THE EDITOR, Madam:
I READ with interest Mr. Roy Reynold's views on the threat by landslides to Kingston in today's Gleaner of October 7. I agree with much of what he says. However, there are statements in the article that need comment.
Firstly, the 600-ton boulder scenario. It is true that large boulders occur on the surface of the Liguanea Plain, technically an alluvial fan, as it is not horizontal, but slopes towards the sea from a high point at Papine. Boulders are also found partly or completely buried in the sedimentary rocks making up the fan. It is also certain that these have been transported to their present positions by mudflows, perhaps also landslides, emanating from a point near to Papine. We know this because the boulders consist of a rock type known as Newcastle Porphyry, a rock type that only outcrops in the Port Royal Mountains, including parts of the Hope River drainage basin.
But here the similarity stops. These boulders were brought down through the Hope River drainage system before the Hope River ceased to flow across the Liguanea gravel fan, and found its modern path down to Harbour View. The date of change in direction of flow of the Hope River is not known precisely, but it probably occurred in the Pleistocene period, that is during the height of the Great Ice Age. This would put the age of the change in direction at some half million years ago or more. The boulders must be older than this, the date of change in flow direction.
Since the Hope River started to flow down the August Town gorge to Harbour View the danger from large boulders has ceased to exist, at least for Kingston, although perhaps not for the residents of Kintyre, Hope Flats and Harbour View. This is not to say that a landslide danger does not exist for Kingston. There is ample geological (not geophysical) evidence of landslides and debris flows originating from the slopes below Skyline Drive. Some of these reached as far as Hope Gardens. But they were not the carriers of the large boulders. The first person to write scientifically about these boulders was Charles Matley, who was Government Geologist in the 1920s.
Secondly, it is not at all clear what is meant by cataclysmic disasters running in cycles of 50, 100 and 500 years. It just doesn't happen this way, and I trust that the Environmental Foundation of Jamaica is not feeding this kind of misinformation to an unsuspecting populace. I believe that what is actually being (mistakenly) paraphrased here is the concept of the probability that an event of a given magnitude, such as a landslide, will occur in a given period of time. Thus, small landslides occur frequently, while larger landslides occur less frequently. A very large landslide or debris flow, such as reached the area of Hope Gardens, or produced Judgement Cliff in the Yallahs River valley, might be likely to occur, for example, about once in every thousand years (we don't really know).
Small landslides will probably occur once or twice every few years. Thus one can estimate, in theory, the size of landslide that might be expected to occur once every 50 years, or every 100 years, and so on. This is not to say that a very large landslide will not take place tomorrow, but the probability that it will is very small.
This concept of a probable recurrence period for an event of a particular magnitude is commonly applied to river flooding. It is also applied to the possibility that we will be hit by large meteors from space, such as the one reputed to have extinguished the dinosaurs. Its application to the probable occurrence of landslides relies on much sparser documentation of known examples than does that for floods, so that we are much less certain of the probabilities involved.
Also landslide probabilities are linked to the geological conditions at the sites involved, and the probable recurrence of such events as earthquakes and hurricanes, that are likely to promote landslides. But I agree with Mr. Reynolds that the spread of built up Kingston into the mountains can only increase the possibility that landslides of all sizes are more likely to occur more frequently in the future, because buildings invariably render steep slopes more prone to failure, through such changes as the additional weight of the house, the cutting down of vegetation, and provision of access roads.
The records of the Port Royal earthquake are as described by Mr. Reynolds. The trees brought down to Port Royal were brought via the Rio Cobre. The earthquake caused landslides that blocked the Rio Cobre Gorge, allowing trees and other debris to back up until the landslide dam collapsed, allowing a torrent to bring the debris into Kingston Harbour.
Similar occurrences must have happened widely across Jamaica. Because of the spread of roads, buildings and dense populations over the Kingston and St. Andrew region, there is every likelihood that a future earthquake, of the same magnitude as the one that destroyed Port Royal, will do untold millions of dollars worth of damage and cost many more lives than were lost in that event of three hundred years ago.
I am etc.,
EDWARD ROBINSON
Professor of Geology,
UWI Mona, Kingston 7
E-mail
tedrob@cwjamaica.com
Tel. 927-2728<P>
