Landslides: Their extent and significance in the Caribbean

Landslides are a significant process shaping the landscape of many Caribbean islands. Only on islands with low relief and limestone bedrock such as the Cayman Islands or the Bahamas can the process be considered irrelevant. The geology and climate of the Caribbean contributes to the prevalence of landslides. The ubiquitous steep slopes resulting from tectonic and volcanic forces provide abundant locations for landslide occurrence. Other erosional processes such as wave action along island coasts and stream erosion provide a continual means for maintaining many slopes at inclinations close to the angle of repose for the materials underlying them. Tectonic and volcanic activity also creates lithologic and stratigraphic conditions favoring the occurrence of landslides. The warm, wet climate influences the material involved with landsliding and serves as the most common mechanism for their initiation. The rapid weathering of bedrock under humid conditions creates a regolith generally weaker than the parent rock. The presence of clay either inherited from parent sedimentary bedrock or derived from weathering of metamorphic and volcanic bedrock contributes to the tendancy of soil masses to fail. The seasonal pattern of rainfall punctuated with intense storms serves as an efficient means for triggering landslides in this region. Most landslide types are found within the Caribbean. Debris flows and slides are by far the most prevalent form. Many factors contributing to general slope instability favor development of these landslide types. Earthflows, rockslides, rockfalls, and slumps are other common but less frequently occurring landslide types. Fatalities and injuries are a primary loss attributable to landslides in the Caribbean. In 1938, three debris flows in two days at Ravines Poisson and Ecrivisse in St. Lucia killed sixty people and injured 32 others. Estimates of the missing ranged as high as 250. More recently the catastrophic landslide in the Mameyes district of Ponce, Puerto Rico claimed the lives of at least 129 residents. Smaller landslides causing fewer individual fatalities yield a tragic toll over time. Between 1925 and 1986, twenty-five people in Dominica lost their lives to landslides in five separate events. Destroyed or damaged structures, especially roads, are another major loss attributable to landslides. Clearance of slide debris and repair of damage are the main impacts to roads. On the narrow roads often found in the Caribbean, it does not take a large landslide to create a major impact. In St. Vincent, St. Lucia, and Dominica of the Windward Islands, the average annual cost for landslide damage to roads ranges from $115,000 to $121,000 in normal years. The average annual cost of landslide investigation, repair, and maintenance in the larger islands of Trindad and Tobago is $1.26 million and $0.96 million, respectively. On an average year, it is estimated the cost of repairing landslide damage to roads throughout the Caribbean amounts to $15 million. Other structural effects include the loss of homes. Seventeen families in Jamaica lost their homes due to a year of slow deformation by a large earthflow near Preston, St. Mary’s Parish. On average, perhaps tens of houses are destroyed, and hundreds are damaged by landslides each year in Puerto Rico. In St. Vincent, water lines severed by landslides in 1981 left nearly 40 percent of the population without water for periods varying from a few days to six months. In 1986, landslides damaged pipelines to hydroelectric generating stations and reduced the total electrical generating capacity of St. Vincent by 36 percent until repairs could be made. Landslides in the vicinity of Peligre reservoir in the Dominican Republic (Hispaniola) are contributing to premature filling of the impoundment and threaten the powerhouse. Agriculture, a major economic activity throughout the Caribbean, is impacted by interference with transportation of perishable goods to market and loss of crops. Figures for agricultural losses gathered for major storms affecting Jamaica, Hispaniola, Puerto Rico, and other islands with agrarian economies rarely distingush between losses due solely to landslides and those due to other storm effects. The magnitude of agricultural losses attributable to landslides relies on individual examples where this loss can be quantified. The Good Hope landslide in Dominica carried away or buried 3.8 hectares of crop land worked by six small farmers. An immediate economic loss of $8,000 and a delay of a few months to a few years before replanted crops are harvestable were the consequences of this event. Losses of bananas to smaller landslides on St. Lucia and St. Vincent yield losses of $4000 to $250 for individual farmers. Because agriculture is carried out mainly by individual farmers with per capita incomes ranging from hundreds to a few thousand dollars, losses of this mganitude represent a severe burden to the economies of both individual families and island nations.