Debris slides and debris flows are common on mountainous slopes in the Windward Islands. Walsh (1985) noted the many debris slides and flows triggered by Hurricanes David and Frederic involved failure at depths of 2 meters or less. Similarly shallow depth to the failure plane for these landslide types is documented for Martinique (Faugeres, 1966). Prior and Ho (1972) found the majority of theses slides on St. Lucia originated on slopes steeper then 35 percent.

Rockslides and rockfalls are widespread but less abundant. Bedrock escarpments on mountain slopes, steep-sided valleys, and coastal cliffs are typical localities for these landslide types. Failure usually involves a competent rock type which often has well-defined joints or similar discontinuities. Ignimbrite defined joints or similar discontinuities. Ignimbrite deposits along valleys near Rouseau, Dominica fail as rockslides and rockfalls where the prominent vertical joints create zones of weakness within the rock mass (DeGraff, 1987a). Failure of coastal cliffs occurs due to the oversteeping by wave erosion.

Slumps and complex landslides are the least common types found in the Windward Islands. Most slumps or rotational failures observed are associated with man-disturbed slopes. In Dominica, small rotational failures triggered by Hurricanes David and Frederic were only noted on cultivated slopes (Walsh, 1982). Rotational failures seen in St. Vincent, St. Lucia, and Dominica are limited to small failures in road cuts. Prior and Ho (1972) describe complex slides involving shallow movement of the soil mantle and Moule a Chique in St. Lucia.

The steep slopes prevalent in the Windward Islands are one of the principal conditions favoring landslide development. The land rises from seal level to 800 meters or more over a distance of 3 to 6 kilometers resulting in steep rugged terrain. Examination of drainage patterns on these islands by Walsh (1985) indicates and early phase of landscape development. The resulting slopes are often at angles close to the angle of repose for the materials underlying them. Only small changes in stability conditions are required to bring such slopes close to failure.

The nature of materials underlying slopes on these islands plays a major role in landslide development. Their volcanic origin creates stratigraphic and lithologic conditions favouring landslides. Layers of alternating ash, lava, and breccia lead to locations on slopes where weaker bedrock weathers faster and undermines more competent bedrock overlying it. Location of volcanic vents in the central parts of the islands results in bedrock layers being inclined outward. The resulting presence of bedrock contacts inclined at angles less steep then the mountain slopes seems to favour landslides development. The bedrock and humid climate combine to cause deep weathering of volcanic bedrock. The resulting soil mass has a lower strength than the original unweathered bedrock. Mehigan and Hartford (1985) recognized this situation for some slopes along roads in Dominica. In many instances, the soil contains significant percentages of clay. Prior and Ho (1972) found montmorillonite clay in soils tended to produce flow-type failures on St. Lucia. The clay characteristics of some soil on Dominica influence the intensity of rainfall needed to induce a landslide (Rouse et al, 1986, Rouse and Reading, 1987)

The principal triggering mechanism for landslides in the Windward Islands is rainfall. Pore-water pressure increases along discontinuities within weathered bedrock and within soil masses leads to decreased shear strength. This loss of strength coupled with the added weight of water within the saturated mass leads to failure on zones of weakness such as the soil-bedrock interface or discontinuities within the bedrock (Faugeres, 1966, Walsh, 1982). Hurricanes are one source of intense rainfall. Between September 1963 and September 1987, the following hurricanes induced landslides on one or more of the Windward Islands: Edith, Beulah, Abby, Dorothy, David, Frederic, Allen, and Emily. Storm events other than hurricanes are capable of inducing landslides. Prolonged rainfall experienced during the rainy season is capable of producing landslides as the Good Hope landslide demonstrates. In St. Vincent, Tropical Storm Danielle and associated rains in September 1986 caused more landslide damage than Hurricane Emily in September 1987.

Earthquakes and volcanic activity, recognized triggers of landslides, affect this region. All of the Windward Islands are known to have experienced earthquakes in the past. However, there are no documented instances of landslides being triggered by ground shaking. Volcanic activity is associated with landslides in the windward Islands (Bolt, et al, 1975). Mudflows are described as part of the eruptive sequence of Mount Pelee leading up to the disastrous nuee ardante on May 8, 1902, which devastated St. Pierre, Martinique. Also in 1902, mudflows affecting the northeast and northwest parts of St. Vincent were among the events associated with the May 7th eruptive sequence of Soufriere, which killed 1,500 inhabitants.

Human activities are another triggering mechanism for landslides in the Windward Islands. Roads cut into steep slopes remove support from the soil or rock mass above. The road prism may interfere with natural subsurface drainage in the slope leading to higher pore-water pressures than would occur under natural conditions. Anderson and Kneale (1985) note the terrain on these islands limits the available routes making avoidance of landslide-susceptible slopes difficult. This was the case for a major road in Dominica constructed to improve access to the southern part of the island. The route crossed a slope segment with unfavourable bedrock lithology, groundwater conditions, and slope form. This slope, near Bellvue Chopin, initially failed during road construction. It failed several times in the following years causing one death as well as impacting the road. Despite stabilization efforts, it remains a potential site for future landslide activity.

Agricultural practices are another human activity contributing to landslides in the Windward Islands. Some farmers slash and burn the rainforest on very steep slopes to clear areas for planting bananas. The shallow-rooted banana plants are unlikely to contribute as much root reinforcement for strengthening the soil mass as the original trees of the rainforest. Rainfall reaching the ground is increased by replacing the solid rainforest canopy with the more open canopy typical of banana fields. It is suspected these differences make the slopes more susceptible to landslides. The consequences of altering the vegetation on a steep slope were demonstrated to a farmer in St. Lucia (The Weekend Voice, September 17, 1988). He maintained a banana field on a 90 to 100 percent sloped cleared of forest vegetation. On September 11, 1988 following a tropical storm which drenched the island he was an eyewitness to destruction of his field by a landslide. About 3:30 p.m., the farmer observed the slope begin to move starting very slowly at the bottom and followed by the upper slope as it accelerated. Nearly 5 hectares including his entire field was carried away leaving a 1.5-kilometer long swath of exposed soil. Chief Forestry Officer Gabriel Charles and his fellow rangers inspected the site and unanimously attributed the disaster to deforestation and a slope too steep to be used for banana cultivation. Even the farmer concluded, "I think it was the absence of trees with firm roots which caused the slide. I also think the squatting and cutting of trees on the hills should be stopped…"

Table 4. Number, Size, and Area Distribution of Past Landslides on St. Vincent, St. Lucia, and Dominica

Table 5. Annual Cost of Landslide Damage to Roads on Dominica