Survey report of Tupul landslide trigerred on 29th-30th June
The team comprising of Prof. Arun Kumar MU, Dr Manichandra Sanoujam MU, Dr Thingujam Dolendro Geoconsult, M Jagajit Singh Computer Gallery Imphal, Dr N Sanamacha Singh ECC, SK Gainaimei ECC,RK Bikramjit ECC visited the spot with Dr RK Ranjan
Introduction: The landslide hazards are being frequently triggered along the slopes which usually fall across the existing roads/highways connectivity in the State and along the hill slopes near the major rivers in the hilly areas due to the tree cutting and other anthropogenic factors.
Geologically, western Manipur consists of Tertiary strata (rock formations) which are most vulnerable to the landslide hazards due to the seismically active tectonic terrain. The rock formations are highly jointed, fractured, faulted and display the neo-tectonic movements. The interbedded shales, mudstone, siltstone and sandstone are the main lithological types found in the study area.
The climate is of sub-humid with high rainfall (1600 mm) in the State, which is one of the triggering factors for most of the frequent landslide hazards with existing geological formations and typically rugged topography. The interbedded shale, mudstone, siltstone and sandstone exhibit weak lithology and existing discontinuities in the forms of joints, cracks, faults etc along the moderate to steep slopes are the main triggering factors for the landslides.
The alignments of the National Highways in Manipur across the terrain have also increased the frequent landslides due to the disturbances in the toe, as most of the alignments traverse along these typical adverse geological and geomorphological situations.
Description of Massive Landslide at Tupul (Makhuam Landslide)
It was triggered on the intermittent night of 29th-30th June 2022. Excessive rainfall was experienced during May 2022 (375.60mm) and June 2022 (329.9 mm) at Noney District, which was approximately 60% 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 5th July 2022 and data analysis was done to generate a DEM of the site.
A large old landslide slump at 1063m msl high hill with steep slope which was quite vulnerable for the landslide hazard. The slump became oversaturated due to the excessive incessant rainfall during May-June 2022 and the landslide was triggered.
The Railway formation at 570 m msl height 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 slided in the Ijai 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 casualties (18 injured, 49 dead and 12 missing till the writing of this report).
The estimated volume of the sliding materials is calculated using the CARTOSAT DEM (9th April 2009) and compared with the Drone generated DEM (after the landslide i.e on 05th July 2022). The approximate volume of the landslide slump (debris) was about 4.3 lakh Cubic meters, which was slided due to the landslide.
The presently piled up slided debris may be in terms of about 6.02 lakhs cubic meters deposited on the slope of the hill from Railway formation till the river bed.
It was also estimated that the volume of the slided debris resting over the Railways formation estimated to about 2.5x105 cu.m (approximate area spread piled up 465 m in length and 30 m in width and thickness from the existing railway formation 40 m).
It is estimated from comparison of DEM of 2009 (Cartosat) and 2022 (Drone Survey after landslide) by taking a section along the slide. It usually happens in case of compact mass before the sliding and after the sliding the volume increases due to unconsolidated piling during the landslide pheno- menon.
The profile along the slope displays the actual thickness of the piled up slurry of landslide as well as few slip surfaces have also been developed after the recent event for future sliding whenever excessive rains are experienced
(To be contd)