S. BHALAI – Landslide susceptibility of Portland, Jamaica
Table 2. Landslide susceptibility of elevation classes
20 18 16 14 12 10 8 6 4 2 0
0 - 100
100 - 200
200 - 300
300 - 400
400 - 500
500 - 600
600 - 700
700 - 800
800 - 900
900 - 1000
1000 - 1100
1100 - 1200
1200 - 1300
1300 - 1400
elevation range (metres)
Figure 13. Elevation ranges and susceptibility. Note that the interval hosting the Blue Mountain Ridge (the area of highest elevation) shows relatively low susceptibility.
values represents lower susceptibility zones and high values represents the zones of higher landslide susceptibility.
The model generated was tested using two methods which also aided in its refinement. The first includes the use of a reserved inventory of landslide sources and the second involves a comparison with areas of known susceptibility based on field experience. For analytical purposes the final inventory of landslide sources was split into two uneven populations where 75% of the landslide sources were used for the analysis, whereas the remainder was reserved. The reserved inventory was randomly selected but carefully chosen so that this population accounted for an area of approximately 25% of the total area of the landslide sources.
The susceptibility model is designed to be used by non-technical professionals hence an understanding of the defined zones is necessary.
Guidelines explaining the characteristics of each zone were created. These guidelines considered the prevailing geological and geomorphical situations in the parish in order to present a simplified understanding of the landslide probability.
Landslide Inventory. The mapped landslides
that comprised the final inventory numbered 1041 and covered an area of 66.24 km2 or 8.13% of the area of the parish. The sources of these landslides covered 29.73 km2 or 3.65% of the parish area. Twenty-five percent (25%) or 260 of the total source areas, covering 6.19 km2 were reserved for testing. Landslides may be grouped according to area, into two general classes; large and small. Small landslides were classified as those that have area less than 100,000 m2. There are 916 small landslides, 50% of which exceeded 10,000 m2. Large landslides are fewer, numbering 124; 80% are less than 500,000 m2. Eleven of these larger occurrences are extremely large and the largest is 3.2 km2, occurring in the Rio Grande Valley. The second largest at Shrewsbury, along the valley of the Back River, is 2.6 km2.
4.2. Geomorphic Causative Factors. A strong correlation was revealed in the individual classes of the topographical predisposing factors. Elevation tested at 100 m intervals showed the highest correlation with the area of landslide sources. High values of susceptibility (>10) were computed over the range between 200 m up to 1000 m above sea level (Table 2; Figure 13). Within this range there is 58% of the parish area and 77% of the landslide sources analyzed. In contrast, for the lower (<200 m) and upper (>1000 m) ranges of elevation decreasing susceptibility values were derived.
Slope gradients divided into ranges of 10° showed the highest susceptibility on gradients steeper than 10° (Table 3; Figure 14). As gradients increased, susceptibility consistently increased with the highest recorded for the steeper slopes (up to 50°). Slopes less than 50° occupied a very small area and susceptibility was derived from field surveys. In consideration of slope aspect, slopes facing easterly directions all showed the highest levels of landslide susceptibility weights (Table 4; Figure 15). Additionally, north-westerly facing slopes also had high susceptibility values comparable to the east-facing slopes. South-facing slopes account for the smallest spatial coverage within the parish. The north-east facing slopes, accounting for 17% of the parish area, hosted nearly 20% of landslide source area. This is also