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Landslide Susceptibility of Portland, Jamaica: Assessment and Zonation - page 12 / 16

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YELLOW LIMESTONE

39.5566

3.0224

26.44

SHALE

127.7602

4.4274

11.99

TERTIARY VOLCANICS

8.1901

0.2084

8.80

COASTAL GROUP

39.0423

0.1288

1.14

GIBRALTAR WHITE

187.2914

6.2035

11.46

MONTPELIER WHITE

94.2267

0.5668

2.08

CONGLOMERATE

12.3843

0.1857

5.19

IGNEOUS INTRUSION

32.8550

0.4587

4.83

ALLUVIUM

25.0663

0.0648

0.89

CRETACEOUS VOLCANICS

161.0578

5.4518

11.71

CRETACEOUS SEDIMENTARY

68.9166

0.9608

4.82

METAMORPHICS

0.5042

0.0160

10.98

COLLUVIUM

18.2420

1.8492

35.08

50

111.6070

3.2739

10.15

100

101.3532

2.9324

10.01

150

88.6597

2.6460

10.33

200

76.0916

2.1780

9.90

250

64.4274

1.8606

9.99

300

53.8286

1.5575

10.01

350

45.1130

1.3554

10.40

400

37.8970

1.2442

11.36

450

32.2589

1.1371

12.20

500

27.3190

1.0088

12.78

550

23.0559

0.7687

11.54

>550

153.5429

3.5347

7.97

S. BHALAI Landslide susceptibility of Portland, Jamaica

Table 6. Calculation of the landslide susceptibility of the distance from major faults classes. Susceptibility values are generally between 9 and 12 suggestive of no correlation at this distance range.

CLASSES

AREA OF

SCARP IN

CLASS (km ) CLASS (km )

SUSCEPTIBILITY

Table 5. Landslide susceptibility of the geotechnical groups

AREA OF SCARP IN

CLASS

CLASS

(km )

(km )

CLASSES

SUSCEPTIBILITY

Figure 16. Susceptibility of the geotechnical groups of the rock types in Portland. Colluvium has the highest weight indicating the proportion of source areas developed in this material.

Figure 17. Susceptibility of the classes of distance from major faults. Note that with increase in distance, there is no correlation. The increase in susceptibility at and after 400 m is irrelevant, considering that the parish contains a dense fault pattern and at this distance, the zone of influence of another fault becomes active.

the three highest zones of susceptibility. With the second technique selected districts of known landslide susceptibility (areas having either high or low susceptibilities) were used and these include the Rio Grande Valley, Claverty Cottage, Buff Bay

Valley,

Passely

Gardens,

Manchioneal

and

Shrewsbury. The model favourably predicted the respective level of landslide susceptibility as interpreted from field observations (Table 8). Table 9 shows the distribution of the refined susceptibility zones.

susceptibility between 1000 m are possibly conditions prevailing at

elevations of 200 m to related to the climatic this topographic level. It is

highly likely that humid conditions dominate, encouraging rapid physical and chemical breakdown of the local rock types. The rock types at these ranges may also be responsible. The Shale and

Yellow dominant

Limestone geotechnical groups are at these elevations and these rock types

have also been found to susceptibility, as observed in in the Rio Grande Valley.

have high landslide the Moore Town area

5. DISCUSSION Both geomorphical and geological predisposing factors all show correlation with landslide events. An exception to this is the distance from geological faults, where no correlation was seen. The geomorphical factors all show positive correlations with landslide occurrence. High levels of landslide

Lower levels of landslide susceptibility occur at the low and high extremes of the elevation in the parish. The extent of flat areas and alluvial plains account for the low susceptibility at elevations less than 100 m. At the higher extreme of elevation, as the rocks are decomposed, the residue is eroded during periods of rainfall, removing the weathering

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