Sedongpu Mass Wasting

Why in news?

A new study highlighted the impact of high frequency of mass wasting events in the Sedongpu Gully to Northeast India.

What is mass wasting?

• Mass wasting - A geological event, of gravity-influenced movement of rock and soil down a slope.
• These events may occur very rapidly and move as a flow.
• Causes – They are often lubricated by rainfall or agitated by seismic activity.
• Others includes rapid snowmelt, volcanic eruption, storm waves, rapid-stream erosion, or human activities like grading a new road.
• Types – It involves the slipping, slumping, falling and flowing of soil or rock down a slope.

• Determining criteria - There are various factors which increase the likelihood and rate of different types of mass wasting
• Steepness of slope gradient – The steeper the slope, the greater the influence of gravity on particles.
• Rock structure - Bedding planes and faults can increase vulnerability to movements.
• Rock type – Sedimentary and unconsolidated sediments are more vulnerable to movement than igneous rocks.
• Level of water saturation – The greater the saturation, the lower the friction between particles.
• Vegetation cover – Plant roots help to hold surface together
• Human activity – Mining can cause vibrations which trigger movement/poor slope management can lead to undercutting slopes

What is sedongpu mass wasting?

• Sedongpu wasting – More than 700 million cubic metres of debris have been mobilised in the Sedongpu gully catchment since 2017.
• Patterns of sedongpu wasting - From satellite images, 19 large mass-wasting events were grouped into three sub-patterns
  • Ice-rock avalanche (IRA)
  • Ice-moraine avalanche (IMA)
  • Glacier debris flow (GDF)

A moraine is a mass of rocks and sediment deposited by a glacier.

What are causes for Sedongpu mass wasting?

• Long-term warming - Annual temperature in this area increased at rates of 0.34º to 0.36º C during 1981-2018, which is higher than the global average.
  • Sedongpu basin consists mostly of Proterozoic marble whose land surface temperature ranges from -5º to -15º C, rarely exceeding 0º C before 2012.
• Thus, there is extensive permafrost degradation occurring in the catchment, resulting in the progressive destabilisation of the slopes.
• Dams – Big dams such as the Zangmu on the Tsangpo accentuate the threat of mass wasting.
  • China plans to set up a 60-GW project on the Tsangpo, which will have thrice the capacity of the Three Gorges project on the Yangtze, the world’s largest hydropower plant.
• Steep Topography – Due to its steep topography, it is one of the most landslide-prone locations on the planet.
• Geographical Instability – It is prone to earthquakes.
• The 2017 Nyingchi earthquake destabilised the slopes and initiated the current period of instability.
• Other factors – They have a highly fractured rock mass and the presence of large volumes of sediments from recent glaciations.

What are its impacts in India?

• The Sedongpu mass wasting has serious transnational implications for the Tsangpo-Siang-Brahmaputra-Jamuna basin, especially in India and Bangladesh.
• Sedimentation - Addition of major amounts of sediments to the course of the river.
  • River Brahmaputra carries more than 800 tonnes of sediment at Pandu in Guwahati, becoming more than a billion tonnes at Bahadurabad in Bangladesh.
• River Blockage – Added sediments blocks the river at different locations.

Debris flows have occurred in two adjacent gullies, namely Sedongpu Gully (SDP) and Zelongnong Gully (ZLN), since the 1950s.

• River bank erosion - Increasing sedimentation may make the river more intensely braided in the Assam plains, which could lead to more bank erosion.
• Flooding Hazard - The sedimentation can elevate the river beds more, accentuating flood hazards.
• Glacial Lake Outburst Flood- Breaching of the blockages leads to catastrophic flash floods in the downstream.
• These floods were triggered by the outburst of a dam created by the glaciated debris and rock materials generated during landslides.

• Livelihood Impact - The channels of the river in Assam and Bangladesh may get choked with sand and silt in the lean season making navigation difficult and also affects fishing.

What lies ahead?

• Monitor the status and trends of geophysical events leading to landslides, rock falls and other erosional processes.

Reference

The Hindu| Frequent mass wasting in Tibet could Impact India