DISASTERS 2004

The unprecedented occurrence of earthquakes, hurricanes, and floods in the Caribbean region in 2004, notwithstanding the 9.0 Richter magnitude earthquake that wreaked devastation on the Southeast Asian islands, and India on Boxing Day, naturally prompts the question — are we ready for 2005, and what can we do? In this article an attempt is made to summarise the issues of highest priority that need to be addressed both by individuals and governmental agencies, with a focus on earthquakes and hurricanes.

One can scientifically and correctly conclude from the Caribbean’s recent past that there is a new trend of significantly increasing levels of natural but undesirable events, especially earthquakes, hurricanes, and the attendant floods, and landslides.  One can similarly conclude that if not sufficiently mitigated and managed, then in the near future the severity of these events can be of sufficient magnitude to cause mass damage and destruction. With respect to the prediction of earthquakes, the science has not as yet advanced sufficiently to enable forecasting that is as reliable as for hurricanes.  However, the significance of the occurrence of the Richter magnitude 6.3 off Dominica, a 5.3 in Trinidad, and a 6.4 in the Cayman islands within seven weeks of each other, needs to be put into perspective.   The numerical differences in these numbers unfortunately do not give an adequate representation of the situation. 

A magnitude six event releases 32 times more energy than a magnitude 5 at the epicentre, and likewise a magnitude 7 event releases 32 times 32, or 1024 times more energy than a magnitude 5.  The calculation of earthquake hazard for a region is based on a database of recorded ground motions. All of the Caribbean databases in use prior to these events do not contain records for any event of magnitude greater than 5.0 simply because they are so rare.  That we should experience three events above 5.0 in such a short time frame is so rare indeed that it invites speculation on whether the tectonic plates that are relevant to the region are in some phase of readjustment.  Nevertheless, we have been aware of our earthquake vulnerability for some time and now is as good a time as any for asking key questions related to our preparedness. For earthquakes the highest vulnerability continues to be houses built with unreinforced masonry walls supporting the roof.  

These structures do not have the required structural properties to withstand the likely earthquake.  If the house is 2-storey the situation is worsened since the ground vibration is amplified placing even higher stresses on the structure.  Another critical issue that applies to multi-storey buildings as well, is the lack of movement joints between the wall and frame when this area is infilled with blocks, as is typically done to enclose the building.   This causes the earthquake force to be concentrated in the walls and columns which are typically too weak to withstand the force and they collapse under the stress.  These are the main causes of the observed damage to structures after the recent Dominica earthquake.  A procedure has been developed at the University of the West Indies for strengthening existing single-storey masonry houses and is available at the Department of Civil and Environmental Engineering, UWI.  The approach recommended can also be used for strengthening the aforementioned infilled frames.

With respect to hurricanes, on 13 December 2004 at the Caribbean Development Bank Conference Centre in Barbados, the Caribbean Division of the Institution of Structural Engineers, in collaboration with the CDB/Disaster Mitigation Facility for the Caribbean, USAID, PAHO/WHO, UWI, and others, held a one-day Conference.  The theme of the conference was — The 2004 Hurricane Season: Implications for Structural Engineering Practice in the Caribbean.  In the paper and presentation by Philip Klotzbach, a forecasting specialist with an impressive record, he states that “...it is likely that we will continue to see active tropical cyclone seasons and increased landfall probability in the Caribbean for the next 10-20 years.  A great increase in hurricane-spawned economic loss is to be expected in the next few decades.”   Although Klotzbach indicated that the causes for the high degree of hurricane activity in 2004 was due to a rare combination of factors, based on extensive modeling the forecast for 2005 indicates that there will be 11 named storms that will probably take more southern tracks.  Of these storms, 6 will develop into hurricanes and 3 of these will be intense.  It should be noted that for each unit increase in the category of a hurricane, say from 3 to 4, the damage potential increases by 400 percent.

For hurricanes the most vulnerable area is the existing housing stock and in Trinidad and Tobago in particular, the roof, and the window and door openings.  Regardless of the wind direction, all the faces of the house including the roof surface, collectively called the envelope, are subjected to wind pressures.  This pressure increases with the square of the wind speed so for example the 25 percent increase in wind speed from 20 to 25 metres per second will increase the pressure by 56 percent.  In terms of the location of the house, valleys, hillsides, and areas within about 2 kilometres of a coastline naturally attract higher wind speeds and special attention needs to be paid to these houses.  For the roof, the tendency is for the pressure to lift the roof and the pressure is much higher at the edges.  A proper roof has an adequate number and type of screws holding the sheets to the timber laths, or steel purlins, or timber nail strips, which are typically used in the Caribbean.  In the interim, while more exact recommendations are being formulated, during the coming dry season the homeowner can employ a contractor to double the number of screws currently installed and use extra screws at the edges, say 2 rows every 75 millimetres.  

If even one screw is dislodged the increased stress on nearby screws that must now do the work can be too much so the sheeting progressively unzips.  This issue is the factor that mostly contributed to damage to houses in Grenada during Ivan.  Even with adequate screws, the laths must also be adequately strapped to the rafters, and the rafters to the wall.  If the house does not have these straps, they can be obtained locally and installed by a contractor.  This will probably entail removing and replacing a significant area of the ceiling but without it, the purlins and everything above it can be ripped from the rafter, or the rafter from the wall. 

The envelope must remain intact at all times but during the hurricane, the winds blow horizontally and carry poles, broken branches, cans, etc, as missiles that impact the walls of the house.  If a window is broken or a door unhinged, the wind rushes in and this increases the pressure inside the house in such a manner that an additional pressure is added to the roof to causing it to be dislodged. 
Suzanne Mills’ column resumes next week