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Bhalai, S. 2010. Landslide Susceptibility of Portland, Jamaica: Assessment and Zonation. Caribbean Journal of Earth Science, Volume 41, 39-54. Available online: 14th January 2011. © Geological Society of Jamaica.

Landslide Susceptibility of Portland, Jamaica: Assessment and Zonation

SURESH BHALAI

Mines and Geology Division, Hope Gardens, Kingston 6, Jamaica. sbhalai@mgd.gov.jm

ABSTRACT. The Government of Jamaica commissioned landslide susceptibility assessment of the parish of Portland represents a response to the continuous landslide problem experienced there. Portland is known for the frequent occurrence of landslides, induced by rainfall or seismic activity. Losses amounting to millions of dollars and numerous disaster-related deaths are attributed to landslide activity. Medium-scale (1:50,000) landslide susceptibility analysis through a combination of direct (field) and indirect (statistical) methods is used to generate a landslide susceptibility model for the parish. Using an inventory of known landslides, the bivariate statistical method, specifically simple map combination, is applied to compute weightings for the predisposing factors of landslides originating from topography and geology. The methodology used attempts to maintain full objectiveness in the model. Field experience is introduced where statistical methods fall short of realistically predicting the natural environment. Experience is also used to test the practicality of the map, bridging the void from the ideal theoretical model to the realistic ground truth. The susceptibility model ideally predicts existing landslides with approximately 80% accuracy; a value that is highly acceptable. The model may be employed as a tool for rural and physical planning, engineering works, building and infrastructure developments, and importantly natural hazard mitigation.

Key words: landslide; susceptibility; Portland, Jamaica; bivariate statistics.

1. INTRODUCTION

Increasing loss of property and lives from landslide disasters has led the Government of Jamaica, through the Mines and Geology Division (the local Geological Survey) to undertake landslide susceptibility analysis of different regions of the island. The eastern parishes of Jamaica are historically known for the frequent occurrences of damaging landslides. Examples include the popular Judgment Cliff landslide of St. Thomas of 1692 (Zans, 1959), the Millbank landslide of Portland of 1937 (Harris, 2002) and the Preston landslide of St. Mary of 1986 (Bryce et al., 1987). These have induced large losses and caused numerous deaths and have highlighted the urgent need for zonation of the land surface according to the probability of occurrence, that is, the propensity or the susceptibility for land slippage. Under the landslide susceptibility assessment programme started by the Government in 1998, a pilot study was done on the Rio Grande Valley (Mines and Geology Division, 2000) and St. Thomas and St. Mary parishes were subsequently zoned (Mines and Geology Division, 2004a, b). The study of Portland followed (Bhalai, 2007; Mines and Geology Division, 2007).

Landslides in Jamaica are generally triggered by earthquakes or during heavy rainfall events. Sometimes both processes work in tandem. The Judgment Cliff landslide is one such case which occurred either during or shortly after the Great Port Royal Earthquake of 1692, when there was also a period of torrential rains where the Yallahs River was in spate. Jamaica is tectonically active and the eastern end shows dominance in seismic activity. In terms of earthquake frequency western Portland is classified as having the potential to experience 8 to 15 damaging earthquakes of MMI VI or greater per century as computed by Pereira (1979). Central Portland has a frequency of 6 to 7 events per century whereas east Portland may experience 4 to 5 events per century. Other regions in Jamaica have an average expected frequency of 3 to 5 events per century. The January 13, 1993 earthquake of MMI VII had epicentre at the Blue Mountain Ridge between St. Andrew and Portland. Forty landslides were triggered in these parishes, many of which were in Portland. Roads were blocked and water pipes and electricity poles damaged (Ahmad, 1996). In one case, at Greenhill, on the Newcastle to Buff Bay road, 100 m of roadway was destroyed (Harris, 1996).

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