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:

Upslope Factors – Conditions on the upper part of the slope that contribute to instability.
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.

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.

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