Landslide Upslope Downslope Factors

Landslides occur when the driving forces (gravity, water saturation, seismic activity) exceed the resisting forces (cohesion, friction, vegetation cover) on a slope. To understand landslide dynamics, we classify contributing factors into:

1. Upslope Factors – Conditions on the upper part of the slope that contribute to instability.
2. Downslope Factors – Conditions at the base that further exacerbate landslides, often by removing support or altering water flow.

1. Upslope Factors

(a) Steep Slope Angle (Gradient Effect)

• A higher slope gradient increases shear stress, making the slope more susceptible to failure.
• Angle of Repose: The maximum angle at which a material remains stable; exceeding this angle leads to landslides.
• Example: The Himalayan region has high landslide risks due to steep slopes and tectonic activity.

(b) Weak Soil or Rock Type

• Lithology (rock type) determines slope strength.
• Clay-rich soils (e.g., montmorillonite) expand when wet, reducing stability.
• Weathered rocks (e.g., shale, phyllite) lose cohesion over time.
• Example: Western Ghats experience landslides in lateritic soils after monsoon rains.

(c) Vegetation Cover

• Roots reinforce soil and absorb excess water.
• Deforestation increases erosion and runoff.
• Example: Amazon Basin has stable slopes due to dense tree cover, while Haiti suffers from deforestation-induced landslides.

(d) Water Saturation (Pore Water Pressure Effect)

• Hydrostatic pressure increases soil weight and reduces internal friction.
• Infiltration capacity varies; sandy soils drain better than clayey soils.
• Example: Uttarakhand floods (2013) triggered landslides due to excessive rainfall.

(e) Joints and Fissures

• Geological discontinuities (faults, bedding planes, joints) act as failure planes.
• Example: The San Andreas Fault in California increases landslide risks due to active tectonics.

(f) Human Disturbances

• Construction (roads, dams) alters load distribution.
• Mining induces vibrations and weakens slopes.
• Example: The Malin landslide (Maharashtra, India, 2014) was worsened by hill cutting for agriculture.

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2. Downslope Factors

(a) Erosion at the Base

• Undercutting by rivers, waves, or glaciers removes slope support.
• Example: Konkan coast, India experiences coastal landslides due to wave erosion.

(b) Changes in Water Table

• High groundwater levels increase pore pressure and reduce cohesion.
• Example: Oso landslide (Washington, USA, 2014) was linked to high water table changes.

(c) Steep Channel Gradients

• Accelerated water flow scours the base, destabilizing the slope.
• Example: Teesta River valley, Sikkim has landslide-prone zones due to steep stream gradients.

(d) Presence of Debris

• Accumulated sediments create barriers, leading to sudden failures.
• Example: Landslide dams in Nepal Himalayas cause flash floods when breached.

(e) Previous Landslide Activity

• Remobilization of old debris increases future risks.
• Example: Darjeeling region experiences recurring landslides due to historical instability.

• Interplay Between Factors: Upslope and downslope factors often act together.
• Mitigation Strategies:
  • Upslope: Reforestation, slope drainage, terracing.
  • Downslope: Retaining walls, river training, landslide barriers.
• GIS-Based Risk Mapping: Combining remote sensing and Digital Elevation Models (DEM) helps in hazard prediction.