the village of Kinsale in the north to 1 km south of the White River. The direction of the PDC could be inferred from the bent steel reinforcement bars (1.5 cm diam.) on partially built houses, and the flattened trees and utility poles; fences and their steel poles were also bent in the direction of the flow (Fig. 5; Ritchie et al, 2002.). A general finding was that the vertically placed steel poles used mainly in fencing became increasingly more angulated across the study area. The grain-size variations of the deposits and directional data indicated that the main part of the PDC was directed radially SW from the dome with its axis 1.5 km north of the main valley of the White River, with most of the volume depositing in the sea, and taking much of the building debris with it. The maximum deposit thickness was extremely variable in the axial areas due to the current causing mainly erosion with variable local deposition. The deposit thickness was affected by topography with the development of streamlined hummocks and accumulation in the lee of obstacles and convex breaks of slope. In the peripheral areas, including the area of study, less erosion occurred, with deposit thickness ranging from 0–10 cm in the area where buildings suffered the least damage (Fig. 6a). A coarse-grained basal layer was overlain with a layer of finer ash which was in turn covered by accretionary lapilli derived from the fallout from the PDC ash cloud. The deposits generally became finer grained from the central axis to the periphery, rather than with radial distance from the vent (Fig. 6b). The temperatures of the deposits suggested that
298 Fig. 5 Map of the deposits of the 26 December, 1997, PDC. Deposits showing mean directional indicators and inferred axis of the PDC
the temperature of the PDC was likely to have been around 300C (Sparks et al., 2002), lower than that estimated for the June 25 and September 21 events, which may be explained by the incorporation of the flank-rock material in the collapse event. The area of building damage covered by the PDC falls into four main zones which are most conveniently demarcated by the topography of the valleys (ghauts) and the White River (Fig. 5): 1. Outer zone. The PDC cloud went further than was shown by the deposit, at least 100–200 m beyond Aymer’s Ghaut, where minimal heat effects were observed on the trunks of some trees facing the crater and leaves were singed. Bitumen shingles and PVC gutters were melted in places, indicating that the temperatures of parts of the surge cloud were raised (>200C) despite the absence of damage due to its momentum. The houses here were intact and there were no fires. 2. Peripheral zone. The peripheral building damage area within the PDC deposit lay between Aymer’s and Gingoe’s Ghauts. The main visible effects were the blowing out of windows by blast and the fire damage to most houses due to the high temperature of the ash. Few wooden structures, including outhouses or sheds, resisted burning. Trees and utility poles remained mostly standing. Many external wall surfaces facing the crater showed signs of abrasion from particles in the flow, or from material such as stones picked up in the moving current. Objects such as wood and gal-
vanised sheets collected at the foot of the facing walls and provided obvious evidence for their entrainment in the PDC. The PDC impacts showed many localised anomalies, depending upon the sheltering provided by topography or adjacent houses, strongly indicating the directionality in the way the PDC moved in relation to buildings (in contrast to an explosive blast wave). In several houses, the windows were effectively protected against the flow by being boarded up as a hurricane protection measure. 3. Intermediate zone. An intermediate building damage area between Gingoe’s and German’s Ghauts contained numerous buildings which suffered obvious damage to external and internal walls, with some buildings collapsed. Most trees, utility poles and fences were flattened or badly damaged. Extensive fire damage to all buildings was visible. There was also clear evidence for large missiles (tree trunks, heavy building elements) being entrained in the PDC and inflicting damage to the walls. 4. Central zone. The central zone received the main force of the PDC and here the destruction of buildings was complete with the foundations or only sheltered basements remaining. Every standing object was razed to the ground. Large boulders embedded into the remains of one structure had acted as missiles, and most of the debris had been blasted into the sea.
299
Types of building damage The area south of the White River valley had not been built upon and was more mountainous. In contrast, the main study area (Fig. 6a,b) contained mostly residential houses, including villas, and was located between 4–6 km from the dome and on the sloping flank divided by the three narrow valleys (ghauts). Within this 2 km band it was not possible to see an obvious gradient of damage increasing towards the crater, whilst the sheltering effects of topography were clearly visible protecting some houses compared with those most exposed (see below). The severity of the building damage increased over the 2– 3 km distance from the periphery towards the axis of the PDC, with each zone having a predominant level of visible destruction mixed with examples of lighter or more severe damage, until the Central zone where near total destruction of buildings was found. The houses were mostly aligned to face the sea with the backs of the houses towards the crater, so many of the windows faced the full force of the PDC. The damage to buildings was found, on inspection, to be mainly caused by the heat of the ash causing fires where the ash was capable of penetrating the building openings (the windows being the weakest of these); the dynamic pressure or lateral loading of the PDC; and the missiles entrained by the flow movement. As described above, inspection showed that the damage increased in severity from the periphery to the central axis of the flow. Although the temperature of the PDC was potentially uniform across the area, apart from at the periphery where it would be cooler due to mixing with entrained air, the heat transfer to building elements and outdoor objects would have increased with the velocity. The lateral loading or dynamic pressure increases in proportion to the square of the velocity using the standard formula, following Valentine (1998): Dynamic pressure ¼ 0:5 density velocity2 The damage gradient from the outer to the inner parts of the PDC could therefore be mostly explained by a gradient in velocity, with a maximum in the central axis of the PDC. Analysis of the building damage
Fig. 6 Maps of the deposits of the 26 December, 1997, PDC. a Maximum deposit thickness isopachs and directional markers. Black dots represent locality and numbers refer to actual deposit thicknesses in cm. b Median grain-size contours for Layer 1 deposit of the PDC and directional markers (see Ritchie et al., 2002). Black dots represent locality and numbers refer to the actual median diameter
The following examples of the buildings are intended to be descriptive only and to show how the severity of damage increased towards the flow axis, without, at this stage, attempting to quantify the PDC parameters. 1. Outer zone. Our observations at Streatham village (June 25 event) and Tuitt’s (September 21 event) showed that even where the flow was coming to a halt the temperature was high enough to cause some external heat effects on buildings and
300
trees before the ash cloud convected away or mixed with air (Figs. 2a–d; 4a–c). Where there was sufficient momentum or turbulence for the ash to gain entry through breaking windows or penetrating louvers then the temperature was high enough to ignite furnishings and cause serious fire damage. We were not able to study these effects further except in the December 26 event because of the hazardous state of the volcano until, that is, after the dome collapse on 12 July, 2003, when it became safe to visit the buildings shown in Fig. 2a–d. The house in Fig. 2b is unusual for Montserrat in that it has glazed windows, but it is of special interest in showing the impact on window panes. The PDC broke the windows of both upper bedrooms facing the direction of the crater, but the house was unfurnished and there was little evidence of the PDC having entered the building beyond causing heat damage to the frames themselves. The other windows remained intact and the rooms behind were undamaged. The wooden board roof was burnt through in two localised areas facing the direction of the PDC (one can be seen in Fig. 2b) and this was undoubtedly due to the heat transfer from the flow itself acting against the tall roof of this two-storey house. The house in front had had its facing windows protected by hurricane boards (see below), and the PDC cloud had not disturbed any of the furnishings or contents, or caused fires; very little ash had entered the building. 2. Peripheral zone. Here, where the PDC was moving slowly, the behaviour was governed by topography and sheltering effects, unless the building was exposed, when directional effects were observed with the flow forcing its way through a building by entering the facing windows and exiting by the back windows without shattering the building structure. The house in Fig. 7a shows how the exterior of the house was undamaged in the shelter of a neighbouring house located up slope, but the PDC had gained entry through the side front corner window. Inside, the current had apparently exited in a directional manner through the adjacent corner window and ignited the TV standing in the corner (Fig. 7b). The rest of the room and its furnishings were left undisturbed, apart from a thin layer of ash covering everything. Paper and furnishings showed no heat damage. The most significant findings, as far as mitigation is concerned, were several buildings which had had their louver windows covered with thin (1 cm) plywood boards as hurricane protection when the area was evacuated. In Fig. 7c, one of these is shown located about 150 m from the house in Fig. 7d. The effects of heat from the current were visible as blackening of the wood from scorching on the surface of the boards facing the crater and on the bitumen roof tiles. Inside, we found only a thin layer of inflated ash (1–2 cm.) over the furniture and papers, with no evidence of burning. The effectiveness of the boards in preventing the ingress of ash was very striking.
The typical appearance of an unsheltered house with unprotected windows in this zone is shown in Fig. 7d. Both facing and back windows and their frames are blown out and the inside of the house has been completely gutted by fire from the hot ash deposits that filled the rooms to several centimetres depth. Furnishings would be instantly ignited by contact with the hot ash. The wooden board roofs would have been ignited by the interior fires. Where the building was not directly facing the crater, or it was less exposed, the ash would enter through the openings (windows, doors) on the most exposed side and leave directionally through the opposite openings, with other parts of the building receiving less of the impact. The greater in-filling of ash on the side of the house facing the crater resulted in fires being more frequently localised on that side of the building, and this provided the explanation for the frequently made observation from the air that partial burning of roofs facing the crater was the only external sign of damage in many of the houses. The alternative explanation, that the external heat transfer from the fast moving PDC directly burned away the roofs in these one storey buildings, can be seen to be disproved in the example in Fig. 7e. The construction of this house, which was exposed and close to the house in Fig. 7d, had only been just completed and it contained no furnishings or furniture, so the damage to the roof was entirely from the heat of the passing current. Although the heat had burned the wood away, a large opening has not been formed, unlike the combustion caused by internal fires. The ash on the floor of the room below was 15 cm. deep but the radiant heat would not by itself have given rise to this degree of damage. Other examples of buildings without furniture, or with little furniture, were found which showed similar effects. The effects of the PDC moving directionally through houses which have weaker, galvanised steel sheets for roofing are shown in Fig. 8a. The interior has burnt out with the sheets being blown off the rafters on one side and the rafters have burnt away on the other, allowing the remaining sheets to fall inside. The abrasive action of the particles and entrained fine material on the outside wall surface is also visible. Some occupied houses were only partially completed and were constructed out of reinforced concrete and had concrete roofs: they showed little external damage except for the typical effects of the PDC on window openings (Fig. 8b). 3. Intermediate zone. In this zone the lateral loading of the PDC was more in evidence, and combustion of the inside of buildings and their roofs was more complete. Damage to external and internal wall structures was found with evidence that large missiles, such as elements from other buildings damaged in the PDC, became entrained and assisted in battering the structures (Fig. 9a,b). The house in Fig. 10a–d is shown from all sides to indicate the full extent of the damage to openings and the
301
Fig. 7 a December 26, 1997. A house in the peripheral zone was protected by topography and the neighbouring house which lies slightly above it. The PDC entered by the side window in the porch, igniting the TV in the corner of the room. b The PDC passed out of the room through the opposite window behind the TV. The rest of the room was unaffected, apart from a thin layer of ash over the furniture. c December 26, 1997, peripheral zone. A house was protected from the hot PDC ash by plywood boards nailed over the windows to protect the evacuated house against hurricanes. Some heat damage to the bitumen roof shingles is visible. d A house
located 150 m from the house in Fig. 7c. The PDC has blown out facing and rear windows in the direction of flow from the crater. Note the sandblasting effect on the paint of the walls. e Peripheral zone. A house under construction before the evacuation and without furniture or fitted windows. The combustion of the roof was due to the heat transfer from the moving flow and also the radiant heat from the ash deposit inside the house. Note the limited amount of burning of the roof only, suggesting that the duration of the flow was brief
outer structure. Part of the outer wall facing the volcano had been knocked down and the roof completely burned through. Part of the balustrade on the balcony was missing. Although some windows were completely blown out, the kitchen windows and outer door were mostly intact. Inside the kitchen there was little disturbance to the wooden furniture apart from it and the floor being cov-
ered in a deep layer of insufflated ash (Fig. 10e). In Fig. 10f, the interior of a house in this area is shown, with the effects of fire and radiant heat from the ash on the concrete. In some houses there was little burning in some rooms, but the radiant heat effects on the ceiling suggested that oven-like conditions must have prevailed for some time until the ash deposits cooled.
302
Fig. 8 a Peripheral zone. A house with a corrugated steel sheet roof, with the sheets having been blown off on the right hand side, but burning of the rafters has left the sheets collapsed inside on the left. The utility pole and its wires were flattened. b Peripheral zone.
A house with a reinforced concrete roof and concrete block walls showing the blown out window frames and bent rebars. The concrete roofs on this type of building remained intact even in the intermediate zone
Fig. 9 a Intermediate zone. A house showing more extensive damage and there are abundant projectiles on the ground. A galvanised steel sheet roof has been blown off and deposited on the ground, and a palm tree blown over. b Intermediate zone. A dev-
astated house with large missiles visible against the outer walls. Outer and inside walls intact, but the balustrade was driven against the house. Galvanised sheets blasted and bent around the wall post
A good example of the flow damage in this zone is seen in the house in Fig. 11a. The roof of the collapsed building next to it has flipped over and landed nearby, but the force of the flow has blown off the balcony roof (and probably the main roof as well) and the concrete roof beam was driven outwards by the impact. The example in Fig. 11b,c shows how a tree trunk had smashed its way into a house through the facing outside wall of the kitchen before coming to rest against the inside of the wall overlooking the sea. Interestingly, it covered over some of the wall debris which lay on a layer of ash on the floor, suggesting that the tree was carried in the main part and not at the head of the PDC. Alternatively, this finding provides evidence for two PDC pulses, with the second pulse carrying the tree and possibly being stronger than the first.
4. Central zone. The remarkable evidence for the total destruction that can be wrought by PDCs is shown in Fig. 12a-f. Clearly, whole buildings have been entrained and carried away in the current, and Fig. 12f shows how boulders (1-metre diameter) have been transported from somewhere high up on the volcano (possibly from the dome) to become projectiles. The destructive mechanism can only be surmised (e.g., the head of the PDC and its missiles caused the buildings to fragment and the pieces were then entrained in the main body of the PDC), but the flattened steel poles of the Police Station fence (Fig. 12c) indicate the enormous lateral force involved. Even at the heart of such devastation the effects of sheltering are apparent in Fig. 12d,e. The kitchen basement area of the house was saved by the protection provided by the slope of the volcano at this point, so the ground floor of the house which acted as the basement
303 Fig. 10 a-f Intermediate zone. Views of a single house showing all four outer walls. a The wall facing the volcano was badly damaged by missiles. The door to the kitchen (b) is intact and inside the furniture is not disturbed (e), despite the damage to the rest of the house. f Intermediate zone. Interior of a house showing radiant heat effects of fires and the hot ash on walls
ceiling was level with the slope of the ground and everything above it disappeared. The lack of disturbance of the table and chairs, together with the absence of heat effects, is remarkable and should be compared with Figs. 7b and 10e. As in many basement and garage areas below the houses, the ash deposits were fine and inflated with air, suggesting that the ash had in-filled these parts of the buildings after the main PDC had passed and during the turbulent mixing of the PDC cloud with air. Mapping the building damage in the December 26 event The distribution of the damaged buildings in the outer, peripheral and intermediate zones is shown in the map in Fig. 13. A line (more or less along German’s Ghaut) separates the outer and peripheral zones from the central zone; the two areas have been subdivided by another line from north to south that appears to be a natural spatial divide between those houses closest to the volcano and
those nearer the ocean. Building damage levels were analysed for these areas designated A-D (Table 1). Studies of the aerial photographs taken after the eruption showed the following distribution of the remaining building types: – Timber walls and roof (1%) – Concrete block walls with reinforced concrete roof (12%) – Concrete block walls with timber board roof (33%) – Concrete block walls, or some with rubble masonry walls (the two wall types are indistinguishable from the aerial photographs), with galvanised corrugated steel roof on wooden rafters (49%) – Unknown (5%) This distribution was very similar for each area A-D. The numbers of houses was too small within this narrow belt of settlement to adequately quantify all the structural and damage variables involved, at least for detailed statistical
304 Fig. 11 a Intermediate zone, near the edge of the central zone. The roof was blown off and tipped over, forcing outwards the concrete balcony roof-support. The roof from a nearby collapsed building has been blown over onto the ground. Note the angle of the bent fence poles in the foreground. b Intermediate zone and near the edge of the central zone. The house is severely damaged, with collapsed outer and internal walls, and has been imploded by a large tree trunk acting as a missile. The masonry lies beneath the tree (c), see text
analysis, especially as suitable aerial photographs, or maps, showing the distribution of the houses before the eruption were not available. The distribution of the levels of building damage according to the provisional damage scale in each area is shown in Table 1. Only 3 buildings were in Level 4. The finding of 13 houses at Damage Level 3 in the peripheral zone (Areas A and B) was most likely due to the difficulty of distinguishing the damage caused by fire from that caused by dynamic pressure using aerial photographs alone, so some degree of misclassification has to be accepted when using this technique (the volcanic hazard prevented more ground observations being made to fully validate the conclusions from the aerial photographs). However, this is unlikely to be the explanation for the lack of a more obvious gradient of building damage with proximity to the crater. Thus, combining Areas A and C and comparing with Areas B and D shows that the proportion of houses at damage levels 0–1 is 53% and 61%, respectively, a difference that is not statistically significant (Chi square = 1.19; p >0.05). But the damage gradient from the outer and peripheral zones (Areas A and B) to the zones closer to the central axis of the PDC (Areas C and D) is much more evident, with the proportion of buildings with damage levels 2–4 being 31% and 56%, respectively (Chi-square = 12.94; p < 0.001). In addition, the proportion of houses at damage level 3–4 in Area C (33%) is double that in Area D (17%), a difference which is just statistically significant (Chi square = 3.69; p < 0.05), which suggests that the larger amount of missiles picked up in this high velocity part of the flow was adding
to the damage inflicted by the PDC to houses furthest away from the crater. These results, together with the finding of the presence of level 1 and 2 housing damage throughout the footprint of the PDC up to the edge of the central zone (where all buildings were classifiable as at Damage Level 4) is noteworthy. Most wooden outhouses and sheds had been destroyed by the force of the momentum of the PDC or by fire. The example of the range of building damage within a small area in Area D (Fig. 14) is clear evidence for the conclusion that dynamic pressures can fluctuate in the range of 1–5 kPa, or more, depending upon localised topography and sheltering effects, with variations in velocity and density, even near to the area at which the impact of the PDC becomes totally devastating and the maximum dynamic pressure exceeds 25 kPa. In this particular event and over this locality, then, proximity to the central axis of the PDC is clearly much more important to the destruction of buildings than proximity to the crater. Dynamic Pressure Scale for building damage In Table 2 we present a scale on levels 0–5 based on our observations on Montserrat and our estimated dynamic pressure values from theoretical considerations and our analytical work performed for buildings in the Vesuvian area, which included static destructive testing of building elements. We considered that the results of static testing were applicable to lateral loading estimates of PDCs, with their
305 Fig. 12 a-f Central zone. Devastation complete, only the foundations of any buildings are visible. St Patrick’s police station is shown in b, and a close up of the bent outer fence poles can be seen in c, lying flat on the ground (a thin layer of tar from the combustion of the vegetation is also visible, see Sparks (2002) for explanation). d, e A severely damaged house in the central zone, with its upper structure having been blown into the sea. The slope of the ground has protected the lower kitchen area facing the sea. Remarkably, the furniture is undisturbed with only a layer of insufflated ash on the floor. f Large boulders acted as projectiles and became embedded in the side of the remains of this building in the central zone. This picture provides clear evidence of the destructive impacts provided by missiles entrained in the PDC flow, which greatly added to the destructive force
relatively slow rise to peak dynamic pressure values compared to explosive blasts. The use of a 5-point damage scale (Column 1) and the accompanying descriptions (Column 2) match the scale used in post-earthquake damage and intensity assessment, for example in the European Macro-seismic Scale (Grunthal, 1998). The descriptions of damage (Column 3) refer to the effects on the classes of buildings on Montserrat, mostly with concrete block walls and lightweight roofs. A separation was attempted between those effects which were caused by the direct action of the dynamic pressure and temperature of the PDC external to the building, and those which were secondary effects of ingress of hot ash into buildings (in italics). The expected effects on other building types (not found on Montserrat) of the flow conditions at each damage level were based on calculation (Spence et al., 2004b).
Discussion Although PDCs are frequently produced in explosive eruptions, many aspects of their flow and behaviour remain poorly understood. The December 26 lateral blast and the two preceding main eruptive events at the Soufrire Hills volcano described here have provided the first opportunity to evaluate the impact of a violent surge in a human settlement since Lacroix’s classic study of the destruction of St Pierre, Martinique, the world’s worst volcanic disaster, in 1902 (Lacroix 1904; Scarth, 2002; Zebrowski, 2002). PDCs are high speed, gravity driven flows of hot particles and gas (with or without free water) which can be formed in at least three ways: by gravitational collapses of lava domes, the fountaining of vertical eruption columns, or by lateral blasts (Druitt 1998; Sparks et al., 1997). However they are formed, the present work goes towards confirming that the behaviour and hazards of PDCs can be very similar. The surge cloud of the June 25
306 Fig. 13 St Patrick’s: map of the locations of the houses and their level of damage. Level 1 damage to houses was present throughout the peripheral and intermediate zones, up to the edge of the central zone
Table 1 Numbers of buildings (% buildings in parentheses) with different levels of building damage in the four areas affected by the pyroclastic density current
Fig. 14 An area in the intermediate zone which shows level 1-3 damage to houses located within a short distance from one another. One wooden house at the top of the picture is protected by topography (Level 1), whilst the house in the foreground has suffered severe damage, with the roof blown off, windows and their frames missing, and partial damage to internal walls (level 3–4). These differences reflect a range of dynamic pressure 1–10 kPa within a 200 m. distance and are an illustration of the mixed damage levels seen in Fig. 13
Damage level
Area
0 1 2 3 4 Total
31 17 11 9 0 68
A
Total B
(46) (25) (16) (13) (0) (100)
13 12 9 4 0 38
C (34) (32) (24) (10) (0) (100)
1 20 21 19 2 63
D (2) (32) (33) (30) (3) (100)
4 23 12 7 1 47
(8) (49) (26) (15) (2) (100)
49 (23) 72 (33) 53 (24) 39 (18) 3 (1) 216 (100)
eruption we studied detached from a pyroclastic flow derived from a gravitational dome collapse, and then reformed by rapid sedimentation to flow down the Belham River valley (Loughlin et al. 2002b; Druitt et al. 2002). The December 26 eruption is thought to have resembled in some features the 1980 lateral blast at Mount St Helens, in that a failure of the edifice generated a debris avalanche followed immediately by a high energy PDC, although the volume of material was only 5% of Mount St Helens (Sparks et al., 2002). Devastating eruptions can be judged by the area of destruction that they produce. At Mount St Helens the surge covered 600 km2 as far out as 28 km from the crater in a 180 sector; and at Mount Lamington the footprint of the surge was 176 km2 in a 11 km radius of 3600 around the volcano. At Montagne Pele, the
307 Table 2 Building damage scale for dynamic pressure impact of pyroclastic density currents 1
2
3
4
5
Level
Description
Effects: observed
Effects: estimated
Approx pressure range (kPa)
Level 0
No Damage
Little damage except to occasional window panes.
Less than 1
Level 1
Light Damage
Level 2
Moderate Damage
Level 3
Heavy damage
Windows and roof intact Minimal external burn damage to buildings, including even lightweight wooden houses. No evidence of pushed over fences or posts, but light charring of wooden posts and vegetation, including trees in places No or minimal ash penetration. Infiltration of ash due to window catch or frame in bad condition. One or two windows on side facing crater with windowpane broken. No, limited or even extensive fire damage to roof. Effects mainly due to intense heat, not dynamic pressure. Fencing and posts intact and unbent. PVC guttering melted Thin layer of ash inside. Heat damage localised to impacted room or rooms, and even to within a corner of a room. Old wood window and door frames imploded on side facing crater, exit windows or panes blown outwards, but window frames intact, or roof partially burned through in places from external heat of flow. In surrounding area visible effects of dynamic pressure and heat. Patchy sandblasting of walls facing volcano, scattered flow debris. All ordinary wooden houses consumed Deep layer of ash in rooms where penetration has occurred, but fine layer only in remainder of building. Fire caused by combustion of furnishings by hot ash deposit. If no furnishings, roof stays intact but externally damaged by heat of flow. Complete combustion in rooms where fire occurs, part of roof burnt out from internal fire. All windows on side of volcano imploded, frames missing, and windows on opposite sides blown out or outwards, including frames, roofs lifted off. Evidence of flow missile damage. Missiles such as galvanised sheets and wood more abundant and gathered against walls facing crater. Tops of sturdy trees cut off, most trees and utility poles downed or pushed over. Fences and posts pushed over Widespread internal fire with ash deposit throughout, roof burnt away by internal fires, radiant heat from deposit, or heat transfer from flow
Level 4
Partial devastation
Level 5
Total devastation
As for Level 3, but loss of parts of external and/or internal walls. Large single or multiple small missile impacts to wall facing volcano, most or all of roof missing from fire or lifting off of non-RC roofs. No lightweight buildings left standing. Abundant missile debris. Walls removed, only parts or none of the structure still standing. Multiple large missile impacts. Complete devastation from heat, dynamic pressure and missiles; ground scoured with little deposit or remaining debris.
PVC louvers melted, aluminium frame catches in bad condition break allowing ingress of ash. Window panes break.
1 to 3
Aluminium window and door frames damaged or destroyed.
2 to 6
Some serious damage to masonry structures. Masonry in-fill panels of reinforced concrete (RC) structures fail in numerous buildings Many one- and two-storey weak non aseismic RC structures collapse; some multi storey strong non-aseismic and weak aseismic RC structures collapse.
4 to 10
All masonry in-fill panels collapse. Widespread serious damage to masonry buildings, most RC structures collapse except for strong aseismic RC structures
8 to 25
All masonry and non-aseismic RC structures collapse. Only strong aseismic 1 and 2-storeys RC structures survive, but these are seriously damaged.
>25
Secondary effects from the heat of the ash in italics
wedge-shaped sector covered 20 km2 and extended at least 11 km to the sea, or twice the size of the PDCs we investigated on Montserrat. The value of our study lies in the insights it provides on the causes of building vulnerability in PDCs. The main characteristics of the impacts of the 26 December eruption were the lateral loading and the directionality of the
PDC from the crater; the current flowing like a dense and hugely powerful wind; and the presence of missiles of all sizes from dirt and shingle, including ash and lapilli, to large boulders and trees which greatly added to the destruction. The heat of the ash ignited all flammable materials and caused widespread destruction even when, as shown at Streatham village (Fig. 3), the dynamic pressure
308
of the flow at its limit was too low to inflict mechanical damage. The most vulnerable aspects of the building envelope to these hazards were the openings, in particular the windows. The louver, or lightly shuttered, windows in tropical countries are designed to allow the free movement of air, but this increased the buildings’ vulnerability by permitting the hot ash to enter the closed-up houses even at the limit of the current and ignite the furnishings and fittings. We found some houses had been protected by the simple expedient of covering the windows with plywood boards, a device used to guard against hurricanes when the area was evacuated. It was fortunate that, on the basis of recommendations of the scientists at the Montserrat Volcano Observatory, none of the houses in the Montserrat eruptions described here were officially occupied. However, nineteen people were killed on June 25 when they chose to ignore this advice (Loughlin et al., 2002a). Small topographical variations played a crucial part in sheltering or exposing houses to the PDC. An important finding was the wide variation in building damage across the peripheral and intermediate zones reflecting a dynamic pressure range of 1–5 kPa or more within small, localised areas, but as the dynamic pressures exceeded this range closer to the axis of the PDC then destruction became more complete and less governed, if at all, by environmental influences, with total devastation suggesting dynamic pressures over 25 kPa. The absence of an obvious gradient of damage with radial distance from the crater also suggests that the PDC impact below 10 kPa can be strongly governed by dynamic flow behaviour around buildings and topographic obstacles, rather than attenuation over this distance of the average velocity of the main current. Furthermore, fire could cause as much damage to buildings as dynamic pressure in the peripheral flow areas (Figs. 2a, 3, 4a–c), and thus could lead in some instances to an overestimate of the dynamic pressure in field studies or wrongly attributing damage effects to dynamic pressure alone. The building damage was, in general, compatible with a strongly directional lateral loading without having an obvious diffraction or peak pressure component, as in an explosive blast wave, which would have given a more crushed appearance to the buildings, as happens in a conventional chemical or nuclear explosion (Glasstone and Dolan, 1977). The apparent strong directional flow of the PDC through buildings and yet being deflected in places by buildings and the topography in its path was also unlike the impact expected in a blast wave (Mays and Smith, 1995). Using numerical modelling, Woods suggested that an expansion blast wave from the fragmenting lava dome gave way to a displacing gravity flow front at about 1 km from the crater (Woods, 2000). A further consideration is the effect of the isostatic pressure operating in all directions around the buildings, due to the force from the height of the surge cloud and other pressure oscillations in the PDC (Esposti Ongaro et al., 2002). In the December 26 eruption Woods estimated this could
have been as large as 10 kPa (Woods, 2000), so it too may have been a factor that contributed to the variable building damage impacts seen within the intermediate zone. All our ground observations, however, were compatible with lateral loading and suction effects from the flow around the buildings, although the thin layers of ash in the few intact buildings we came across may have been partly due to isostatic pressure operating as well. In this typical tropical housing with louver windows, we surmised that the isostatic pressure changes would most likely have rapidly equilibrated across the building envelope without causing significant damage. Missiles and other entrained material could also have accounted for the very anomalous evidence for dynamic pressure that was found, e.g., in the bent steel fence posts and reinforced bars (re-bars), indicating that they had been struck repeatedly by missiles rather than being bent by the dynamic pressure of the surge cloud alone. At the Building Damage Scale level 3, the angles were approximately 40–60, at level 4, 70–80, and at level 5, close to 90. The alignment of poles in Fig. 12c is impressive, and suggests an overwhelming combined force of transported material and current. The missiles ranged from fine material from the ground, e.g., lapilli (stones) and small blocks (shingle), and large objects entrained in the flow, depending upon the nature of the local terrain. The role of ground materials and building elements as projectiles is one of the most intriguing of our findings, and it seems of great importance in accounting for the damage in PDCs in addition to the dynamic pressure. Local extreme variations in damage could arise with windows being more readily penetrated if objects become swept into turbulent flows. We also found some evidence in the high velocity part of the PDC that projectiles entrained from the damage to buildings (in Area D) may have contributed to increasing the severity of damage to buildings impacted subsequently by the PDC (in Area C). Interestingly, the part played by windborne flying debris in hurricane damage has only recently begun to be appreciated as well (Wills et al., 1998). The size of windborne missiles of a given density is proportional to the dynamic pressure, and destructive power is proportional to their kinetic energy, which is to the fourth power of the dynamic pressure, and inversely to the square of the object density (Wills et al., 1998). Volcanologists should therefore be cautious in calculating particle concentrations (density) or velocities of PDCs using dynamic pressure figures derived from impacts on individual buildings or objects, at least in the lower dynamic pressure ranges, where a wide variation may be found close to the same location. Valentine (1998) has proposed that the dynamic pressure of the PDC, as estimated from the building damage, could be used to infer PDC particle concentrations from independent estimates of velocity. Sparks et al. (2002) used such assumptions about the PDC of the 26 December and took Valentine’s published dynamic pressure data to derive velocities in the impact area. They estimated PDC velocities of 30–40 m/s at Kinsale, exceeding 40 m/s at Germans Ghaut, and well in
309
excess of 60 m/s in the central area (theoretical models predict peak velocities in excess of 100 m/s: Sparks et al., 2002.). However, the new evidence on the importance of missiles raises doubts about the validity of this approach. The photographic evidence for the December 26 event permits some interpretation of the duration and behaviour of the PDC. Sparks et al. (2002) consider that the most intense activity was pulsatory and lasted about 12 min according to seismic signals, and as a consequence of the retrogressive failure of the dome. Pulses could also account for the erosional and depositional features. However, our findings indicate that the main impact of the PDC was unlikely to have lasted for more than a minute or two for the following reasons. In the more peripheral impact area the roofs of houses which did not contain furniture did not burn through and the sides of trees and shrubs left standing were only superficially burnt. Walls had patchy abrasion marks and were not severely sandblasted in appearance, and they had not been darkened by heat. The accumulated ash inside buildings in the peripheral area, where most deposition had occurred, was not great. The house damaged by the tree projectile had masonry beneath the tree (Fig. 11c), but little debris had accumulated on top of it, suggesting that the current following, or a subsequent pulse, did not continue long enough to cover it with material (or, had erosional features). Thus, the findings could be explained by a single surge of short duration, though several pulses could not be excluded. Montagne Pele and Soufrire, St Vincent 1902, Vesuvius, AD 79 The findings show a remarkable similarity to those documented by Lacroix (1904) at Montagne Pele’s eruption, and by Anderson and Flett (1903) at St Vincent, a hundred years before the present work. The lateral directed blast of Montagne Pele on May 8, 1902, generated at the base of a growing lava dome, bears strong comparison with the December 26 event, but interpretation of the eruption sequence from stratigraphic evidence in the former is complicated by the occurrence of at least six more PDC eruptions in the months following which were poorly observed by local people and scientists. No satisfactory account of the events in terms of deposits and PDCs has been forthcoming despite much research, as controversy still exists over the number of flows that reached St Pierre and the interpretation of the stratigraphic findings (Bourdier et al., 1989). The majority of published photographs of St Pierre were taken after the May 20 eruptive event, which caused most of the destruction of the buildings, and also after the other major PDCs on July 9 and August 30 had completed the levelling of the city. Other, smaller PDC’s may also have reached St Pierre, e.g., on May 28 and June 6 (Boudon and Lajoie, 1989). Of the four main scientific teams to report on the disaster, Kennan’s (Kennan, 1902), which included Hovey (Hovey, 1902) and Jaggar (Jaggar, 1949),
was the first to arrive in Martinique on May 21, followed by Heilprin on May 25 (Heilprin, 1905), Lacroix on June 21 (Lacroix, 1904) and Anderson and Flett (Anderson and Flett, 1903) in early July (they witnessed the July 9 surge at close hand). Thus, accounts of the impact of the May 8 PDC itself are sparse, but some photographs were preserved for the record and photographs taken after the later PDCs do show the course of the destruction. The photographs in Lacroix’s monograph (Lacroix, 1904) clearly show how, in the days after the catastrophic PDC on May 8, hundreds of two-three storey buildings along the shore and in rows inside the central and southern parts of the city were still standing and burning, including one of the two cathedral towers; but their wooden roof structures and tiles had mostly gone (Lacroix, 1904; Kennan, 1902; Heilprin, 1905). The rubble masonry walls (surviving relics of these are a metre wide) were charred and crumbling from the heat of the fires that had consumed the city (Heilprin, 1905). Photographs taken before the eruption show that the windows had heavy wooden shutters, which would have been wide open in the early morning, and moveable slats, which allowed the hot ash to enter and immediately ignite the interiors so the whole city rapidly caught fire (restored examples of these urban buildings can be seen today in the old town area of Basse Terre, at the foot of the Soufrire volcano, Guadeloupe). Subsequent photographs confirmed that the May 20 PDC was responsible for the major levelling of the city and, by the time Lacroix arrived in June, most houses had been reduced to their foundations and heaps of rubble masonry. The PDC formed in the lateral blast on May 8 left a thin deposit only in the city, but the May 20 event left a deposit of decreasing thickness from north to south into the city, with a depth of 30 cm in the centre or Mouillage (anchorage) area. These differences may reflect contrasts in the forces or in the movement of the two PDCs, including their propensity to entrain missiles. The deformed and flattened iron/steel fence poles on Montserrat are reminiscent of the deformed bars that had been the decorative surrounding frames to the graves in the Fort Cemetery and the deformed steel chairs used to restrain patients in the mental hospital (Maison Coloniale de la Sant); the bars must have been deformed in the course of the May 8 and May 20 eruptions (they were buried by the latter) and reflect the directionality of the surges from the crater (Boudon and Lajoie, 1989). Lacroix was able to reconstruct the directional nature of the PDCs leading back to the summit of Montagne Pele, the hurricane-like forces on the structures and the entrainment of missiles from the damaged buildings adding to their destructiveness. His conclusions on the building damage were therefore remarkably similar to our own. The rapid destruction of the city and the loss of lives on May 8 were attributed to the hot ash igniting inflammable objects in the interiors of the buildings and the whole city bursting into flames, which we can confirm occurs in PDCs from our own observations on Montserrat. Lacroix estimated the temperature of the PDC to have been around 450C (Tanguy, 1994), again similar to the
310
temperature range estimated from deposits on Montserrat. Using the Montserrat findings we can infer the dynamic pressures involved, though the rubble masonry buildings of the city would not have been as resistant as most of the structures we studied on Montserrat. On May 8, destruction was greatest at the northern Fort area, which was the closest part of the city to the main axis and greatest velocity of the PDC, with the dynamic pressure attenuated towards the Centre and southern Mouillage areas. The Mouillage was the location for the ships which were also caught in the PDC, and the urban area there contained the lighthouse and the cathedral. We estimate that the dynamic pressure at the Fort would have to be around 25 kPa, falling to 5–10 kPa in the main city in order to account for the loss of roofs and upper storey rubble masonry walls on many buildings of two to three floors in height, and the wide range of building damage (on our scale, levels 1–3, Table 2.). The PDC on May 20 need not have been so powerful to flatten the remaining buildings, as they were already in a much weakened state, but the presence of entrained missiles suggests a dynamic pressure of at least 5–9 kPa. Concrete boulders derived from blown down walls have been found in the deposit (Boudon and Lajoie, 1989), but Hovey also found boulders up to 1 m wide in the city ruins on May 21 (Hovey, 1902). The missiles were mainly from debris, but the boulders had to have been transported to the city from the volcano or its flanks, which implies a much higher dynamic pressure (>25 kPa) and is in accord with the views of some authors that the May 20 PDC was more powerful than on May 8. However, such a force would have razed buildings to the ground and blasted the debris into the sea if the mechanism of entrainment had been the same as that observed at St Patrick’s in our study (Fig. 12f). Our findings are also consistent with the classical accounts of the August 30, 1902, eruption of Montagne Pele, when a PDC struck the villages of Morne Rouge and Ajoupa-Bouillon on the south-east flank of the volcano and not far from St Pierre. About 2,000 people had returned to the villages following an evacuation after the May 20 eruption despite the obvious danger, and 1,200– 1,500 died when the PDC struck (Lacroix, 1904; Scarth, 2002; Zebrowski, 2002). In Mourne Rouge, the houses were made of wood with galvanised sheet roofs and most of these collapsed, with some catching fire, and just the stone-walled church and its steeple, together with one or two other large buildings were left standing. Within 1 km of the edge of the PDC the more robust housing kept out the surge and people survived inside, but where the doors or windows flew open the hot cloud killed or badly burned the occupants. At Ajoupa-Bouillon a similar story was told, with houses and trees blown down by the scorching cloud, but there were no fires, and the cottages and their inhabitants survived the dilute PDC if the doors and windows were tightly closed (Lacroix, 1904; Heilprin, 1905; Kennan, 1902). Anderson and Flett’s description of the May 7, 1902, eruption of La Soufriere on St Vincent also contains graphic accounts of survival in houses which kept out the PDC cloud whilst people less
protected or who were outside were all killed, with the main causes of death being due to heat and asphyxia (Anderson and Flett, 1903). As seen in the dome collapses of June 25 and September 21, 1997, on Montserrat, in eruptions where dilute PDC clouds are slowing and have insufficient momentum to damage even weak building structures, the hot ash (>300–400C) will ignite interior fires and cause lethal burns on direct contact with the skin even though the thickness of the deposit on the ground is less than one centimetre. The temperature of the ground deposit will remain high and radiate intense heat for a period depending upon its thickness, but the duration of the hot PDC cloud itself in the Montserrat examples would probably have lasted no longer than 1–2 minutes at most. Another example of a PDC where heat was more important than dynamic pressure was during the AD 79 eruption of Vesuvius when hundreds of people were killed as they sheltered in boat chambers on the shore at Herculaneum, as the dilute PDC (S1) had insufficient force to do much damage to the buildings along its path (denoting in our view a dynamic pressure < 3–4 kPa for ancient Roman structures). However, on coming to a halt at the beach the PDC rapidly underwent sedimentation and filled the chambers up to 1 m depth with hot ash (>400C.), killing the people instantly (Mastrolorenzo et al., 2001). Mount St Helens, 1980 and Mt Lamington, 1951 The May 18 eruption of Mount St Helens was in a forested, wilderness area, but left 58 dead out of those who had been present and unprotected in the open at the time. An important feature of the impact of the PDC was the stripping of the mainly conifer trees of their branches and bark, and the levelling of their trunks in the zone of tree blow down (indicating a dynamic pressure of at least 2–3 kPa). The downed trees were combed into flow patterns, but were mainly aligned in the general direction of the PDC which, whilst moving unconstrained over ridges, was controlled by topography in places (e.g., Waitt, 1981). The PDC cloud was opaque, rapidly moving and ground hugging, and observers reported it had a leading edge that was less than 200 metres high followed by a billowing, convective cloud. The trees were wind-snapped (stem fails above ground level) or wind-thrown (stem and root plate overturned) and some were also entrained as missiles in the PDC, whilst the upright stumps were damaged by projectiles and the sides facing the crater were blackened by the heat, as well as being abraded or sand blasted (Waitt, 1981). In the scorch zone extending for 1–3 km at the periphery of the PDC margin, where people survived with no or minor injuries, the foliage of trees was badly singed; here, the temperature of the PDC would have fallen from >300C to 50–200C and lasted less than two minutes (Moore and Sisson, 1981). The dense forest could have acted as a rapid heat sink for the hot ash, and provoked more mixing with air, which may
311
explain why many people survived being enveloped in the periphery of this PDC (Baxter, 1990), whereas the temperature of the PDCs on Montserrat remained very high and lethal up to almost the limit of their course. A similar consideration probably also explains Taylor’s findings of variable temperature effects on trees in the devastated forest zone and scorch zone around Mt Lamington (1951), in addition to his evidence for the destructive role of planks and corrugated sheets as missiles (and his conjecture about the deflection of the steel flagpole at Higaturu being caused by flying debris impacts; Taylor, 1983). At Mount St Helens, the role of explosions in adding to the destructiveness of the May 18 PDC is debatable (Hoblitt, 2000; Kieffer, 1981), though terms used to describe this type of eruption, such as a lateral or directed blast, add to the confusion. Local blast waves could have been generated at or near the crater, or further into the channelled flow area, but blast waves would have rapidly attenuated and it is widely agreed that the main PDC was gravity driven (Sparks et al., 1997). The blow-down and delimbing of trees, and the patterns of the downed trees on the ground, are in keeping with the ways in which the PDCs in our study flowed with such striking directionality through the openings in the buildings. In addition, the increase in damage towards the centre of the flow axis in our study would be hard to explain as being caused by a blast wave, which would instead be expected to attenuate with radial distance from the crater. Valentine’s theoretical study (Valentine, 1998) does not differentiate between peak overpressure and dynamic pressure, the forces in conventional weapon and nuclear blast waves, which is one possible reason why his paper reports higher dynamic pressures for given building damage levels than ourselves. Thus Glasstone and Dolan (1977) provide a table showing the separate contributions of peak and dynamic overpressures, which shows that at low dynamic pressures (1– 10 kPa) the peak overpressure is about an order of magnitude greater than the peak dynamic pressure due to the wind velocity behind the shock front. In contrast, our observations on the impact on buildings in this study can all be convincingly explained by lateral loading by dynamic pressure and the kinetic energy of entrained loose materials and projectiles. In the PDCs described in this paper, the dynamic pressure is solely from the gravity current and there is no peak overpressure, but pressure perturbations acting on the building envelope due to the internal dynamics of the current need to be considered (Esposti Ongaro et al., 2002), as well as the variation with flow density, velocity, and hence dynamic pressure, with the height of the PDC. The relatively slow rate of rise of the dynamic pressure in PDCs also ensures that the results of quasi-static loading tests on building elements of actual houses are applicable to our building damage scale. In addition, the nuclear blast damage studies, as summarised by Glasstone and Dolan (1977), do not include estimates of damage due to entrained particles or missiles. This could also partly explain the higher dynamic pressure values they suggest compared with those we have esti-
mated in Table 2. Although we found no field evidence for a diffraction shock front preceding the PDC in any of the three PDC events on Montserrat, as illustrated by Valentine (1998), this could be present in PDCs with higher velocities than those studied here (Wohletz, 2001).
Conclusions The similarities between our findings on Montserrat and the reports from these previously recorded eruptions support the devising of a Building Damage Scale for the dynamic pressure effects of PDCs which could have general applicability. Our findings provide a consistent description of the behaviour and movement of violent PDC’s of small volume and permit for the first time the quantification of building vulnerability in PDCs from field studies of an actual destructive event. The results do have to be qualified in that the behaviour we have observed is part of a continuum of processes and complex phenomena, not all of which will have been incorporated in our observations of impacts on the built environment. Furthermore, the behaviour of PDCs will vary between eruptions and at different volcanoes, depending upon many factors including the initial conditions of the magma, which can only be surmised, the volcanic conduit and vent dynamics, the density stratification and particle sizes of the PDCs, and so on, all of which may change in the course of the eruption (Druitt, 1998; Wohletz, 2001). The topography and other ground features may also modify the behaviour of the PDC in the course of its run-out. Nevertheless, the implications of our conclusions need to be considered for disaster mitigation in other volcanic eruptions. For example, our findings illustrate the problems of micro-zonation strategies in populated areas close to volcanoes, especially when such energetic PDCs are also capable of overcoming topographical barriers. The dramatic increase in the activity leading to the June 25 PDC and the formation of the surge-derived PDC which flowed down the Belham Valley were unexpected by scientists monitoring the volcano (Loughlin et al. 2002b). Our work shows that small to moderate sized PDCs behave like gravity-driven, suspension currents (Simpson, 1997), and have lateral loading effects that are not analogous to man-made explosions. On the available evidence, we consider that explosive blast waves in general, and nuclear blast waves in particular (Valentine, 1998), provide only a weak analogue with the flow of a PDC. The high temperature of the ash in PDCs (300–450C) greatly adds to the damage to buildings by igniting interiors if it can gain entry, even at dynamic pressure values too low to inflict damage, as clearly shown here in the June 25 and September 21 events. The variability of dynamic pressure impacts in the 1–5 kPa, or more, range, due to such factors as turbulence, the effects of the environment on dynamic flow behaviour, and the entrainment and transport of objects as projectiles in PDCs, is another important consideration. In the urban environ-
312
ment the sheltering effect of adjacent buildings may profoundly attenuate the lateral loading impact, but houses better sealed against the weather than those on Montserrat may be more susceptible to damage from the isotropic pressure effects of PDCs. These aspects are best addressed in future studies using 3D modelling. We have already used our findings in a two-dimensional numerical simulation model of a future eruption of Vesuvius, Italy, for developing new methods for predicting building losses and potential human casualties, incorporating the key physical parameters of dynamic pressure, temperature and particle concentration (Baxter 2000; Spence et al., 2004a, 2004b). To date, no widely agreed criteria have been used for classifying the violence of individual flows or surges. Our work provides for the first time a conceptual basis for classifying the energy of dilute PDCs according to dynamic pressure, which can be developed in risk assessment and mitigation for explosive eruptions in populated areas. Acknowledgements We thank the British Geological Survey for funding the digital elevation model. We are grateful to the many scientists at the Montserrat Volcano Observatory, especially Steve Sparks, Simon Young and Sue Loughlin, for field support and many helpful discussions, and to Lucy Ritchie, who assisted Paul Cole in the field work and the preparation of the maps of the deposits. Andrew Woods provided valuable ideas on the fluid dynamics of the 26 December 1997 eruptive event. Partial funding was also provided by the European Community, Project No. ENV4CT98-0699.
References Anderson T, Flett JS (1903) Report on the eruption of The Soufrire in St Vincent 1902 and on a visit to Montagne Pele in Martinique. Phil Trans R Soc London 100: 353–553 Baxter PJ (1990) Medical effects of volcanic eruptions. 1. Main causes of death and injury. Bull Volcanol 52: 532–544 Baxter PJ (2000) Human and structural vulnerability assessment for emergency planning in a future eruption of Vesuvius using volcanic simulation and casualty modelling. Final Report EC Project ENV4-CT98–0699, Bruxelles Baxter PJ, Neri A and Todesco M (1998) Physical modelling and human survival in pyroclastic flows. Natural Hazards 17: 163– 176 Boudon G, Lajoie J (1989) The 1902 Pelean deposits in the Fort Cemetery of St Pierre, Martinique: a model for the accumulation of turbulent nues ardentes. J Volcanol Geotherm Res 38: 113–129 Bourdier JL, Boudon G and Gourgaud A (1989) Stratigraphy of the 1902 and 1929 nue-ardente deposits, Mt Pele, Martinique. J Volcanol Geotherm Res 38: 77–96 Boyle RD (1999) The use of GIS techniques in assessing building damage caused by pyroclastic density currents: a case study of the 26 December 1997 eruption of the Soufrire Hills volcano, Montserrat. M Sc Thesis, University of Edinburgh Clarke AB and Voight B (2000) Pyroclastic current dynamic pressure from aerodynamics of tree or pole blow down. J Volcanol Geotherm Res 100: 395–412 Druitt TH (1998) Pyroclastic density currents. In: Gilbert JS, Sparks RSJ (eds). The physics of explosive volcanic eruptions. Geological Society Special Publications no. 145. Geological Society, London, 21, pp 145–182 Druitt TH, Calder ES, Cole PD, Hoblitt RP, Loughlin SC, Norton GE, Ritchie LJ, Sparks RSJ, Voight B (2002) Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation
from pyroclastic surges at Soufrire Hills Volcano, Montserrat: an important volcanic hazard. In: Druitt TH & Kokelaar BP (eds). The eruptions of Soufriere Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 263–279 Druitt TH, Kokelaar BP eds. (2002) The eruption of Soufrire Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 1–645 Esposti Ongaro T, Neri A, Todesco M, Macedonio G (2002) Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modelling. 2. Analysis of flow variables. Bull Volcanol 64: 155–177 Glasstone S, Dolan PJ eds. (1977) The effects of nuclear weapons, 3rd ed. US Department of Defense, Washington DC, pp 1–653 Grunthal G ed. (1998) European Macroseismic Scale (EMS 1998). Council of Europe. Cahiers du Centre Europen de Godynamique et du Seismologie, Vol. 15, ISBN 2 87977–008–4 Heilprin A (1905) Mont Pele and the tragedy of Martinique. Philadelphia: JB Lippincott, pp 1–335 Hoblitt RP (2000) Was the 18 May 1980 lateral blast at Mt St.Helens the product of two explosions? In: Francis P, Neuberg J, Sparks RSJ. Causes and consequences of andesite eruptions. Phil Trans R Soc Lond 358, 1639–1661 Hovey EO (1902) Martinique and St Vincent. Bulletin of the American Museum of Natural History 16: 333–372 Jaggar TA (1949) Steam blast volcanic eruptions. Fourth special report of the Hawaiian Volcano Observatory of the US Geological Survey and the Hawaiian Volcano research Association. Honolulu: Hawaiian Volcano Research Association Kennan G (1902) The tragedy of Pele. The Outlook Company, New York, pp 1-257 Kieffer SW (1981) The 1980 eruptions of Mount St Helens, Washington. Fluid dynamics of the May 18 blast at Mount St Helens. In: Lipman PW, Mullineaux DR (eds). The 1980 eruptions of Mount St Helens. US Geological Survey Paper 1250. US Govt Printing Office, Washington DC, pp 379–400 Lacroix A (1904) La Montagne Pele et ses eruptions. Paris: Masson, pp 1-650 Lipman PW, Mullineaux D eds. (1981) The 1980 eruptions of Mount St Helens, Washington. Geological Survey Professional Paper 1250. United States Government Printing Office, Washington, DC, pp 1–844 Loughlin SC, Baxter PJ, Aspinall WP, Darroux B, Harford CL, Miller AD (2002a) Eyewitness accounts of the 25 June 1997 pyroclastic flows and surges at Soufrire Hills Volcano, Montserrat, and implications for disaster management. In: Druitt TH, Kokelaar BP. The eruption of Soufrire Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 211–230 Loughlin SC, Calder ES, Clarke A, Cole PD, Luckett R, Mangan MT, Pyle DM, Sparks RSJ, Voight B, Watts RB (2002b) Pyroclastic flows and surges generated by the 25 June 1997 dome collapse, Soufrire Hills Volcano, Montserrat. In; Druitt TH & Kokelaar BP. (eds) 2002. The Eruption of Soufrire Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 191–209 Mastrolorenzo G, Petroni PP, Pagano M, Incoronato A, Baxter PJ, Canzanella A, Fattore L (2001). Herculaneum victims of Vesuvius in AD 79. Nature 410: 769–770 Mays GC, Smith PD eds (1995) Blast effects on buildings. London: Thomas Telford, pp 1–121 Montserrat Volcano Observatory (1999). Dome collapse and vulcanian explosive activity, September to October 1997. MVO Special Report 5. Montserrat: Govt of Montserrat Moore JG, Sisson TW (1981) Deposits and effects of the May 18 pyroclastic surge. In: Lipman PW, Mullineaux DR (eds). The 1980 eruptions of Mount St Helens. Geol Surv Prof Paper 1250. US Govt Printing Office, Washington DC, pp 421–438 Ritchie LJ, Cole PD, Sparks RSJ (2002) Sedimentology of deposits from the pyroclastic density current of 26 December 1997 at Soufrire Hills Volcano, Montserrat. In: Druitt TH, Kokelaar BP (eds). The eruption of Soufrire Hills Volcano, Montserrat
313 from 1995 to 1999. Geological Society, London, Memoirs. 21, pp 435–456 Scarth A (2002) La catastrophe: Mount Pele and the destruction of Saint-Pierre, Martinique. Terra, Harpenden, pp 1-246 Simpson JE (1997) Gravity Currents. 2nd Ed. Cambridge University Press, Cambridge, pp 1–244 Sparks RSJ, Barclay J, Calder ES, Herd RA, Komorwski J-C, Luckett R, Norton GE, Ritchie LJ, Voight B, Woods AW (2002) Generation of a debris avalanche and violent pyroclastic density current on 26 December (Boxing Day) 1997 at Soufrire Hills Volcano, Montserrat. Druitt TH & Kokelaar BP (eds). The eruption of Soufrire Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 409–434 Sparks RSJ, Bursik MI, Carey SN, Gilbert JS, Glaze LS, Sigurdsson H, Woods AW. (1997) Volcanic Plumes. John Wiley, Chichester, pp 1-514 Spence RJS, Baxter PJ, Zuccaro G (2004a) Building vulnerability and human casualty estimation for a pyroclastic flow: a model and its application to Vesuvius. J Volcanol Geotherm Res 133: 321–343 Spence RJS, Zuccaro G, Petrazzuoli S, Baxter PJ (2004b) The resistance of buildings to pyroclastic flows: analytical and experimental studies and their application to Vesuvius. Natural Hazards Review 5: 48–59 Tanguy J-C (1994) The 1902–1905 eruptions of Montagne Pele, Martinique. J Volcanol Geotherm Res 60: 87–107 Taylor GAM (1983) The 1951 eruption of Mount Lamington, Papua. Department of Resources and Energy, Bureau of Mineral Resources, Geology and Geophysics, Bulletin 38. 2nd. Edition. Canberra: Australian Governemnt Publishing Service, pp 1–129
Todesco M, Neri A, Esposti Ongaro T, Papale P, Macedonio G, Santacroce R, Longo A. (2002) Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modelling. 1. Large-scale dynamics. Bull Volcanol 64: 155–177 Valentine GA (1998) Damage to structures by pyroclastic flows and surges, inferred from nuclear weapons effects. J Volcanol Geotherm Res 87: 117–140 Waitt Jr RB (1981) Devastating pyroclastic density flow and attendant air fall of May 18 - stratigraphy and sedimentology of deposits. In: Lipman, PW, Mullineaux DR eds. The 1980 eruptions of Mount St Helens, Washington. Geological Survey Professional Paper 1250. United States Government Printing Office, Washington, DC Wills J, Wyatt T, Lee B (1998) Warnings of high winds in densely populated areas. In: Forecasts and Warnings. UK National Coordination Committeee. Thomas Telford Publishing, London Wohletz KH (2001) Pyroclastic surges and compressible two-phase flow. In: Freundt A, Rosi M. From magma to tephra: modelling physical processes of explosive volcanic eruptions. Elsevier, Amsterdam, pp 247–312 Woods AW (2000) Dynamics of hazardous volcanic flows. In: Francis P, Neuberg J, Sparks RSJ. Causes and consequences of andesite eruptions. Phil Trans R Soc Lond 358: 1639–1661 Woods AW, Sparks RSJ, Ritchie LJ, Batey J, Gladstone C, Bursik MI (2002) The explosive decompression of a pressurized volcanic dome: the 26 December collapse of Soufrire Hills Volcano, Montserrat. In: Druitt TH, Kokelaar BP (eds). 2002. The eruption of Soufrire Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, pp 457–465 Zebrowski Jr E (2002) The Last Days of St Pierre. Rutgers University Press, New Brunswick, pp 1–201
