Sikkim: Two landslides and a subsidence area on NH10 between Rangpo and Gangtok

NH10 is the lifeline of Sikkim and of much of West Bengal’s Kalimpong district. Of the 114km of NH10 between Siliguri and Gangtok, roughly 90 km (between Sevoke–Gangtok) runs through mountainous terrain, and a large share of it hugs the Teesta river. 
The highway is most vulnerable to landslides, rockfalls and subsidence between Coronation Bridge and Teesta Bazar. Here, erosion and subsidence have worsened since the construction of Teesta Low Dam Projects III (27th Mile, near Rambi) and IV (Kalijhora), which local stakeholders blame for siltation and altered river behaviour. The October 2023 GLOF from South Lhonak Lake added new hazards along the Teesta corridor.

Though the Teesta Bazar–Gangtok section is generally more stable, it has its own nightmare zones. The 20th Mile landslide at Bardang, between Singtam and Rangpo, has recently caused repeated closures of NH10 but the Martam landslide, once a major trouble spot, appears to have stabilized through engineering treatment and natural consolidation working in tandem.

Martam landslide (27°15’34.4’’N,88°33’02.2’’E)
The Martam landslide located approx 9km from Ranipool towards Singtam was a very trouble some slope failure which affected transportation and the lives of people for years. It was a very active and large (190mX75m) landslide which seems to have now healed thanks to a combination of technology and nature.

Google Earth Images of the Martam landslide of 2022 and 2025 which show a less active landslide in the image below.

My images of 2019 and Oct 2026 show that the Martam landslide has indeed become more stable.

 

A technical study on the Martam landslide is available here:
(IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 11, Issue 4 Ser. I (July. – Aug. 2023), 52-56 www.iosrjournals.org).

The 'forever' sinking area near Martam (27.252609N 88.5641663E)

This area of subsidence on NH10, a short distance towards Singtam from Martam (see Google Earth satellite image of 2025 above) has been there for a long, long time and as the photos below show nothing much has changed over the years. 

 

 

The 20th Mile Landslide at Bardang (27.206257N 88.489384E)

20th MIle landslide at Bardang on 16Oct2026

 

The Bardang area near Singtam in Sikkim has long been an area of slope instability and has disrupted traffic and caused economic losses to Sikkim during the annual monsoon seasons for decades. The initial trigger of the landslide appears to be natural with heavy rain and geology being the primary causes.
In 2010, the Border Roads Organization constructed an 80m long, cut and cover tunnel at Bardang as a short term measure to allow safe passage to those using the roadway, rather than stabilizing the the slope. However, the landslide area has progressively expanded up the hillside and the existing protection has been overwhelmed by the sheer volume of debris (see above photo of 16Oct2026).
In the monsoons of 2026, the NH10 (Sikkim section) was crippled by multi-day closures and  traffic restrictions stretching over four months (June to October 2026) with traffic being diverted thru alternate routes.

I have 'welded' two drone images of the slide to show a contiguous image, while doing so there maybe alignment errors- 16Oct2026.

Drone image of the Teesta river and NH10 on 16Oct2026

Drone image of traffic on NH10 at Bardang on 16Oct2026

Praful Rao
SaveTheHills
savethehills@gmail.com
9475033744

The waxing and waning of Nimbong landslides (Kalimpong district) thru the years.

The ZERO landslide at Nimbong in 2018 

Nimbong in Kalimpong district is no stranger to landslides and one reason for this (besides the geology and anthropogenic causes) could be its location on the sub-Himalayan ridge line overlooking the plains of the Dooars, where it is the first mountainous obstruction to the SW monsoon currents rolling in from the Bay of Bengal. 
As such this region receives the full blast of the SW monsoons each year - annual rainfall is around 4000mm with the area being prone to major thunderstorms.
Being around 70 kms from Kalimpong, I have visited the area quite often and the roads which were a mess once, are now good with the newly constructed NH 717A running thru Nimbong.
My first visit to Nimbong was on 06Jul2009 and I have chronicled the same here. 
In that report, there is no mention of the ZERO landslide because the slope failure had not occurred then and my focus was on the huge CENTRE gaon landslide. Google Earth images from 2010 to 2015 seem to bear me out and show the area below Anuj SUMI school was stable with no trace of any landslide (see below).

The slope failure at ZERO seems to have commenced in 2016 and increased to its maximum size of approx 800m X 150m in 2018 (see top image). Fortunately, I visited Nimbong on 08Jan2018 and have uploaded a drone video showing the gigantic size of the landslide in YouTube. 
Photos of the landslide (of approx 2018), near Anuj SUMI school showing the soil texture and proximity of the landslide was to human habitation are placed below:

 

 

My most recent visit to the  ZERO landslide at Nimbong was on 03Nov2026 and drone photos of the above area near Anuj SUMI school reveal how the entire area has been re-vegetated  and the landslide seems to be healed (see below) and with it human habitation in the area has increased.

The re-vegetated and 'healed' ZERO landslide

Other landslide areas in the vicinity of NIMBONG

The image shows: ZERO landslide, the Centre Gaon landslide and a new landslide (A) on NH717A which seems to be growing.

New Landslide (A). 26° 58' 11.046" N 88° 34' 1.962" E, Elevn 1287.50 m
This landslide is a fresh landslide and is not visible in the satellite image of 2022. It is undoubtedly caused by the road construction and seems to be growing upwards - Google Earth dimensions of the slide are approx 86mX51m.

New landslide on NH717A below Anuj SUMI school at Nimbong being checked by Prof Balamurugan (CUTN) and STH member Tashi Ongmu

Centre gaon landslide 26.963255 N 88.579111 E.
My personal observation seems to be borne out by the Google Earth image of this huge landslide which seems to getting re-vegetated although at a much slower pace than the ZERO landslide. 
Drone images taken on 03Nov2026 also show fairly extensive vegetation surrounding the slide and within it. Hopefully this landslide will also be controlled like the ZERO landslide.

Crown of Centre Gaon landslide at Nimbong.

Midsection of the landslide.

 

Our visit on 03Nov2026 showed that nature working in tandem with technology and some luck from the monsoons can bring even truly massive landslides undercontrol within a finite amount of time. This is good news for us living in the mountains.

My thanks to Dr Peter Mcgowran and Tashi bhai of DDMA, Kalimpong,who's photos I have used here.

Praful Rao
SaveTheHills
savethehills@gmail.com
9475033744.

 

Lest we forget : the disasters of October the fourth

 

A photo of the Andersen Bridge at Teesta bazar which I took in Oct 1968 one week after the disaster (restored using AI without making any major changes). The two figures are Kalyan (my cousin) and a friend - we had all walked down from Kalimpong to see the destruction.

 

 The Fourth of October

I'm not superstitious. Yet 04 October brings a certain chill, not from belief in some omen, but because three major disasters have struck our region on that date, and I lived through and witnessed all of them. 

4 October 1968: the mother of all disasters
 
This was the worst calamity the Darjeeling-Sikkim Himalaya has experienced, and the roots of many of our unstable slopes can be traced back to it.
I clearly remember a steady, heavy downpour that lasted four days, from 2 to 5 October. When it stopped and the blue autumn sky returned, all one could hear was the sound of water: gushing down drains, flowing along streets, seeping out of buildings, bursting out of springs that had never existed before, and in the roar of the river in the Teesta valley.- which I heard from my home situated almost 4000 feet above on the Tirpai ridge (of Kalimpong).

Rainfall data: about 1,000 mm, with the peak falling in roughly 52 hours
Cause: a depression that moved in from the Bay of Bengal
Fatalities: estimates range from about 600 to around 1,000
Damage to Infrastructure: hundreds of landslides, the majestic Anderson Bridge at Teesta Bazaar destroyed, and the highways to Kalimpong, Darjeeling and Sikkim washed away in many places
Isolation: Kalimpong, Darjeeling and Gangtok were all marooned. Food and relief materials were airdropped in Kalimpong.

4 October 2023 GLOF: the Teesta strikes again

The river in spate  at the Teesta Bazar, during the GLOF. Some of the most iconic photos of the GLOF have been taken by STH photographer Praveen on 04Oct shortly after the floods hit Teesta bazar.

A glacial lake outburst at South Lhonak lake in North Sikkim triggered the largest GLOF in the Himalayas so far on 04Oct2023. A wall of water raced down the Teesta in the early hours, wrecking the Chungthang dam and washing away bridges. It was the deadliest flood in the region since 1968.
Fatalities: 92 or more, with many more missing
Bridges lost: 14, which cut Sikkim off from Siliguri
STH has covered this in detail in our numerous Oct2023 blogs on the GLOF.

4 October 2025: Mirik and Sukhia Pokhari landslides

Fatal landslide at Toklang, Mirik on 04/05Oct2025 which killed 4 people.

Rain from a Bay of Bengal depression brought landslides across Darjeeling district. Mirik and Sukhia Pokhari were hit hardest, and the Dudhia iron bridge on the Balason, the main link between Siliguri and Mirik, collapsed.
Rainfall data: 

Fatalities: at least 28 
We have covered this event in detail in the Oct2025 blog

Different decades, different places, same date. That's the chill.

Praful Rao
SaveTheHills
savethehills@gmail.com
9475033744