Ilan,

Good question, but there are additional complexities. Isabel struck an area of the USA where hurricanes are not common, where the usage of electricity is ubiquitous, and flood prevention measures are well-developed. Hence the vulnerability to wind damage. You compare with Cuba - there hurricanes are common, so there is a well-developed emergency plan, which revolves round early warning activating centrally planned and co-ordinated action including evacuation. There are few societies where such plans could be made and acted upon effectively, but in Cuba, it comes from the top - I saw a photo of Castro in the National Meteorological Centre when the first warnings for Mitch were issued. As in previous discussions, there is a balance to be struck between probability of occurence, cost of mitigating measures, and preparedness. Mitigation depends on early warning - it is important to have a warning scheme which includes a forecast probability of events occuring as well as the inte!
nsity.

The major damage done by most hurricanes is due to the extreme rainfall, and consequent flooding and land-slips. Remember also that damage can be done by flood water and debris in areas not affected by the storm itself (as in the Algiers flood)

Wind-loading on trees is only one of a number of factors in causing damage; in the October 1987 storm in the UK, the wind speeds were high, but not significantly outside probabilities. The trees fell also because the storm was preceded by a long wet spell, so the soil was at field capacity (again near the limits of probability), and therefore had little strength, and an unusually warm spell meant the deciduous trees still had full leaf cover. Interestingly, we did not forecast the storm properly, but one can argue that the death and injury count was very low because most people were in bed - if we had produced a better forecast, more people would have been outside trying to stop the damage and at risk of being hit by debris!

One can also argue that it is better to leave the electricity on - then everyone will be expecting the cables to be live and dangerous and will keep their hands off. I suspect that the dangers of electrocution are much less than being hit by falling objects, even for utility workers doing repairs. One of the problems with re-starting power is being sure that it is safe - in the recent London power cut, it was stated that power to the Underground railway could have been restored within 20 minutes, but did not happen for over 3 hours because passengers left the trains and were walking on the rails.

Steve
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Stephen G. Palmer
Technical Co-ordination Manager
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