19 September 2014
Moving to the University of East Anglia
Regular readers will have noted my lack of blog posts in recent weeks, for which I apologise. As you may be aware I have been changing jobs – I gave up the Wilson Chair in Hazard and Risk at Durham University last Friday, and on Monday took up the position of Pro-Vice-Chancellor (Research and Enterprise) at the University of East Anglia. The last week has been a whirl of introductory meetings, all of which have been great fun. This hasn’t left much time for the blog, but as I now feel established I will be returning to previous levels of activity.
So to get things underway again, two new landslide videos have appeared on Youtube in the last week:
A lucky escape in Costa Rica
In the last few days very heavy rainfall in Costa Rica has triggered extensive landsliding. This video shows a very lucky escape for a motorcyclist:
It is of course only a small landslide, but illustrates nicely the dangers posed by even apparently superficial soil slides.
Ongoing landslides in northern India
Meanwhile northern India and Pakistan have been affected by heavy rainfall that has caused extensive damage (though has received very little coverage). This landslide in Ramban District appears to be causing considerable disruption to the road:
8 September 2014
The Sunkoshi landslide dam breach event
On Saturday night, apparently without warning, the landslide dam on the Sunkoshi River in Nepal abruptly breached, allowing the lake to mostly drain. This event appears to have occurred without warning, such that one of the Nepal Army excavators was washed away. The breach event appears to have been natural – a response to heavy rainfall – rather than a result of the attempts by the Nepal Army to widen the channel. Whilst some damage has been reported downstream, the flood does not appear to have been exceptionally large, so no casualties have been reported. Republica has a very nice set of photographs of the breached lake and dam:
Whilst Kapil Dhital (@bewitchkapil) tweeted this image of the lake this morning:
So what caused the breach? This is the gauging data for the Bharabise station (upstream of the dam) and the Pachuwaghat station (downstream). Note that the two sets of data use different y-axes. The flood peak generated by the breach event is very clear at Pachuwarghat, although this is not a very high peak. The Bharabise station probably responded to rainfall – there is a dramatic increase in discharge a few hours before the flood wave reached Pachuwarghat. Thus, it appears that the breach was initiated by this sudden increase in river flow in response to heavy rainfall:
This is clearer when the data for the few hours before the breach event are plotted:
In many ways this is the best possible outcome. The release of most of the water means that the hazard has in effect been removed, but without causing a huge amount of damage downstream. Unfortunately the huge amount of resource and effort that has gone into building the two new roads has been wasted. Once again I would reiterate my opinion that a focus on deepening the channel, rather than widening it and building new roads, would have been a much more effective way to manage this hazard. That the dam breached in a rainfall-induced event suggests that a targeted effort to deepen the channel could have resolved the crisis much earlier.
24 August 2014
Sunkoshi landslide – draining the lake
The Nepal Army is continuing in its attempts to drain the Sunkoshi landslide-dammed lake. Whilst the threat of a breach event has now subsided (though it has not gone completely of course), the major emphasis is on re-establishing the trade link to Tibet. There can be little doubt that considerable effort has been expended, but the results are shown in these two photographs of the lake, tweeted by Kapil Dhital. This is the lake on 6th August:
And this is the image he tweeted yesterday – 17 days later:
One really does not need to be a trained photographic interpretation expert to see that the lake level has not fallen, despite the best efforts of the Nepal Army. I have done a quick search on the Republica and other Nepal websites for news stories about the lake level behind the Sunkoshi landslide dam:
On 8th August, Republica reported that “The water level in the Sunkoshi River in Sindhupalchowk that increased last Saturday after a massive landslide blocked the river, has started to increase even after the Nepal Army blasted outlets…The water flow in the river was recorded at 180 cusec on the day of the landslide, but now it has reached 300 cusec, Singh said quoting the army technicians.”
On 9th August: “The inflow of water in the artificial lake was 205 cusec while the outflow was 210 cusec, and the water level has decreased by seven centimeters by Saturday, according to a statement issued by NA´s Directorate of Public Relations. The army plans to release accumulated water in the artificial lake, and resume the obstructed Kodari highway at the earliest, the statement added.”
On 10th August: “The water level in the artificial lake formed after the recent landslide occurred at Jure, Sindhupalchowk blocking the Sunkoshi River has increased. The Nepal Army (NA) team carried out three blasts targeting the blocked sites today to ease water flow of artificial lake, said Deputy Superintendent of Police Bharat Bahadur Bohora.”
On 11th August: “If we managed to reduce water level by 10 meters, we can bring the submerged road into operation within another three days,” Sitaula [secretary of the Ministry of Physical Infrastructure and Transport] said. “As the dam was filled in 11 hours, we can drain it out by doubling water flow downstream.”
On 13th August: “Nepal Army on Wednesday carried out eight blasts to resume the regular course of the Sunkoshi River that was blocked following a massive landslide at Jure of Ramche VDC-5 in Sindhupalchowk district on August 2…As per the measurement carried out in the afternoon the internal water current in the artificial lake is 255 cubic sec while the external current stands at 397 cubic sec.”
On 17th August: “The water level in artificial lake formed after massive landslide at Jure blocked Sunkoshi River, is gradually decreasing with increase in water outflow from the reservoir after Nepal Army (NA) constructed another channel to drain out dammed water…According to NA Brigadier General Ashwin Kumar Thapa, the water level has decreased by 75 centimeters after excavators were used to increase water outflow. “The water level in the dam is decreasing at the rate of two centimeters per hour,” he said. He further informed that water inflow in the artificial lake is 180-200 cumec, while outflow of water from the reservoir is 350 cumec “The current water inflow rate is normal rate for Sunkoshi River during Monsoon,” he said. During three-days of torrential downpour both water inflow and outflow were high with outflow and inflow rate standing at 330 cusec and 530 cusec respectively.”
On 19th August a different message emerges: “Although 900,000 kg/meter water is exerting pressure on the landslide dam [note - this statistic cannot be correct for a lake less than 50 m deep], the huge rock material is strong enough to withstand the pressure, the NA said organizing a press conference at its headquarters in Kathmandu. “We cannot open large outlets for the pooled water as that may cause more flooding downstream,” Geologist from the Tribuvan University Ranjan Kumar Dahal said. Also, it is very costly to completely remove the landslide debris, he said. He said it might cost Rs 450 million to remove the whole debris, which is almost impossible. The water level above the landslide dam has so far decreased just by a meter. NA also clarified that it can´t carry any strong blast to make the water exit from the dam.”
On 20th August, eKantipur reported that: “The Nepal Army team, entrusted to drain the landslide-dammed lake in the Sunkoshi river, plans to bring down the water level by 15 metres in the next 30 days, said army officials…“In the last few days, we’ve been able to reduce the level of water by a metre,” said Niranjan Kumar Shrestha, assistant spokesperson for the Army. The Army plans to widen the smaller of the channels currently draining the lake to bring down the water volume.”
On 23rd August the Himalayan Times reported that: “In a bid to ease outflow of water…Nepal Army carried out two controlled blasts on Saturday. NA Directorate of Public Relations said the average inflow in the artificial lake as of this afternoon is 187 cusec whilst the average outflow is 280 cusec”
On 24th August the Himalayan Times reported that: “Issuing a statement on Monday, NA’s Directorate of Public Relations said the Army’s efforts to remove the obstruction…are also ongoing. According the the statement, while the inflow of water in the river was measured at 193 cusec this afternoon, the outflow was recorded at 303 cusec”
In recent days the media has repeatedly reported that the outflow is greater (often much greater) that the inflow. If so the water level should be declining quickly, which is clearly not the case. Thus, the figures must be wrong or misleading, so I am bemused as to why they are repeatedly reported in this way.
The sad reality is that all of the immense effort that has gone into building the two channels has had no impact on the net lake level. The reasons are probably demonstrated in this image from the Nepal Army:
Widening the channel is of course no bad thing, but to achieve a drawdown of the lake level behind the Sunkoshi landslide dam the channel bed needs to be deepened. The lake level will not change in aggregate without deepening the channel.
Finally, Kapil Dhital tweeted this image of the private road being constructed by local entrepreneurs:
Whilst the frustration that has driven the local people to do this is understandable, the environmental consequences of this road are clear. And, of course, this road will be very prone to landslides, especially as the lake will have elevated the water level in the slopes. I would not want to be on this road is heavy rainfall.
23 August 2014
Dariali Gorge landslide number 2
In May this year a very large landslide occurred at Devdoraki in Georgia, blocking the strategically-important Dariali Gorge. This transpired to be a large rockslope failure from directly below the glacier, which turned into a high speed, long runout flow. I was lucky enough to be able to feature some images of the landslide.
On 20th August, another landslide occurred at the same site, once again blocking the Dariali Gorge. This landslide, which is reported to have originated at the glacier, is reported to have been larger. The trigger was reportedly heavy rainfall. The landslide, and the resulting damage, are sufficiently serious to have needed a visit to the site by the President of Georgia. Two people lost their lives in the landslide, both workers from the nearby HEP project.
There are some images on the internet of the landslide. The best that I have found are on this Georgian news website. At times it is not entirely clear what they are showing, but they are interesting nonetheless. I suspect that these two images show the landslide deposit itself – it appears that the landslide blocked the river, which has then breached the dam to create an outburst flood(?):
An interesting aspect of this is the landslide deposit marks on the valley walls, which suggest that a very substantial volume of material has been eroded away. It is not clear to me whether this is from the May or the August landslide events.
This image seems to show downstream damage from the outburst flood:
In many ways this image is the most interesting. It is possible that it shows the channel through which the landslide travelled to reach the Dariali Valley, although this is speculation at best:
It would be fascinating to see a set of images of the source and track of this landslide – to have two very mobile, long runout landslides in the same valley in a short period is both unusual and intriguing. Does anyone know if such a set of images has been posted online?
21 August 2014
On Wednesday, a suburb of Hiroshima in Japan was struck by a series of catastrophic rainfall-induced landslides, which resulted from a period of prolonged and exceptionally heavy rainfall. These landslides are reported to have killed 39 people, with as many as a further 43 people reported to be missing. Sadly, some of the deaths appear to be rescue staff who were overcome by subsequent slides.
From a technical perspective, the best images I have found of these landslides are on the IBT website, which includes this overview of the site:
These landslides appear to be channelised debris/mud flows. It appears that they have started as small failures along watercourses that have entrained large volumes of material to become highly destructive flows. It appears that they have mostly consisted of very fine-grained materials, which is consistent with the bedrock geology, which is reportedly weathered granite.
Unfortunately, there is further heavy rainfall forecast for this area, and at the time of writing Robert Speta tweeted the following:
Line of storms approaching Hiroshima. Rescue efforts have been suspended. pic.twitter.com/6nHPe9UADN
— Robert Speta (@robertspeta) August 21, 2014
The image shows a line of storms approaching the area:
Whilst it seems unlikely that this rain will cause as much damage in Hiroshima itself, landslide and flood warnings are in force across the region and there have been many evacuations.
18 August 2014
ICIMOD has released a report about the Sunkoshi landslide, which has been reported in The Himalayan Times. The most interesting aspect may be some statistics on the landslide, which appeared in a tweet by Kanak Mani Dixit, which said:
ICIMOD collation re Sunkosi landslide: dam length 409m, width 106m, height 55m, volume/material: 6mill m3, lake length 3km, volume 8mill m3.
— Kanak Mani Dixit (@KanakManiDixit) August 18, 2014
These statistics are in line with those generated elsewhere, but are the first to provide a complete set. The main thrust of the ICIMOD report (which I’ve not seen) seems to be that more investigation of the Sunkoshi landslide itself, and more detailed landslide hazard mapping in general, are required. It is hard to disagree with these suggestions.
Meanwhile, the Nepal Army continue to try to drain the lake, but the degree to which they are being successful is very unclear. There is a profound deficit of reliable information about the state of play at the dam site – over the last two weeks there has been a number of reports that the lake level was falling, all of which have proven to be incorrect. The latest reports, on Sunday, were that:
The water level in artificial lake formed after massive landslide at Jure blocked Sunkoshi River, is gradually decreasing with increase in water outflow from the reservoir after Nepal Army (NA) constructed another channel to drain out dammed water. Decline in water level could be clearly witnessed on Sunday as clouds separated revealing blue sky after three days of torrential downpour came to an end.
According to NA Brigadier General Ashwin Kumar Thapa, the water level has decreased by 75 centimeters after excavators were used to increase water outflow. “The water level in the dam is decreasing at the rate of two centimeters per hour,” he said. He further informed that water inflow in the artificial lake is 180-200 cusec, while outflow of water from the reservoir is 350 cusec. “The current water inflow rate is normal rate for Sunkoshi River during Monsoon,” he said.
During three-days of torrential downpour both water inflow and outflow were high with outflow and inflow rate standing at 330 causes and 530 causes respectively.
But what does this mean? The figures above suggest that during the heavy rainfall the inflow exceeded the outflow, which means that the lake level must have risen. Then, when the rain stopped the inflow reduced to a level below outflow, so the lake level must have fallen. This is quite normal dam hydraulics. A key question is whether the net (i.e. low flow) level of the lake has fallen. The data being provided are not adequate to judge this.
The key to reducing the level of the lake has to be to lower the bed of the channel. Widening the channel, or creating a new one, will make the discharge of storm flow more efficient – i.e. the lake level won’t rise so much in the next heavy rainfall event. But to meaningfully draw down the lake the channel bed has to be lowered. Now this is of course risky – in particular if the bed of the stream is being armoured by large boulders, then these have to be blasted. But, this could be the case that beneath the boulders lie fine-grained deposits that might erode quickly once exposed. So this sort of operation is very sensitive and challenging, and requires high levels of expertise. And this is why I still contend that the Nepal Army should be seeking assistance from groups that have managed these problems before, most notably the Chinese. They might note the speed and efficiency of the operation to drain the valley blocking landslide in Yunnan Province, despite the very challenging access problems.
If the Nepal Army wants to drain the lake then the engineers have to deepen the channel. The challenge is to do this is a safe way. Why not get assistance from the Chinese team whose raison d’etre is to manage valley blocking landslides? After all, China wants this important trade link open too.
Finally, an additional dimension is the rapidly escalating pressure to reopen the road to Tibet. The Nepal Engineers Association is pressing for concerted efforts to drain the lake and construct an alternative road to bypass the Sunkoshi landslide. In a separate development, a group of local entrepreneurs have started to construct a new alignment with a bulldozer:
Whilst I can understand the frustration that is driving this, the track shown above is both environmentally catastrophic and a recipe for further landslides. It seems unlikely that it is a sustainable solution.
15 August 2014
The Sunkoshi landslide in Nepal – an update
Landslides continue to cause major disruption in Nepal as the summer monsoon intensifies. At Sunkoshi, the major river-blocking landslide continues to confound the attempts by the army to draw down the lake, despite the presence of heavy machinery and now over 50 controlled blasts by the Nepal Army. Indeed, in the heavy rainfall of the last few days reports suggest that the lake level has risen by 50 cm.
I think that there is little doubt that the Nepal Army has been working very hard on this problem, and deploying resources extensively. However, the communications side of the work has gone less well, with repeated statements that the lake level is falling and/or that the problem would be solved within a day or two. This has been proven to be erroneous and overly-optimistic. Unfortunately, the press have taken such statements at face-value, which in turn has generated a huge backlash when the problem could not be resolved. eKantipur today has an article entitled “Shame On You!”, which is strongly critical of the government’s efforts in dealing with all aspects of the landslide. As Nepal moves towards the Desain holiday, which is important for shopping, and imports from the north remain impossible, these pressures are going to rapidly grow.
The bottom line is that there is now a need to both implement a proper communications plan, which should emphasise the need to view this as a long term project (whilst recognising that a rapid breach event can still occur), and to call for expert assistance, in particular from China, where there is considerable expertise in managing these issues. Based on experience elsewhere I suspect that there will be a need to either block the existing channel to blast the base (not an option during the monsoon, and probably not in the dry season either because the dam has so little freeboard across the entire width) or to build a new channel from the foot of the dam upwards. This is a major project that will take some time, assuming that the dam does not breach first. As usual the lack of information about the structure of the dam is a major issue, meaning that its behaviour is unpredictable.
Heavy rainfall across Nepal and N. India causes high levels of loss
Meanwhile, very heavy monsoon rainfall across Nepal and northern India over the last few days has generated high levels of losses from landslides and floods. For example, Republica is reporting 53 people killed and 75 missing in Nepal, although this figure may change considerably as news filters through. Chisapani in West Nepal reportedly received 545 mm of precipitation in 24 hours. At least 23 people have been killed in northern India too. TRMM highlights the areas likely to have been affected by the landslides, which extends along much of the Himalayan Arc:
12 August 2014
The Eaglepointe landslide
One of the many landslide events in the last few days was a slide in the Eaglepointe subdivision in Salt Lake City in Utah. The development of this landslide was brilliantly caught on a time-lapse video by KUTV reported Holly Menino and 2News photographer Mike Stephen:
Jeff Moore of the University of Utah has very helpfully put together the following incredibly useful briefing on the Eaglepointe landslide:
- On Tuesday morning Aug 5, around 6 am, a large landslide underwent rapid and sustained failure, displacing some tens of meters over a period of minutes.
- One house was destroyed and three others imminently threatened. A nearby gas pipeline was undamaged.
- Movement had initiated at least four months prior back in April (our wet season), by some accounts back in fall 2013.
- In the week before the slide, large tension cracks had been growing at the crest of the slope, and the toe area was bulging creating some distress to adjacent properties.
- The developer hired geotechnical consultants to install inclinometers at the crest, and homeowners were diligently observant of the slope.
- Residents had received a letter indicating that there was a slide and to be prepared, but most seem to have already known due to the obvious signs of ground distress.
- During the morning of the failure, rolling boulders from the toe woke one of the 12 residents in the destroyed home, who then woke the rest of his extended family and everyone evacuated from the house before it collapsed.
- Movement later slowed but the scarp and slide mass threaten more homes.
- A heavy monsoon rain storm the previous evening is the ascribed trigger of ultimate movement, although slow displacement had been ongoing for months.
- My preliminary volume estimate of the slide is ~300,000 m3, based on the aerial extent and an assumed average depth of 10 m.
- The slide mass consists of sands and gravels of the Lake Bonneville highstand, overlain atop Tertiary volcanic tuff – the latter is a notorious material for landslides in the northern Utah, the so-called Norwood Tuff. It weathers to swelling, water-sensitive clays. The condition or depth of the sliding surface is however, not known at this time.
- Another slide just 1 km away in similar stratigraphy (named Springhill) destroyed several homes over an extended period in 1998 and during later wet years. Yet another slide in the same geology is progressively destroying several residential lots in nearby City Creek.
- The development was a gravel quarry until the late 1990′s, and the area that slid was the highwall or maximum extent of quarrying. It had been graded at 2:1 (~25 degrees). The houses threatened were built in the mid-2000′s. New development is currently underway at the crest of the slope that failed, including new roads and several new homes.
- Geotech reports used by the developer and city to permit the previous and new construction deemed the slope globally stable. These can be downloaded and freely reviewed here: http://nslcity.org/index.aspx?NID=331
- Some smaller sluffs and slides had been observed in previous years especially around the base of the slope where small cut-backs had been made.
- Location: https://email@example.com,-111.9022917,295m/data=!3m1!1e3 – to the north of the slope you can see the covered tennis courts and home immediately west that sustained the most damage, as well as some clearing for development at the crest of the slope (south).
Standard.net has a great gallery of images of the landslide. This image from that set shows the state of the eaglepointe landslide site now:
Given the history of the site I think that it is unlikely that the Eaglepointe landslide can be considered to be a natural event. However, it is not clear to me who, if anyone, might be to blame.
7 August 2014
The [Bhote Kosi landslide] Sunkoshi landslide
[The Bhote Kosi landslide now seems to be the accepted name for the landslide that I have been calling Sunkoshi, so I am adopting the local name. So let's start by looking at what we know about the Bhote Kosi landslide]. [EDIT: The comment below by Ranjan Dahal suggests that the correct term is Sunkoshi, so I am reverting to this terminology] Colin Stark, Goran Ekstrom and Clement Hibert have been looking at the seismic signals generated by the earthquake – it was recorded as an Msw=4.7 (approx.) event. The signal suggests that the landslide occurred at 20:51:44 UTC on 1st August (which is 02:36:44 on 2nd August local time). The landslide force inversion suggests that the landslide mass was about 13.5 million tonnes, which corresponds to a volume of about 5.5 million cubic metres, give or take a bit. This image, kindly provided by Kunda Dixit but taken by Tuk Lal Adhikari and used with permission, shows the [Bhote Kosi landslide] Sunkoshi landslide and the deposit:
Based on this it appears that the [Bhote Khosi landslide] Sunkoshi landslide started as a rockslope failure in the upper reaches – there may be a rotational component, although this is hard to tell, which then triggered a deep failure in the lower section (below the bedrock bench). It is interesting to note the freshly fractured rock in the upper scarp and the flow of water over the face in this region. Note that the landslide spread laterally by some distance to generate a broad, low angled landslide mass.
The lake level at the [Bhote Kosi landslide] Sunkoshi landslide
Clearly in Nepal there is a great deal of concern about the barrier lake that has formed, both because of the potential for a breach flood and because the lake has closed the Araniko Highway to Tibet. The image below, also by Tuk Lal Adhikari, shows the blockage and the channel:
A few things to note here:
- The channel has a reasonably low gradient, reflecting the broad, low gradient landslide deposit. This means that the chances of a sudden breach event are lower, though cannot be ruled out. A sudden large inflow into the lake would be dangerous;
- The channel does not seem to have incised significantly (reflecting the bedrock nature of the landslide), but has eroded laterally through the weaker surface materials;
- The constraint on the channel is the very narrow section at the dam crest, which has a very rocky surface form.
- If a road is to be constructed over the landslide then it will be either pushed against the foot of the cliff (very hazardous as rockfall activity will continue for a long time), or it will have to cross the river twice, or the river will have to be moved. None of these options are straightforward.
- There is no sign from this image that the lake level has decreased substantially.
So at present the landslide appears to be in a state of status quo. Ripendra Awal of Prairie View A&M University in Texas has taken a look at the gauging station data for Baharabise (4.2 km upstream of the landslide) and Pachuwarghat (38 km downstream) for the period since the landslide. This data is available online here. Ripendra has demonstrated that before the landslide the flows at the two stations were highly correlated – in other words if you know the flow at one you can, using a simple equation, determine the flow at the other (there is a three hour lag between the two stations that needs to be built into this). Ripendra has then generated a graph of the flows at the two stations during the period of the landslide crisis. He sent me the graph yesterday – I have updated it with today’s data and reproduce it below:
The blue line is in effect the inflow of water to the lake, the red is the discharge downstream (much higher because there are other rivers feeding into the Sunkhosi downstream of the dam). The black line is the expected discharge for Sunkoshi as determined from the dam inflow, based on the relationship before the landslide. So, three hours after the dam formed (shown on the graph as the first vertical line) the flow in the Sunkoshi dropped as water was impounded by the blockage. A few hours later the water overtopped, and the flow came back to the expected level.
The difference between the red and black lines thereafter is an indication of the behaviour of the lake trapped behind the [Bhote Kosi landslide] Sunkoshi landslide. If the red line is above the black one then the lake might be draining, if it is below then it might be filling (i.e. the water level is rising). For most of the period since the overtopping the two lines have been very close, suggesting little change in lake level. On 4th and 5th August the red line is a little above the black, so the lake might have been emptying by a small amount, which is consistent with media reports. Yesterday the two were in close lockstep, suggesting little change, which is also consistent with reports. Strangely, today the red is below the black, which might mean that the lake is filling. Don’t read too much into this though – it could also be that the data are not yet reliable or that the Sunkoshi has received a big flow of water from another source.
The key conclusion is that the lake level is not falling appreciably. I would think that this is because the gradient of the landslide body is so low that the channel is not eroding downwards. This was also the case at Attabad. This might suggest that to clear the Bhote Kosi landslide blockage large-scale blasting and/or excavation is going to be needed, but undertaken very carefully of course to avoid a rapid breach. And in turn this suggests that the army have a long and difficult task ahead of them. Meanwhile the road to Tibet remains closed and will be hard to reopen until the lake level is drawn down, and people upstream face difficulties travelling.
The [Bhote Kosi landslide] Sunkoshi landslide represents a very major challenge for Nepal. Unfortunately this is not going to be easy to fix or to manage.
6 August 2014
There is so much going on in terms of landslides at the moment that it is hard to know where to start! However, a key issue is the valley-blocking landslides and associated “quake lakes” triggered by the Yunnan earthquake of a few days ago. Yesterday, Xinhua posted a story about the impacts of landslides within the Yunnan earthquake zone that starts to shed some light on the issue:
Landslides have blocked roads leading to Longtoushan Township in Ludian County, epicenter of Sunday’s tremor, which killed at least 410 people and injured more than 2,300 others. A road leading to a large barrier lake on Niulan River was made passable after 38 hours of repairs by an armed police squad. Repair work is underway on three other roads in the quake zone.
As of 6 p.m. Tuesday, accumulated water volume of the barrier lake had reached 49.6 million cubic meters, and the water level is increasing at a speed of 16 cm per hour, said Sun Huaikun, a technical specialist in charge of the barrier lake response. Sun told a press conference late Tuesday that 4,200 residents living on the lower reaches of the Niulan River had been evacuated. A reservoir, located on the upper reaches, is required to intercept river water and reduce pressure on the barrier lake.
Barrier lakes, formed by landslides blocking rivers, are a recurrent threat to the lives of victims and rescuers alike. A 24-year-old border policeman is missing after he was hit by a falling rock as he attempted to swim a barrier lake in search of survivors. More than 60 soldiers were trapped by a sudden flood from a barrier lake Tuesday afternoon and were rescued by another team of armed police.
The most useful information though has come via tweet from Armand Vervaeck:
— Armand Vervaeck (@ArmandVervaeck) August 5, 2014
The link is to a web page of the National Administration of Surveying, Mapping and Geoinformation, which provides a series of satellite images of landslides triggered by the earthquake. One of the images is this one of a valley blocking landslide:
This appears to be a very large landslide dam that still has the potential to impound a great deal more water. The slide appears to have been quite mobile, showing signs of flowing down the valley (towards the left on the above image). If so, this should mean that the downslope gradient is less steep, which will reduce the chance of a catastrophic breach event. The landslide and lake have destroyed a large amount of infrastructure, including what appears to be a hydroelectric barrage and a bridge, judging by this before-and-after image pair from the same site:
The site of this landslide is at 27.036 N, 103.409 E if you want to take a look on Google Earth. This is a perspective view of the landslide site:
About 22 km downstream there appears to be a very large dam and long reservoir. This will catch the flood if a catastrophic breach occurs, so long as the water level can be drawn down in time. I would imagine that this is a very pressing imperative at the site at present. The timing of the earthquake, right in the middle of the monsoon, means that landslide occurrence will have been higher than would have been the case in the dry season and the rate of filling of the barrier lakes will also be high.