Landslides, heavy rainfalls and hurricanes in Dominica, West Indies
In 1979, the volcanic island of Dominica was severely affected by Hurricanes David and Frederic. Together they devastated vegetation and buildings and resulted in innumerable landslides on the predominantly very steep slopes of the deeply dissected landscape. This thesis examines the major medium to long term effects of these hurricanes and in particular assesses their role in initiating rapid mass movements. Despite the massive disturbance caused by the 1979 hurricanes, and also by a third, Allen (in 1980), evidence suggests the landscape is rapidly reverting to its pre-hurricane state. Variations in the frequency of tropical cyclones will clearly have great effect on the denudation characteristics of the Dominican (or similar) environments. The thesis, therefore, first examines the changing frequency of West Indian tropical cyclones. Cyclone frequencies were particularly high in the periods 1780-1840, 1880-1910 and 1930-1959. Large troughs in the frequency of cyclones are noted in the mid 17th century and since 1960. Superimposed upon these peaks and troughs have been marked variations in favoured tracks. During the 20th century there has been a definite shift in mean tracks, northwards and eastwards. The unusual physical, geotechnical and hydrological characteristics of the island's residual clay soils are investigated and the considerable problems which arose in the measurement and geomorphological interpretation of soil properties are stressed. A detailed understanding of soil formation and runoff processes, notably depth of soil, zones of percolation impedence, throughflow, and positive pore water pressure development is found to be essential to explanations of slope failure patterns. Although only a small increase in positive pore water pressure would be necessary to initiate failure in the deeper soil horizons, the amount of additional water necessary to achieve this is large. It is suggested that such an increase could only occur in the type of exceptionally prolonged and high intensity rainfalls associated with tropical cyclones.