S. BHALAI – Landslide susceptibility of Portland, Jamaica
Figure 7. Age distribution of rock types in Portland. The Blue Mountain Inlier is a tectonic window of Cretaceous rocks surrounded by younger lithology. It is enclosed in the south (not shown).
future. These conditions are topographical or geomorphical, and geological in nature. Topographical causative factors are elevation, slope gradient and slope aspect. Geological factors are rock type (and their respective geotechnical behaviour) and structural features, which in the case of Portland, is the network of faults.
The assessment process, consistent with the usual
practices, involves combining direct (field) indirect (statistical) methods (Soeters and
Westen, 1996; Hansen, following steps:
i). Remote sensing interpretation and records of landslide occurrences; ii). Data collection (topographic and causative factors);
iii). Field surveys and confirmation checks;
susceptibility model (including testing);
comprise an inventory of all the landslide occurrences that can be mapped, and local terrain conditions such as elevation, slope gradient and slope aspect.
A preliminary inventory of landslides was generated primarily from remote sensing imagery interpretation, specifically aerial photographs. Monochromatic (e.g., Figure 8) and panchromatic photographs of varying scales (1:12,000 – 1:25,000) spanning a wide temporal range (1953 - 1992) were used. Landslides post-dating the photographs were added to the inventory based on the descriptions provided in the documented damage assessments. Landslide boundaries were plotted on 1:12,500 topographic maps. The source area, loosely defined as the upper half of the landslide (Parise and Jibson, 2000), or that part of the scarp and zone of depletion where the material originated, was defined on the maps and carefully confirmed in the field.
v). Compilation of supporting guidelines for using
the susceptibility model.
Documented evidence of landslide events in the form of damage assessments and reconnaissance field reports were extremely useful in determining the magnitude of the landslide problem. The local Parish Council and the Mines and Geology Division are the main bodies that generate these reports. Data collected for the susceptibility assessment may be classified as topographical or geomorphical, and geological. Geomorphical data
Field surveys and checks aided the addition of new landslides and confirmation and adjustment of the landslides in the preliminary inventory. This corrected and updated preliminary inventory represents the final inventory (Figure 6) used for analysis to generate the susceptibility model. Additionally, a part of this inventory (25%) was reserved for use in testing the new susceptibility model.
While the landslide inventory is important, for the analytical stage specific input parameters of the