4.0. LANDSLIDES AND FLOODING - HYDRO-GEOLOGIC HAZARD

An analysis of all the natural hazards that have affected the island since the turn of the century suggests that landslides together with floods account for most of the recurrent-economic losses, human miseries and social hardships, and environmental degradation. Table 5 lists some of the significant slope movements that have occurred on the island since prehistoric times and includes data on fatalities, injuries and economic costs. In Jamaica, landslides have caused death and injury and have damaged or destroyed rural settlements, schools, public and private property, roads, bridges, culverts, retaining walls, agricultural lands and crops, water pipelines, telecommunication lines, submarine cables and electricity transmission lines.

The indirect economic costs of the damage due to landslides are several orders of magnitude higher than the direct costs. However, the cumulative economic costs cannot be quantified due to lack of information. Moreover, much of the damage remains undocumented because comprehensive damage assessment surveys have generally not been done in the aftermath of a significant landslide-producing event.

The damage due to landslides is often wrongly ascribed to damage caused by floods. According to available information, the overall costs are undoubtedly very high. Hazards related to slope instabilities are, therefore, an important social and environmental concern to Jamaica.
Landslide processes observed on the island are a natural geodynamic process through which hill slopes evolve. They also result from the modification of natural slopes for various human activities. Ahmad (1995) has provided a comprehensive review of landslides in Jamaica, their extent, significance and geological zonation.

The most effective mechanism for triggering widespread landslides activity on the island is provided by the seasonal pattern of rainfall punctuated with intense storm events. In general, landslides follow sustained rainfall that recharges the soils and deeply weathered and highly fractured bedrock, which make up the steep slope. This condition favors the development of excessive pore-water pressures, which tend to reduce the shear resistance in slope-forming materials, leading to slope failures. Earle (1991) estimated that approximately 300 mm of rainfall in 48 hours initiated widespread shallow landslide activity in central Jamaica during 1986 flood rains.