Landslide Susceptibility of the Western Ghats

Landslide susceptibility is the tendency of terrain to experience slope failure because of its inherent landform, material and structural characteristics. The Western Ghats form a high, deeply dissected escarpment along the western margin of the Peninsular Plateau, where steep slopes and weathered material create widespread slope instability.

Steep and dissected relief

The Ghats have a pronounced western escarpment, high local relief and numerous steep valley sides. Gravity produces greater downslope stress on such slopes, while irregular profiles create sites where unstable debris can accumulate.

• Dense stream dissection produces narrow valleys, spurs, ravines and steep side slopes.
• Concave hollows and natural depressions collect water and loose slope material, favouring failure.
• River incision and erosion at the slope toe can remove support from the material above.

Deep weathering and unstable slope material

Warm, humid conditions promote deep chemical weathering of bedrock, producing thick regolith, soil and colluvium. These materials can lose shear strength when saturated, particularly where relatively permeable debris overlies less permeable weathered rock.

• Lateritic caps and weathered mantles may develop sharp contrasts in permeability and strength.
• Loose colluvium and older landslide debris resting on steep slopes are susceptible to renewed movement.
• Weathered rock retains inherited joints and fractures that can become potential failure surfaces.

Structural and drainage controls

Rock masses in the Western Ghats contain joints, fractures, foliation and lithological contacts. Where such discontinuities are unfavourably oriented relative to a slope, blocks or debris can slide along them.

• Closely spaced drainage channels concentrate surface runoff and encourage saturation of slope material.
• Short, steep west-flowing streams have strong erosional gradients and can undercut valley sides.
• Intense monsoon rainfall is the principal trigger because it raises pore-water pressure and adds weight to already susceptible slopes.
• Road cutting, quarrying and vegetation removal can disturb slope geometry and drainage, thereby amplifying natural susceptibility.

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