Report of the Expert Team On landslide affected areas in Manipur
- Part 1 -

Prof Arun Kumar *

Landslide Tragedy : Recovery works begin at Joumoul in August 2015 :: Pix - HL

During the first week of August, 2015, there was a massive landslide in Manipur where more than 20 people buried alive. Considering the general problem of landslide in North East India, particularly, in Manipur, Department of Science & Technology, Govt. of India has constituted a team of experts for visiting the landslide affected areas in Manipur and undertake on the spot preliminary studies/ investigations about the underlying causes of these landslides and suggest short term and long term measures so that problem of landslide are minimized in this part of the country.

2. Accordingly an expert team was constituted by DST. The team visited the landslide affected areas in Manipur during 14-16 Sept, 2015; the following experts visited the landslide sites:

1. Prof. S.M. Ramasamy, Former VC, Gandhigram Rural University, Dindigul, Tamilnadu.
2. Prof. Arun Kumar, Deptt. of Earth Sciences, Manipur University, Imphal
3. Dr. Bhoop Singh, Head (NRDMS & NSDI), DST, New Delhi
4. Dr. M. Chandra Singh, Lecturer, Imphal College, Imphal
5. Dr. Dinachandra Singh, Manipur Council of Science & Technology, Manipur
6. Dr. S. Manichandra, Deptt. of Earth Sciences, Manipur University, Imphal.

Prof. Anirudh Sengupta, Deptt. of Civil Engineering, IIT, Kharagpur would not join the team due to his indisposition.

3. Landslide sites

The Expert Team visited two important landslide sites i.e. Phesama Landslide and Tengoupal Landslide. The detailed observations of the team area as under :

(i) Phesama Landslide

A major landslide occur on 15-17 August 2015 with located at 139 kms between on the highway between to Imphal-Kohima. In addition to making field observations all along the highway, the team has also made detailed observations of the landslide on 15 September 2015.

All along the highways was almost fringing the easterly slope of the N-S trending hill ranges, constituted by Disang and Barail group. Disangs shales are inter-bedded with mudstone, siltstone and sandstone. They are highly jointed, and shales are black to brown and splintery in nature. In many places, because of the rains the shales have spread clays along the roads indicating more clayey contents of the Disangs. The easterly slope of the hills located to the west of Highways under discussion, developed lot of gullying due to the drainage flowing down the slopes which have produced lot of spurs on the slope.

On the right side highway, there is moderate to deep valley, with northerly southing flowing drainage showing over all rectilinear flow with meandering patterns at many places. Wherever, these meandering are souring the slope, which is destabilised at places causing subsidence and at places landslides. That to wherever, these sinus drainage are cutting the toe, the highways development authority have done additional cutting of the slopes in the little upslope region, which is causing the landslides. This is the overall scenarios of the highways such as NH 37, NH 150 etc In Manipur and Nagaland.

Near the Phesama, where landslide occurs, the road is taking an arcuate turn westerly and again gets northerly alignment at Phesama village. BRO has developed arcuate highways, because the down slope region is braided valley which forms the catchment of for the streams. The morphology of the slide is as follows:

Crown arc length with up scarp with depth of sliding and distance travelled by the debris flow up to the valley. Now the entire slope is made up of landslide occur exposes Disang shales which are highly jointed and splintery and inter-bedded with highly jointed siltstone ranging from 1 mts at bottom slope to 30 mts at crown. The width of scarp is 190 mts; Depth of sliding 190 mts; Distance travelled by sliding materials is approximately 500 mts.

Beside the inherent softer, splintery and jointed and clayey nature of the Disang shale, the slope also has huge debris cover appears to be old landslide slump which formed the earlier landslides.

Above crown arc of the present landslide two more crown have developed indicating that the active sliding is in progress.

While the bottom of the landslide is cut by the highways development on the top of the crown, there was a village, which has constructed a huge tank for storing the rain water, which could also add the aggravate the seepage besides already occurring number of streams which submerged along the top slope and re-emerges along the foot hills just above the road level.

Over and above all the CMF which is all along fringing and running parallel to the highway is located enclosed proximity to the crown of present slide. The subsidence is also occurring as addition to the major landslide because the down the slope region is a broad arcuate valley acting as catchment for the steams.

(ii) Tengoupal Landslide:

The Expert committee visited the Tengoupal landslide on 14 September 2015 and it is located near the Tengoupal village in between 68 km to 71 km on the NH 02 leading to Myanmar via Moreh.

The geological formations under tectonic features are almost similar to Phesama with Disangs shales forming the slopes and barails forming the top cap rocks. The slide occur on first week of Augustr due the road cut made by the BRO for the widening the NH 02.All along the road from Imphal to Tengoupal in the N-S trending hill ranges are forming a steep slope of 60-70 degrees especially at where landslide occur.

In general, the slope of the Disang slope is gentle to moderate, and formations are dipping towards hill. Though over all it is splintery shales, it has got pockets of massive zones at places. While the western side of the road, where landslide occur, eastern side of the road is broad arcuate valley forming the catchment of major streams.

In addition to the inner fragility of the rocks, toe removal by the road widening the head ward erosion by the steams are also caused toe removal. And in conjunction with rainfall caused the landslide in the area.

To be continued ..