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Landslide Susceptibility of Portland, Jamaica: Assessment and Zonation - page 11 / 16

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S. BHALAI Landslide susceptibility of Portland, Jamaica

Table 3. Landslide susceptibility of slope gradient classes

Table 4. Landslide susceptibility of slope aspect classes

CLASSES

FLAT

  • 0

    - 10

10 - 20 20 - 30 30 - 40 40 - 50

(km )

SCARP AREA AREA OF CLASS IN CLASS

(km )

SUSCEPTIBILITY

5.10

0 (undefined)

0 3.13 10.75 17.95 24.30 26.75

303.58 295.59 159.19 49.13 1.94

2.75 9.18 8.26 3.45 0.15

Figure 14. Slope gradient and susceptibility. Although not shown, the susceptibility of slopes of gradient >50° is low.

Figure 15. Slope aspect and susceptibility.

comparable to the east-facing slopes which accounted for 13% of the parish area but which hosted 20% of landslide sources. The north-easterly facing slopes correspondingly had slightly lower susceptibility values.

4.3. Geological Causative Factors. Of the thirteen geotechnical groups, six had high landslide susceptibility weights, two of which were exceedingly high (Table 5; Figure 16). The Shale, Gibraltar White and the Cretaceous Volcanics groups accounted for 16%, 23% and 20% of the source areas respectively. These had approximately equal weights of susceptibility. Colluvium and Yellow Limestone, accounting for only 2% and 5% of sources respectively, had exceedingly high susceptibilities. Alluvial areas showed extremely low landslide susceptibility. No correlation was observed with the distance from geological faults (Table 6; Figure 17). Susceptibility remained unchanged even as distance increased further away from specific faults.

4.4. Susceptibility Model. The susceptibility weights calculated for each class of the predisposing factors for landslides (Table 7) were used as weightings to the respective class for the generation of the susceptibility model. A form of heuristic approach is somewhat introduced where some weighting were adjusted to ensure realistic representation of the parameters. This considers field experience and general knowledge of the respective geotechnical behaviour of the predisposing factors. Summation of the final weighted coverages produced the susceptibility model (Figure 18).

Five zones of increasing landslide susceptibility were identified in the final model. These are zones of Negligible to Low, Moderate, Moderately High, High and Very High landslide susceptibility. Using (i) the reserved source area inventory, and (ii) areas of known landslide susceptibility, the model was tested to determine the reliability of making predictions and provide for refinement. Using the first testing technique approximately 80% of the reserved source areas were confidently predicted in

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