Overnight, it was indicated by the MVO that the runout of the flow yesterday morning was actually 5 km [~ 3 miles], and that it was generally consistent with the model projections from last September for a 5 mn cu m flow.
It is estimated that somewhat more than 5 mn cu m has fallen from the dome, leaving a chute just above Tyre's Ghaut, potentially facilitating further, larger flows into the Belham. It was also indicated by the MVO that what has fallen already represents only a relatively small fraction of the available mass in the dome.
What does this all mean?
A good way to answer that, as we continue to monitor the evolving state of the volcano, it would be instructive to review the Montserrat Volcano Observatory Scientific Advisory Committee's [SAC] views -- NB: then, not fully formalised -- when the volcano was last in a fairly similar condition to what now obtains.
For, instance, the September 2002 assessment, some eight months before the July 2003 collapse is quite illustrative.
Citing the Executive Summary:
With a very large dome and continuing dome growth, situations can arise rapidly where currently populated areas in and marginal to the lower Belham Valley will be at high risk. If the dome switches its direction of growth to the north or northwest then collapses are possible which could generate pyroclastic flows that would reach the lower Belham Valley . . . . For individuals, the risk of death by eruptive activity for persons living north of Lawyer’s Mountain remains classified as NEGLIGIBLE under the Chief Medical Officer’s Risk Scale, and residents of north Woodlands are also assessed to be at NEGLIGIBLE risk; further south, risks range from MODERATE for residents along the northern slopes and ridge of the Belham Valley, to LOW in the Olveston area, and VERY LOW on passing north of Salem.
With a northwest or north direction of dome growth, in which additional material accumulates such that moderate or large collapses could occur and be directed by topography into Tyre’s Ghaut, the risks would change for population areas bordering the Belham Valley. For this situation the exposure would be assessed as HIGH.
Moving north from the Belham Valley, there is a steep decline of risk to residents. If, at any time, it were required to reduce total overall societal exposure, this could be achieved by sequential evacuation of areas bordering the Exclusion Zone. Early warning and other mitigation steps would also reduce exposure. [Bold emphases added]
Thus, we can clearly see that there is a gradation of risk to the vicinity of Woodlands beach under conditions similar to what now obtains. That is related to the unfortunately credible risk -- given, e.g. the May 8, 1902 St Pierre disaster -- that surge clouds connected to pyroclastic flows can spread out well beyond the margins of the valleys down which the main flows tend to rush.
In September 2006, some remarks along these lines in the SAC Technical Report, were:
The dome is currently growing to the east/northeast, but the next time that the locus of lava dome growth shifts to the north or west there will be the potential to infill the moat between the dome and the crater walls and overtop the crater rims above Gage’s and/or Farrell’s Walls. If this happens, the possibility will exist to send dome collapse pyroclastic flows towards Plymouth or into the lower Belham Valley. Our new analysis of this (paragraphs 20-21) shows that the assessed probability by flows down Gage’s Ghaut has fallen relative to the last report. This analysis also shows that the top of the dome must be at least about 950 metres above sea level for such collapses to occur at all, and that most collapses with volumes greater than about 2 million cubic metres occur when the dome is above 1000 metres above sea level. As before, flows of volume greater than 2 million cubic metres could threaten Plymouth . . . .
As we have seen from the PYROFLOW probabilistic simulations of pyroclastic flows and their surges (Fig.4), the 5 million cubic metre simulated flows would reach no further down the Belham River Valley than Cork Hill. Because we have not had such long flows on land on Montserrat we cannot with high confidence accurately simulate equivalent flows that reach all the way down the Belham Valley to the sea. Nevertheless extrapolations of model parameters suggest that such a flow would need to have a volume of about 12 million cubic metres or more. The boundaries of such flows and their surges in the lower Belham Valley simulated by PYROFLOW are shown in Fig.6. [pp. 13 - 15, emphases added.]
The caption to Fig 6, p. 14, reads:
Fig.6. Simulated limits of dome collapse pyroclastic flows and their surges in the lower Belham Valley. The red boundary is for flows of up to 5 million cubic metres such as has been experienced as a result of collapses over the Farrell’s Wall earlier in the eruption. The blue line represents the boundary of a ~ 12 million cubic metre flow. The purple line along Nantes River marks the northern limit of any conceivable pyroclastic flow/surge activity. Note that the pyroclastic surge limits are derived by a very simple method and thus the red and blue boundaries are not necessarily conservative with respect to surge hazard.
In short, the models in use are to be viewed with caution, and the SAC's credible limit for dome collapse or avalanche-induced pyroclastic flow surges is the line of the Nantes River, as noted previously.
That, in turn, suggests to this observer, that the limit for surges due to collapse of high eruption columns is considerably further out from the Belham. (This would account for the estimates that view Lawyers as the line of ultimate safety as noted in 2002. Such a column blast is conceivable as the result of a major dome collapse event. Hopefully, such an event would give sufficient warning - credibly, at least some dozens of minutes -- through the generation of the required high, sustained eruption column that people could flee north of that line.)
But, the above is sufficient for us to draw out some issues and concerns on the announced evacuation of yesterday, which is up to the rim of the Belham valley. That is, it assumes that the potential pyroclastic flows are in effect constrained by the topography of the Belham valley. But, there is plainly a credible enough potential for a wider-spreading surge cloud that it is commented upon in the September 2006 SAC Technical report, complete with a "purple line."
This confirms the validity of the previously posted three-scenario decision-support model for hazards [tied to three decision alternatives -- 1: do nothing assuming a switch is imminent; 2: evacuate up tot he northern rim of the Belham; 3: evacuate up to the line of the Nantes]. Such a model can also be inferred from remarks by SAC members released to the press locally last week:
A: OPTIMISTIC SCENARIO [Wadge], let's say 70% chance: Since a shift in the locus of dome growth to the NW usually lasts up to ~ 4 weeks, maybe the locus of growth will shift away to a more "safe" quadrant soon, so if nothing is done nothing would result. [But if B or C happen, dozens to hundreds of casualties could result.]
Expectation of losses [for option 1]: 0.7 * 0 + 0.27 * 20 + 0.03 * 200 = 11.4
B: INTERMEDIATE [Wadge], let's say 27% chance, i.e. 9/10 of the remaining 30%: A large collapse leading to pyroclastic flows down the Belham but which are more or less confined to the margins of that river. If the margins are evacuated, then no casualties would occur under A or B. But if C happens, up to hundreds could happen.
Expectation of losses [for option 2]: 0.7 * 0 + 0.27 * 0 + .03 * 200 = 6
C: PESSIMISTIC [Aspinall], let's say 3% chance: A very large collapse with unconfined surge clouds similar to May 8, 1902. If an evacuation up tot he Nantes line is done, no casualties would occur. But, if no evacuation or a marginal evacuation is done, up to hundreds of casualties could easily result.
Expectation of losses [for option 3]: 0.7 * 0 + 0.27 * 0 + .03 * 0 = 0
Now, of course, the probabilites and casualty estimates are illustrative only, but the general orders of magnitudes given are enough to make the point.
THAT POINT IS THAT THE LEVEL OF EVACUATION CURRENTLY UNDERTAKEN -- OPTION 2 -- IS NOT THE MOST ROBUST DECISION. (That is, on the evidence in hand, Salem -- up to the line of the Nantes, is still exposed to the possibility of a wide-spreading, sudden pyroclastic flow surge; which is not covered by evacuation option 2, the one taken as at yesterday afternoon, 6 pm, when the Governor's proclamation went into effect.)
For, plainly, Salem and environs, on the best available scientific, historical and common-sense information, are at risk of a sudden, large dome-collapse-triggered pyroclastic flow. So, it would be wise for the people of Salem and environs to move north of the Nantes now, if possible, and/or at minimum, to be prepared to do so on a moment's notice. (NB: Whatever the pressures on the authorities, by virtue of our need and right to know as a directly concerned public, it is plainly incumbent upon them to immediately explain why evacuation to the line of the Nantes has not currently been ordered, given the above.)
Finally, it should be noted that fears that if Salem is evacuated, Montserrat is "finished" are probably groundless.
For, prior to about 1998, the level of infrastructure further North may have lent credibility to such a view. But, in the near-decade since then, considerable infrastructure has been emplaced in the North. Also, as was discussed in this blog Saturday, using modular construction technologies similar to what Mr Ryan has used in his new mini-plaza in Brades, we can rapidly put in warehousing, small-business plazas, evacuation cottages and land for farmers. Indeed, if necessary, similar modular construction could be used to build campuses for the education institutions in Salem, which are on the fringes of the Nantes river line.
The bottomline is plain: with prudence, a sustainable Montserrat can be build starting from where we are now. So, let us join together with one heart and mind to do just that! END