Manipur: Manipur University Earth Science Department report on Noney landslide

Imphal, July 12 2022: Manipur University Earth Science Department has submitted its survey report on the Makhuam landslide in Noney district, on July 9 citing multiple reasons for triggering the tragedy.

The report stated that excessive rainfall was experienced during May 2022 (375.60 mm) and June 2022 (329.9 mm) in Noney district, which was approximately 60 per cent higher than the normal earlier recorded rainfall.

The data was recorded at the nearest station from the landslide site.

In order to assess the dimension of the massive landslide, a drone survey was conducted on July 5 and data analysis was done to generate a DEM of the site.

The survey team found that a large old landslide slump was present in the area and the slump became oversaturated due to excessive incessant rainfall during May-June 2022, which triggered the landslide.

The railway formation was developed for the yard, railway station and open cut for the railway track.

The slide triggered from the crest of a hill and debris slid into the Ijei River bed.

An area of 31 hectares approximately was affected by the massive landslide in which the open cuts and major part of the railway yard was devastated along with causing casualties.

The approximate volume of the landslide slump (debris) was about 4.3 lakh Cubic metres, which slid due to the landslide.

The presently piled up debris may be in terms of about 6.02 lakh cubic metres deposited on the slope of the hill from the railway formation till the river bed.

Citing four possible causes of the landslide, the report stated that geologically, non-resistant lithologies (shale inter bedded with mudstone, siltstone and sandstone) of Barail Group exposed in the study area after the modification of the slope geometry that impedes the mountain’s ability to resist the existing gravitational force may become one of the causes of mass wasting/landsliding.

The process might have started long back, and the entire mass wasting slump was settled along the slope of the hill.

It seems that the recent precipitation during May-June 2022 became the threshold for the ongoing massive landslide hazards in the study area.

Secondly, the fluctuation in the ongoing precipitation pattern is the source of availability of more water along these slopes to percolate below the slope surface.

The fine grained lithologies (clay and silt) being less permeable become more plastic and reduce the stability of the slope and initiate the sliding, while the resistant lithologies (sandstone and mudstone) still hold water in their pore spaces.

These two contrasting combinations of litho units in the western Manipur as well as in the study area seem to be one of the causes for slope instability.

Thirdly, Manipur is included in the High Seismic Hazard (Zone V, Seismic Zonation Map of India) and hence, the micro seismicity is active besides a large earthquake had occurred in 2016 (M6.7) whose epicentre was near the recent landslide area.

Based on the fault plane solutions (Global CMT Project) of earthquake data, the principal P-axis is towards north, indicating the compression direction, resulting in the extension along east-west, the resulting creeping of micro deformation towards the western slopes of the terrain including the recent landslide area is aligned with the principal T axis.

It is also seemed to be one of the triggering factors for the hazard as few minor cracks are observed in the cut slopes of the railway yard.

Fourthly, changing in the land use patterns on the top of hill like deforestation, Jhum cultivation, banana cultivation, which also accelerated the soil erosion process and might also be the secondary factors for the recent landslides, the report said.

On the basis of finite element analysis with shear strength reduction method, it can be concluded that the existing yard cutting area is not stable as upper slope of the area may undergo slid in a rainfall event.

And the landslide area was earlier investigated and identified as high to very high hazard under an unpublished doctoral thesis submitted and awarded by Manipur University in the Department of Earth Sciences in 2007.The Jiribam-Tupul-Imphal Railway Project is progressing well and the recent landslide has affected the railway formations in the proposed Tupul Station yard.

Since, it will be one of the major life-lines for the state in order to transport goods and passengers from rest of the country to the state, and considering the success and longevity of the ongoing project for several decades, it is necessary to adopt the regulatory monitoring of the vulnerable slopes along the railway line where the cut portion and yards are exposed.

The report also made some recommendations suggesting reassessment of slope stability (Factor of safety) analysis for open cuts and yards of the Jiribam Tupul Railway Line; regular drone survey and monitoring to detect the mass movement along the open cuts and yards as well as beyond railway land boundary before and after monsoon season without a significant failure event, the detection of subtle slope movements across a short survey interval using DEM-differencing will likely prove challenging to identify.

It also recommended installation of automatic weather stations for monitoring of weather data at major Railway Station sites from Jiribam to Tupul (precipitation and soil moisture); installation of slope failure monitoring with web enabled equipment (Borehole extensometers, optical targets, tiltmeters, piezometer, crack meter, soil moisture meter at a depth of 5-10 meters) at open cuts and yards.

Geology of the area cannot be changed, however, the innovation of new and emerging technologies and ideas will mitigate these disastrous situations, the report said and suggested to encourage the state and Central organisations, to engage locally available individuals, who are working in the relevant fields and familiar with the terrain, and geological conditions of the area for the recommendations.

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