11 May 2015

Jalbire, Sindhupalchok: the best landslide video from the Nepal earthquake to date?

Jalbire, Sindhupalchok landslide video

I think that this film, from Jalbire in Sindhupalchok, is the best landslide video to have emerged from the Nepal earthquake so far:

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The location is Jalbire, which is a village in Sindhupalchok:

Jalbire

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The yellow marker on the above image is a landslide mapped by the combined US / UK / Nepal team.

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Gorkha earthquake update

Gorkha Earthquake update

The landslide situation in the aftermath of the Gorkha earthquake in Nepal is steadily becoming more clear due to the efforts of multiple parties.  Importantly, the main groups working on mapping the landslides have now joined forces and are indeed generating a common dataset.  There is a nice summary of their efforts on the ICIMOD Gorkha Earthquake portal, which notes that:

“To date, the response teams have identified over 3,000 landslides, and assembled a database of over 250 identified landslides and other large mass movements, focusing specifically on those that were generated by the earthquake and its aftershocks or other secondary effects.”

The Google Earth map of the 250 landslides is available to download as a kml file.  The map looks like this:

 

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As the BGS / EWF team noted earlier, the landslides are concentrated in the northern area of the earthquake affected region, and many are along highways.  Apart from the high losses associated with Langtang moraine and ice avalanche, the most serious landslide identified to date seems to be a valley-blocking slide on the Marshyangdi River upstream of Lower Pisang.  NASA have a nice post describing this landslide, including this satellite image.  This work was undertaken by Dan Shugar from the University of Victoria in Canada:

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As of 6th May this landslide dam had not breached.

Meanwhile, news reports are describing the impact of landslides at various locations.  For example eKantipur has an article outlining the loss of life from landslides in Rasuwa:

“Scores might be missing in landslides triggered by the devastating earthquake on April 25 at Mailung, which lies between Dandagaun and Haku VDCs in Rasuwa district. Rescuers have fished out 12 bodies from the debris as of Sunday.  The Mailung settlement has been buried by landslides. Locals said around 250 workers at the 60 megawatt Upper Trishuli III A hydroelectricity project, access road of Upper Trishuli I and locals have been buried in landslides…However, the construction company has the record of 32 workers missing. Sixteen workers are from Rolpa, two each from Dang and Bardiya. The address of 12 others is yet to be verified.

Two bodies of workers engaged in constructing the Upper Trishuli I road track, have been found. Project chief Uttam Amatya said they had recovered the body of one Chinese and another Nepali whereas eight others are still unaccounted for.

The District Administration Office said one each from Nepal Army and Nepal Police were also killed in landslides. Police found six bodies of people who had reached Mailung for a picnic. Eight other picnickers are missing.”

The magnitude of landslide damage to new and under construction hydroelectric stations in Nepal is the subject of an article in the Himalayan Times, which details extensive landslide impacts at a number of sites.

Finally, to date 120 bodies have been recovered from the Langtang ice and moraine avalanche.

 

The devastating earthquake of April 25 has badly hit the country’s power sector, causing damage to more than 20 hydro projects that were either generating electricity or were under construction.Data provided by the Independent Power Producers Association of Nepal and Nepal Electricity Authority show that the 7.6-Richter quake affected at least 23 operational hydroelectric projects with installed capacity of at least 176.88 megawatts.Although exact financial loss inflicted by the disaster is yet to be ascertained, the damages have reduced the country’s power generation capacity, which is reeling under power cuts of around six hours per day. – See more at: http://www.thehimalayantimes.com/fullNews.php?headline=Earthquake%20hits%2023%20hydro-electric%20projects%20&NewsID=454378#sthash.dzOZIiwV.dpuf

 

The devastating earthquake of April 25 has badly hit the country’s power sector, causing damage to more than 20 hydro projects that were either generating electricity or were under construction.Data provided by the Independent Power Producers Association of Nepal and Nepal Electricity Authority show that the 7.6-Richter quake affected at least 23 operational hydroelectric projects with installed capacity of at least 176.88 megawatts.Although exact financial loss inflicted by the disaster is yet to be ascertained, the damages have reduced the country’s power generation capacity, which is reeling under power cuts of around six hours per day. – See more at: http://www.thehimalayantimes.com/fullNews.php?headline=Earthquake%20hits%2023%20hydro-electric%20projects%20&NewsID=454378#sthash.dzOZIiwV.dpuf
The devastating earthquake of April 25 has badly hit the country’s power sector, causing damage to more than 20 hydro projects that were either generating electricity or were under construction.Data provided by the Independent Power Producers Association of Nepal and Nepal Electricity Authority show that the 7.6-Richter quake affected at least 23 operational hydroelectric projects with installed capacity of at least 176.88 megawatts.Although exact financial loss inflicted by the disaster is yet to be ascertained, the damages have reduced the country’s power generation capacity, which is reeling under power cuts of around six hours per day. – See more at: http://www.thehimalayantimes.com/fullNews.php?headline=Earthquake%20hits%2023%20hydro-electric%20projects%20&NewsID=454378#sthash.dzOZIiwV.dpuf

 

 

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6 May 2015

Gorkha earthquake in Nepal: a landslide map and update on the landslide hazard

Gorkha earthquake in Nepal: update on landslide hazard

Vanessa Banks, Alex Densmore, Tom Dijkstra, Colm Jordan, David Milledge, Dave Petley, John Rees, Nick Rosser, Jack Williams
British Geological Survey, Durham University & University of East Anglia

This article was first posted to the EWF Blog yesterday

 

Key messages (all observations based on available cloud free satellite imagery)
  • Major or disruptive landsliding is limited to a zone that runs east-west, approx. parallel to the transition between the Lesser and High Himalaya. This zone includes parts of the districts of Gorkha, Dhading, Nuwakot, Rasuwa, Sindhupalchok, Dolakha, Ramechhap, and Khavre.
  • We observe several large valley-blocking landslides. Most observed landslides comprise comparatively small rockfalls and debris avalanches that have a significant local impact and loss of life.
  • The need to plan measures to mitigate landslide deaths and disruption in the forthcoming monsoon season must remain a priority. Many more damaging landslides may occur in the 2015 monsoon, likely to start in late June, than would be expected if a major earthquake had not occurred.
Status

We were working to three objectives: identification of large landslides (1) blocking valleys and potentially causing significant secondary hazards; (2) affecting villages; and (3) disrupting infrastructure, such as roads. The team has undertaken a thorough investigation of available satellite imagery from multiple sources. The imagery has been investigated at 1:5,000 to 1:10,000 scales (with smallest features that can be distinguished ranging from about 5 to 15 m). Image resolution ranged from 2 m to more than 22 m. The team has obtained access to satellite imagery covering c.55,000 km2 with approximately 25% cloud-free. Analysis of imagery from the high Himalayas by the Charter Project Manager’s institution (NRSC-ISRO) has also been evaluated by the UK team.

We summarise some preliminary conclusions:

Siwalik and Lesser Himalayan regions
  • We have high confidence that there are few major or disruptive landslides. Most landslides are small; many are re-activations of pre-existing failures whose locations will be known to local populations.
  • Analysis of villages/infrastructure known to have been damaged by landslides shows that the latter are almost all small (> 10-50 m wide) and extremely difficult to identify using the available imagery.
 High Himalayan region
  • We have identified several large valley-blocking landslides (landslide dams) on the basis of satellite imagery (see Appendix 1). We stress that further valley-blocking landslides may still be found as additional imagery becomes available; other valleys in the High Himalayas should be assumed to be at risk until the existence of landslide dams can be confirmed or ruled out.
  • A zone of relatively intense landsliding and disruption can be mapped in every valley for which cloud-free imagery is available, north and northeast of Kathmandu (red lines in the above map). This zone contains numerous rockfalls and debris avalanches, with many events following pre-existing pathways or channels and having long runout distances (in most cases, all the way to the valley floor). These events are individually localised but together are extremely widespread, and have had catastrophic impacts on roads and villages. It is these small, but widespread, landslides that are likely to have probably caused the largest loss of life due to landsliding.
  • Cloud-free images are not yet available for all valleys within this zone; until they are, we must expect that similar levels of disruption are present in ALL valleys within the zone, not just those indicated in the accompanying map.
  • Where visible, high-elevation areas (> 3,500 m) show evidence of some large rockslides, along with extensive avalanching (the latter especially above 4,500 m). Most of these areas are far from permanent settlements or infrastructure, and pose no direct threat to the population.
  • Large areas of the High Himalaya, including valley floors and also most areas above 2,500 m, have not yet been examined because of cloud cover. We are continuing to monitor all possible sources of data.
Entire earthquake affected region
  • The zone of intense landsliding corresponds to areas with high rates of seasonal rainfall-triggered landslides. We must thus expect that the risk due to continued landsliding in this area will escalate significantly as the rains begin. This continued landslide occurrence will threaten already-affected areas with continued failure, and will wash landslide sediment downstream onto valley floors and floodplains.
Landslides and major roads

The Arniko Highway (north to Tibet) is reported to be blocked from near Larcha to the Tibetan border by numerous rockfalls. This area has had chronic landslide problems prior to the earthquake, and the road is regularly impassable during the monsoon.

The Prithvi Highway (the main route from Kathmandu south to India) has also experienced ongoing landslide problems since the earthquake, including large rockfalls onto the road. This road has suffered extensive landsliding in the past. In August 2000 a single landslide blocked the road for 11 days, leading to severe shortage of fuel and other commodities in the metropolitan area.

 

Recommendations
  1. Before the onset of the monsoon, the potential valley-blocking landslides and valleys at risk discussed in the appendix should be inspected and monitored to assess the risk of landslide dam formation and further risk of failure during the monsoon. At present, there does not appear to be a high risk of imminent failure in any of the dams that may severely impact valleys downstream, but this may change as the monsoon develops.
  2. Road access to, and through, the zone of intense landsliding is going to be very challenging at least through the 2015 monsoon season. It would be sensible to undertake a survey of the major transportation corridors to identify where the problems are most likely to arise, and to prepare for the possibility of BOTH immediate action to clear rockfall debris AND continued clearance and maintenance of major roads after future landsliding.
  3. Collation of observations of the distribution, character and impact of landslides from people in the field is required to refine landslide hazard and risk assessments and validate the findings of the satellite-based survey we have undertaken. We have already instigated a social-media call to initiate this, but additional measures may be required, and a method of collation agreed.
  4. Users of post-event map products should be aware that several factors, unrelated to the actual distribution of landslides, may have influenced their apparent distribution: i) large areas of Nepal (such as most of those east of the map above) remain unsurveyed – largely because of cloud cover and image availability; ii) some other published maps are based on pre-earthquake landslide susceptibility evaluations rather than post-event observations; and iii) different methods have been utilised by respective agencies to map landslides; for instance, some do not appear to have taken full account of the occurrence of pre-existing landslides in their assessment of landsliding associated with the earthquake.
Appendix 1: List of confirmed or potential valley-blocking landslides
  1. Therapati, in the south of the Langtang National Park, in the Melamchi catchment. A landslide deposit has been identified on the edge of a high-resolution optical DG image (03/05/2015), and shows surface features characteristic of a rock avalanche (> 1M m3), which appears to toe-out across the valley base and likely causes a blockage. There is no evidence of a lake forming, but the valley at this time of year does not appear to have an active channel. The valley drains into the Trisuli River. If this interpretation is correct, this blockage may be a concern during melt / rainfall.
  2. Satellite images suggest multiple large valley blocking landslides in the main stem of the Trisuli, between Resuo (Nepal) and north to Jilong Community (Tibet). Some of these have already been identified by ICIMOD. This area presents concern.
  3. Video footage from the Upper Bhote Koshi suggests several reaches of rivers with abnormal standing water, notably immediately south of Chaku – most probably from extensive rockslides which appear to have removed extensive sections of the Arniko Highway, making this impassable. Flow downstream from these sites continues, so these blockages are unlikely to be high risk.
  4. Satellite images and news reports describe large post-seismic landslides in the 20 km north of Chaku to the Tibet – Nepal border at Kodari. Prior to the earthquake this road section had numerous deep-seated landslides in colluvium which tend to be highly active during the wet season. These sites pose significant risk of valley blocking should these landslides move. In addition, satellite imagery indicates extensive rockfalls and channelized failures on this stretch of road. Indications are that this area is still cut off. The Chinese army has cleared the road to Zhangmu (sometimes referred to as Zham) on the northern side of the border, but the communities of Larcha, Liping, Tatopani and Kodari appear still to remain isolated.
  5. Satellite images north of the Tibet – Nepal border at Nyalam show extensive areas of landsliding and avalanching. Rockslides have in places crossed the road and have reached the river, but none have generated lakes that are visible in the presently available imagery in the river network.
  6. Near Poki: a 1 km long rockslide (identified also by ICIMOD), appears to have blocked a valley, but has now been breached by the river. This area presents concern, but reports and imagery suggest that water is now flowing across this dam.
  7. Langtang: extensive landslides, rock avalanches and possible glacier snout collapses appear to have traversed the valley floor, and are likely to inhibit flow during melt. The situation in Langtang is well-documented elsewhere.

Note that the 2014 Sun Koshi landslide which cuts the Arniko Highway just upstream of the Bhote Kosi Hydropower plant, and the rock dam that it created, appear to be intact. The river and lake levels do not appear to have changed during the earthquake.

 Appendix 2: Maps

High and low-resolution versions of the above map in both colour and black & white can be downloaded as PDF files from: ftp:\\topdog.dur.ac.uk\njr\nepal_eq

This article was first posted to the EWF Blog yesterday

 

 

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5 May 2015

The Langtang ice and rock avalanche in the Gorkha Earthquake

The Langtang ice and rock avalanche

Over the weekend the picture is slowly becoming clearer in terms of landslides associated with the Gorkha Earthquake in Nepal.  It is now apparent that the level of landsliding in the Hill Districts at least is rather lower than we might have feared, although it is still far from insignificant.  Understanding the situation in the higher mountains remains very challenging due to the amount of cloud cover that is still preventing satellite images from being available.  We have to hope that a window opens up before the monsoon.

So far the stand our landslide event appears to be the ice and rock avalanche at Langtang.  This appears to have caused substantial loss of life.  Not for the faint-hearted, this BBC report from the scene leaves little to the imagination in terms of the likely losses.

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NASA have managed to collect some imagery of the site.  This is a post-event Landsat 8 image that clearly shows the avalanche:

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This is the Google Earth image of the village of Langtang before the ice and rock avalanche:

Langtang

Google Earth

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The Digital Globe satellite imagery of the site captures the avalanche track and deposit pretty well:

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This is a slightly odd event because of the lack of a clear large deposit.  My initial (and very provisonal) interpretation would be that a chunk of ice and rock broke off from the slope above the village – a comparison of the Google Earth image from January with the Digital Globe image suggests this strongly:

Before:-

Langtang

Google Earth

After:

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I suspect that this mass then went through freefall / rapid sliding into the valley below (removing rock from the surface en route – the planed off surface is clearly apparent in the image above). Google Earth suggests that the fall was about 700 or 800 m vertical distance – and that the mass then fragmented on impact to create a massive blast of air, ice and rock.  This is similar to the Seti River rock avalanche that I featured three years ago, although on a smaller scale.  This mechanism would account for the huge damage that occurred in Langtang.

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2 May 2015

Gorkha Earthquake – the slowly emerging landslide picture

Gorkha Earthquake

The official name for the earthquake in Nepal a week ago is now the Gorkha Earthquake.  The casualty count slowly continues to rise – 6659 fatalities at the latest count, but this is likely to rise significantly in the coming days.   We are slowly getting a better understanding of the earthquake and its impacts, although the picture in the rural districts remains worryingly unclear.  My sense is that we are now starting to get information about rural settlements on the major highways, but the situation in the many communities away from the main roads (which is the vast majority) remains unclear.

Concentrating on landslides, which is the focus of this blog, I thought I’d pull together some snippets of information that are now available.  This is far from comprehensive though.

1. The dreadful damage in the Langtang region

There have been various reports of the impact of landslides in the Langtang region, a popular trekking area, and I wrote about this earlier in the week.  Whilst the situation is desperately confused, it appears that there may have been two large rock/ice avalanches, one affecting Langtang itself and the other, a few days later, destroying Ghotabela.   Himali Sherpa has posted some images of the destruction in Langtang via Facebook:

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It is hard to believe that these will be many survivors from this.  It appears that this may have been a rock/ice avalanche, but information is still scarce.

2. ICIMOD analysis of valley blocking landslides

Meanwhile ICIMOD have been leading the analysis of satellite imagery of potential valley-blocking landslides.  They have identified a number of large slides, although fortunately to date these do not seem on first inspection to be too problematic.  Note though that very large areas remain cloud-covered so we are not out of the woods as yet.  The largest to date is in a remote area in the mountains:

Gorkha earthquake

ICIMOD analysis of seismically-induced landslides

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3. Landslides on the Arniko Highway

Meanwhile, Kantipur today has an interesting article about efforts to clear the Arniko (sometimes called Araniko) Highway, which is the road that links Kathmandu to Tibet – a vital trade route.  It includes this image of landslide damage:

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The reports states that:

It is suspected that scores of people, including some foreigners, visiting Tatopani and around 25 vehicles were buried in landslides triggered by Saturday’s massive earthquake. According to witnesses, scores of people were buried at Miteripul, Chaku, Jhyalebhir, Nayapur, Daklang Paharo and Jhirpu. “Ten to fifteen people working at caterings and restaurants in Khasa were buried by landslide at Chaku,” said Shyam Shrestha, who witnessed the tragedy. Some buses, private vehicles and motorcycles were also buried in the area.

 

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30 April 2015

Upper Bhote Kosi: videos of the damage and rockfalls in northern Nepal

Upper Bhote Kosi

The Last Resort is a bungy jumping centre located in the Upper Bhote Kosi valley northeast of Kathmandu.  This is to the north of the area affected by the massive landslide last summer. This is an area that I know well as we have used the resort as a field base for many years. Although there is still a great deal of uncertainty as to the nature of the impact of the earthquake in this area, it is reasonable to assume that the effects have been very severe.

Bhuwan Sharma was at the resort at the time of the earthquake, and has posted a series of videos on his youtube channel.  I thought I’d highlight two of them here.  First, this one shows rockfall activity in the Bhote Kosi gorge during what I assume is an aftershock:

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Note the extensive rockfall activity both upstream and downstream of the bridge across the Bhote Kosi.  If you look carefully you can also see the substantial piles of debris in the river channel from the earlier shocks:

Bhote Kosi.

Whilst this video shows the damage in the surrounding villages in the Upper Bhote Kosi valley:

 

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Clearly there is extensive destruction to the houses, some of which have collapsed.  This suggests that the damage in the Upper Bhote Kosi valley is likely to be very serious.  This is important because the reported losses for this earthquake, although terrible, continue to be lower than we would have expected and seem to be at odds with reports from other parts of the mountain area.  Others are equally confused as to what is going on – indeed some models suggest that the losses should be an order of magnitude higher.  It is very perplexing.  Meanwhile, as usual the media seem to be fixated on individual rescues rather than on the big picture.  Each of these “miracle” survival stories are wonderful, but to report them so assiduously is to miss the big picture in my view.

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29 April 2015

Landslides in Langtang during and after the Nepal earthquake

Landslides in Langtang during and after the Nepal earthquake

The situation in the rural areas of Nepal after the earthquake is slowly, very slowly, becoming apparent. At the moment the clearest indications of serious problems are in Langtang – an area to north of Kathmandu that is popular with trekkers.  A search on twitter using the term *langtang is chilling, with long lists of missing people:-

Langtang

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It is clear that landslides and avalanches have been a very significant factor in this area, both during the earthquake itself and in the aftermath.  There is a very interesting account of this on the Facebook page of Robbie Barnett from Austin Lord, posted on the Modern Tibet Facebook page.  I won’t repeat the whole post, but will highlight a couple of key sections:-

“The village of Langtang was the site of the largest single catastrophe, as the entirety of village was completely buried by an avalanche that came from thousands of feet above on the southern slopes of Langtang Lirung and Langtang II. Smaller settlements on the outskirts of Langtang, such as Chyamki, Thangsyap, and Mundu were also buried. It is impossible to determine exactly how many people died there, but the estimate is perhaps over 300 people in total. The handful of survivors, roughly twelve locals and two foreigners, walked down to Ghodatabela below after spending the night of the 25th in a cave – thus there is no one at Langtang itself. This avalanche is perhaps 2-3 kilometers wide, and is obstructing movement within the upper valley corridor. Currently two large groups are stranded above and below (due to several intensive and recurring landslides in the steep sections between Ghodatabela and Lama Hotel).”

“Below, at Ghodatabela (where I was located during the earthquake for roughly 55 hours following the event) several large landslides were triggered from all directions, the largest from perhaps 1,500 meters above just below the settlement, completely obstructing passage. The two guesthouses there were partially destroyed by large boulders, and the army checkpoint barracks collapsed during the earthquake. The night after the quake, there were two groups sleeping in separate fields by the river, keeping distance from ongoing landslides and rockfall that continued throughout the night.”

“Along the Trishuli River valley itself, there are other smaller landslides and reports of significant collapse at Mailung, Simle, and Archale. In Betrawati, Gerkhu, and Mhanegaun in bordering Nuwakot district several houses also collapsed, and there were casualties in all of these places.”

“The current data on total casualties following the earthquake here in Nepal is approaching 4,500. However, this is probably an underestimate considering that the estimate for the entire district of Rasuwa is 250, and there are at least 300 dead in the area around the village of Langtang alone. Across the board, it is still very difficult to determine exactly what has occurred in remote areas off the road system, as communication is down. Considered in terms of the percentage of total population, my sense is that Rasuwa has likely the third highest rate of fatality, behind Sindhupalchowk (at the epicenter of the major aftershock) and Dhading (closer to the center of the earthquake). Again, Langtang is probably one of the greatest single tragedies of this earthquake.”

“This is the latest news that I have as of 5pm on April 28th.”

This is the Langtang valley in question:

Langtang

Google Earth

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I assume that the landslide / avalanche at Langtang is the one that appeared in this tweeted image from early yesterday:

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The Mountain Hydrology blog has a nice analysis of the potential source of this landslide.

And then yesterday came news of a post-seismic landslide or avalanche that struck the village of Ghodatabela, to which Austin Lord refers above.  Information about this landslide is sketchy at present, but the Himalayan Times described it as follows:

“Not far from the quake’s epicenter, 250 people were feared missing after a mudslide and avalanche on Tuesday, district official Gautam Rimal said.  Heavy snow had been falling near the village, Ghodatabela, and the ground may have been loosened by the quake. Rimal said officials received initial reports of the disaster by phone but then lost contact.  The village, about a 12-hour walk from the nearest town, is along a popular trekking route, but it was not clear if the missing included trekkers.”

However, beyond this the picture remains worryingly unclear. These most recent events highlight the acute threat that landslides and avalanches will pose in the post-earthquake period, especially in the monsoon.  There is an urgent need to start to systematically analyse the hazard.  We hope that the EWF landslide guidelines will be a helpful first step

Not far from the quake’s epicenter, 250 peoplewere feared missing after a mudslide and avalanche on Tuesday, district officialGautamRimal said.Heavy snow had been falling near the village, Ghodatabela, and the ground may have been loosened by the quake. Rimal said officials received initial reports of the disaster by phone but then lost contact.

The village, about a 12-hour walk from the nearest town, is along a popular trekking route, but it was not clear if the missing included trekkers.
– See more at: http://www.thehimalayantimes.com/fullNews.php?headline=New%20mudslide%20hits%20Gorkha%20village%20near%20epicentre%20&NewsID=453075#sthash.ZHxlGRJQ.rYw0H6Jr.dpuf

Ghodatabela
Ghodatabela
Ghodatabela

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Landslides in the Nepal earthquake: preliminary guidelines for relief and response activities

Guidelines

Pakistan 2005

Landslides in the Nepal earthquake: preliminary guidelines for relief and response activities

Alex Densmore, David Milledge, David Petley, Nick Rosser, and Katie Oven

Originally posted at the EWF Blog

The extent and effects of landsliding triggered by the 25 April 2015 Nepal earthquake remain unclear. However, since relief and response activities have already begun, and will accelerate in the coming days, we have assembled preliminary guidelines for relief and response workers and emergency planners in Nepal. They are informed by initial models of landslide likelihood, some fragmentary mapping of available satellite imagery, and experience in the 2005 Kashmir (Pakistan) and 2008 Wenchuan (China) earthquakes.

General pattern

Landslide susceptibility maps for the entire region have been posted at http://ewf.nerc.ac.uk/2015/04/25/nepal-earthquake-likely-areas-of-landsliding/ and http://blogs.agu.org/landslideblog/2015/04/26/nepal-earthquake-2/, along with some initial interpretations. These models are highly generalized and should not be used as predictions of the occurrence of individual landslides; instead, they give an idea of the areas where landslides are most likely. A brief summary interpretation of the key results:

  • We expect many landslides in the high Himalaya of northern Nepal, where valley floor elevations exceed c. 800 m. These areas are particularly important because they have very long, steep slopes and narrow valleys, so that large landslides are likely to block valleys (see section on ‘Landslide dams’ below). The valleys of the Kali Gandaki, Marsyandi, Trisuli, and Sun Kosi rivers, and their major tributaries, are at greatest risk, but large valley-blocking landslides are also likely elsewhere. These areas also have little space for roads, so are likely to be cut off with overland access taking days (to weeks). Experience in Kashmir and Wenchuan suggests that some settlements will be completely disrupted or destroyed, with local death tolls in the hundreds to thousands.
  • There is also a high chance of landsliding in the foothills – e.g., south of Kathmandu and the epicentral area, including the Siwalik Hills. These areas have shorter slopes (up to 1000 m relief) and broader valleys, so there is a lower chance of valley blocking. Preliminary reports are that roads across the Lesser Himalaya to the south of Kathmandu are open, but landslides and debris are hampering movements.
  • Based on previous comparable EQs, it is highly likely that many relatively small landslides (1000s of m3 in volume), up to one if not more large valley-blocking landslide (> 1 million m3), will have occurred. Initial efforts should focus upon identifying the largest landslides, as they pose significant secondary risks if they dam rivers and subsequently fail (see ‘Landslide dams’ below).These models could be revised or run on more targeted areas if there is interest from stakeholders in specific areas, but they should NOT be used as predictions of landslide occurrence at particular sites. Our preliminary image mapping shows fewer landslides than the models anticipate, but imagery to date is patchy, with much cloud cover showing only the foothills. There is not yet any systematic information from the high Himalaya but mapping is ongoing. Experience in Wenchuan (China) shows that the earthquake will have weakened the hillslopes, so further landslides should be expected in the coming days to months – especially, but not only, during rainfall and aftershocks of M > 5. The locations affected by these post-earthquake landslides will be similar to those already affected most severely by the landslides. Wet landslides can be very rapid, and unpredictable in direction and runout.

 

Landslide dams

Most landslide dams fail within a few days of filling; thus, it is critical to establish where they are located and how rapidly they are filling. Periodic measurements of water level relative to dam crest are invaluable. In the absence of information to the contrary, it should be assumed that landslide dams have formed in all of the major river valleys or tributaries draining the area between the epicentre and Kathmandu. Key observations that a dam has formed upstream include: decreased river flow independent of any changes in rainfall, or changes in colour – especially a muddier-than-usual appearance. Flow gauges in Nepal, if still serviceable, should be monitored for rapid changes in flow on all rivers : http://hydrology.gov.np/

The effects of a landslide dam failure depend on the height of the dam and the volume of the lake behind it. As a general rule, valley floors and river channels should be avoided until it has been confirmed that no possible cause for flooding (via failed landslide or hydroelectric dams) exist upstream. Any temporary infrastructure in the valley floor should be sited as high as possible and as far as possible from the channel; a minimum elevation of AT LEAST 10 m above the active channel is recommended. Risk should also be minimised by positioning a spotter at the landslide dam with a comms link to all VDCs downstream and a clear protocol for communities to move to higher ground if there is a dam failure.

Operational guidance

Temporary infrastructure (temporary housing, hospitals, roads, etc.) should be sited to avoid further landslide hazards, flooding from potential landslide dams, inevitable increased river flow in the monsoon, and (over a period of weeks to months) river bed deposition from landslide sediment. Some general guidelines include:

  • Site infrastructure in the widest possible part of the valley
  • Avoid piles of large boulders or other signs of previous landsliding
  • Stay at least 10 m above the active river channel, and as high as is practicable
  • Stay away from steep hillslopes. Try to minimise the angle between a site and the local skyline. This angle should be <20 degrees if possible, although this may not be practical in many valleys
  • If (re)cutting roads for access, minimise gradients, the number of switchbacks, and traverses across loose ground. Cut slopes that are just stable in dry conditions will be unstable in the monsoon.
 Exposure

If further landslides occur, it is important that exposed field workers are briefed on what to do:

  • In broad terms, follow the guidance for earthquakes. If you sense something happening or moving, ‘duck, cover, hold’, and importantly don’t run.
  • Be aware of your surroundings. Unusual noises, tremors, or dust are all possible indicators of an earthquake OR a landslide. Landslides are surprisingly loud, especially in mountain valleys.
  • A key component of risk is exposure, so minimising the time spent in areas that are apparently dangerous and susceptible to landsliding is essential.
  • Landslides on steep hillslopes are often the source of more landslides over time. Don’t assume that because something has already failed, it will not continue to do so in the coming hours and days.
  • Local communities will have a sound knowledge of where landslides have occurred prior to the earthquake. This information is invaluable and should be used wherever possible, as these slopes may be most susceptible. At the same time, be aware that the earthquake may have changed the landscape dramatically, so that other areas may also be at risk.
  • Very generally, earthquakes trigger landslides on ridges, sending debris downslope. Many roads in the Himalaya follow the valley floors. It is therefore important to be aware of the terrain above, and the condition that it is in, even if you cannot see it. While shaking may be more violent on ridges, the chance of being struck by falling objects in valleys is much greater. When in doubt, move up, as ridges will be the safest parts of the landscape.
  • Landslides can occur with no apparent trigger. Even if there is no rainfall or no further earthquake, a slope can still collapse. Keep away from steep slopes wherever possible.
  • Mountain streams (large and small) will be choked with loose sediment and probably subject to debris flows, particularly in the monsoon. As a result, flows will be less predictable and more prone to flooding, even after moderate rainfall. Streams are also likely to change course unpredictably. Stay well away from river beds.
 Next steps

We are currently compiling optical and radar satellite imagery to map large landslides and potential landslide dams across the affected area. We will update these guidelines as those maps become available.

Reference:

Alex Densmore, David Milledge, David Petley, Nick Rosser, and Katie Oven. 2015. Landslides in the Nepal earthquake: preliminary guidelines for relief and response activities.   EWF Blog: http://ewf.nerc.ac.uk/2015/04/28/landslides-in-the-nepal-earthquake-preliminary-guidelines-for-relief-and-response-activities/

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27 April 2015

Nepal earthquake impact – the key questions

Nepal Earthquake impact

Confusion continues to be a factor in determining the Nepal Earthquake impact – not unusual in any way, and not a criticism of anyone involved.  The official death toll continues to increase – at the time of writing it is is 3351 (I strong recommend this twitter account for the latest information), and the total is rising quickly.  But this remains much lower than we had anticipated for an event of this size in Nepal.  This is partly because buildings in Kathmandu have performed far better than we had feared – at the moment the reasons for this are not clear, but it is very welcome.

But the key question is what is happening to the north of Kathmandu in the rural areas.  This is the zone in which the peak ground accelerations are likely to have been highest, and it is also an area in which the houses are vulnerable (typically being constructed of weak materials) and most communities live on, or close to, slopes.    These communities, which are remote at the best of times, may well have been very badly affected by the earthquake.  An example appears to be the town of Barpak, high in the mountains in Gorkha District.  This is the Google Earth image of the town before the earthquake:

Nepal earthquake impact

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Various images have appeared of the aftermath of the earthquake in this town, all unfortunately unverified.  But for example, this image has been widely retweeted:

Nepal earthquake impact

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As has this aerial view:

Nepal earthquake impact.

Both of these images suggest that losses in Barpak should be high.  The key questions are:

1. To what degree are these losses included in the official totals (my guess would be they are not, but I might be wrong)?

2. To what degree is the damage in Barpak replicated elsewhere?

Because if the answer to 1. is that the official death toll does not account for the losses in Barpak, and to 2. is that this is replicated elsewhere then the losses are going to be much higher than at present.  We can only hope that this is not the case.

Either way the pictures above suggest that a massive mobilisation of aid is going to be needed.

 

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26 April 2015

Landslides from the Nepal earthquake yesterday

Landslides from the Nepal Earthquake

Whilst the Nepal Earthquake is now the centre of global attention, the true pattern of loss is not yet apparent.  As usual, the focus in the first 24 hours is on the locations that have both media presence and easy telecommunications – in this case Kathmandu.  The impacts there are undoubtedly serious, especially for historic masonry buildings, and the city will never be the same again.  But the true impacts of this event are going to have been felt in the rural areas to the north of the city.  At present there is little information from those districts, but expect the picture to be tragic as it emerges over the next two days.

Landslides are going to be a very real problem in those areas affected by the Nepal earthquake, and by its aftershocks.  Indeed, just 15 minutes ago there was a very shallow M=6.8 aftershock that is likely to have caused substantial damage in its own right, albeit in a more limited area.  There has been some confusion about the area affected by the earthquake, and in particular the area of most intense shaking.  This is because there is a perception that the epicentre is the centre of the earthquake, and thus that the damage should radiate away from this point.  This is not the case.  The earthquake energy is derived from movement on a fault (or sometimes more than one fault) – a complex three-dimensional structure that extends over a large distance.  The epicentre is just where the rupture initiates – i.e. the location at which the fault starts to move.  The rupture then propagates along the fault, releasing energy as it goes.  Often the rupture propagates quite evenly away from the epicentre, in which case the epicentre is close to the centre of the earthquake-affected area.  But at other times the rupture propagates preferentially in one direction or another.

In the case of the Nepal Earthquake the rupture appears to have propagated mostly towards the east of the epicentre, not to the west.  So the epicentre itself is at the west end of the earthquake affected zone.  This is clear from the USGS shakemap, which shows the epicentre as a star:

Nepal earthquake .

This was good for Pokhara, but bad for Kathmandu.  It also explains why the impacts at Everest were so serious.

Possible landslides from the Nepal earthquake

The most seriously affected area from the earthquake is mountainous and remote, but there are substantial numbers of people living in the valleys and on the hillsides.  This is a typical landscape:-

Nepal earthquake.

Further to the north the mountains become higher and the slopes longer.  Note the terracing in the fields, the steep slopes and the large numbers of houses.  There are many substantial villages, often located high on the slopes.  This is highly landslide-prone territory, and the impact of the earthquake in these regions is going to be dreadful.   Some effort is already being made to analyse the likely landslide effects, although at this stage such efforts are tentative.  Over on the EWF blog, Alex Densmore has posted an initially analysis by Tom Robinson from the University of Canterbury of the likely areas affected by landslides:

Nepal earthquake

Landslide susceptibility in the Nepal Earthquake by Tom Robinson, University of Canterbury

 

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There are a few things to note here.  First, the analysis is dependent on the data quality, and you can see that there are some issues there (this is why there appears to be straight line boundaries in landslide hazard in places) – that is an important caveat.  But the analysis is really interesting and helpful.  It shows that to the north of Kathmandu, the very remote areas are likely to have been severely affected by landslides – but as yet there is no information from up there of course.  And interestingly the hills to the south of Kathmandu are also badly impacted by landslides triggered by the Nepal earthquake – indeed the landslide picture might be as bad there as to the north.  This is significant for two reasons – first, there are lots of people living in this area; and second, the roads that link Kathmandu to the outside world have to cross these hills.

Tom has run a quick analysis of the main roads affected by these landslides – the impacts look to be very serious:

Nepal earthquake

Likely major road impacts of landslides, by Tom Robinson, University of Canterbury

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Kathmandu is entirely dependent on goods being brought into the city on these roads.  If they are badly affected by landslides then the impacts will be severe.

There is a real urgency about the need to assess the actual landslide impacts, especially to the north – we need satellite imagery quickly.  There must be a strong possibility of large valley-blocking landslides triggered by the Nepal Earthquake in the high mountains to the north- if so these will need to addressed without delay.  The monsoon is going to be a real problem this year.

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