SUMMER 2003GEOHAZARDS COURSE AT UWI, MONA - page 016
Prepared and compiled by Rafi Ahmad, Unit for Disaster Studies,
Department of Geography and Geology,
University of the West Indies, Mona, Kingston 7, Jamaica


2.1 (2b) TROPICAL STORMS AND HURRICANES

The island is located in the track of north Atlantic hurricanes passing through the Caribbean . The probability of a hurricane affecting the island in any given year, using a Poisson distribution, is 0.27 (Ahmad, 1993). According to ODP and WMO (1988), if only direct hits are counted which are 16 since 1871, the probability is 0.14.
In Jamaica, however, the hurricane frequency is not uniformly distributed. There were nine hurricanes each during the decades 1910-1919 and 1930-1939, and since 1960 the number of hurricanes affecting the island has dropped off significantly (ODP and WMO, 1988).
The total amount of rainfall associated with hurricanes events show a wide variation. The highest record of rainfall is 2451 mm in four days in November 1909 at Silver Hill in the Blue Mountains (Vicker, 1967), where as 1988 Hurricane Gilbert - described as the biggest hurricane when it made a landfall on Jamaica - produced a maximum of 794 mm of rainfall mostly confined to the mountain ranges and caused floods with only 25- year recurrence interval (ODP and WMO, 1988).
The rainfall in Jamaica is controlled by the orographic trend. It has been estimated by Lirios (1969) that in the Blue Mountains 24- hour precipitation between 250-635 mm can be expected every ten years.
The hazards that affect the island most frequently in relation to hurricanes are damage due to hurricane force winds, landslides and floods. The deleterious effect of these is a function of steep topography, fractured and deeply weathered bedrock, changes in vegetation cover and high intensity rainfall. However, it is more recurrent and non-hurricane tropical storms which are associated with greater daily rainfall and frequent flooding and landslides. The flooding associated with small and localized events are tremendously destructive and costly.
In the mountainous terrain of Jamaica, landslide related mass wasting is probably the most significant process associated with high-rainfall events. The Judgement Cliff Landslide in St. Thomas is perhaps the largest historic landslide in the Caribbean with displaced debris measuring over 80,000,000 m3.
This landslide followed the Great 1692 Earthquake and was perhaps triggered by hurricane related rainfall. The landslide debris dammed up the Yallahs River.
Exercise on Landslides

The most recent event of landslide and flooding on the island is related to the rainfall from Tropical Storm Gordon, November 10-13, 1994. Although the maximum 48-hour rainfall associated with Gordon was measured at 189 mm, with a maximum rainfall intensity of 123 mm/hr, it caused extensive flooding and triggered landslides all over the island resulting in four (4) deaths and over 200 persons left homeless. The direct economic losses were estimated at US$5 Million. The flooding was related to aggrading of the stream channels in lower reaches.
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