Hunza debris flow video

You probably guessed that I am on holiday this week (normal service will be resumed at the weekend), but I thought I'd quickly post this new debris flow video from Hunza in Pakistan. The interesting but starts at about 1 minute 15 seconds:

Debris flow disaster in Leh, Indian Kashmir

Thanks to several people for the heads up on this.  Various Indian news agencies are reporting that the town of Leh in Ladakh, Indian controlled Kashmir was hit by a series of debris flows / flas floods triggered by a cloudburst.  The reported death toll is currently reported to be at least 50, and may increase substantially.

This Panoramio image gives a fairly good idea of what Leh is like:

Impressive mud and debris flow in Oliver, British Columbia

Thanks to a number of people for bringing this one to my attention.  The community of Oliver in British Columbia yesterday suffered an impressive mud and debris flow that is reported to have destroyed five homes (images from the Vancouver Sun):

The flow has clearly come out of a deeply incised gully in the mountains:

A perspective view of the excellent Google Earth imagery of this area is rather helpful:

Two things to note here (the flow hit the area just above the "97" on the image above).  First the community that has been hit by this flow is located on a small fan that has been formed by this type of flow.  Second, the very narrow, deeply-incised channel mouth is the classic location for this type pf flow, where a blockage allows debris and water to accumulate, before collapsing to generate a flow.  It is interesting to note that this area has reportedly experienced unusually heavy rainfall this spring and early summer.

New landslide video from Brazil

Live Leak has an impressive video of a large debris flow striking and destroying a house, reportedly in Blumenau, Santa Catarina, Brazil :

Ten years ago today - the Vargas landslide disaster

Today is the tenth anniversary of probably the greatest landslide disaster in living memory - the 1999 Vargas landslide catastrophe in Venezuela. On 14th-16th December 1999 a coastal storm in Vargas triggered multiple debris flows that swept onto the densely populated river deltas below. It is estimated that 30,000 people died.

The USGS summary of the event, which remains the best starting place to understand the disaster, is here, which is the source of both photos.

Note the size of the boulder wedged into the side of this building.

Debris flow damage from Typhoon Morakot in Taiwan

In addition to visiting the Shiaolin landslide in Taiwan at the weekend, we also managed to visit a couple of other places that had suffered damage during typhoon Morakot. Damage from debris flows and river floods occurred extensively throughout the upland areas of southern Taiwan, as these images show:


The government has set aside about US$5 billion for reconstruction. Unfortunately this area is threatened by another typhoon, called Parma:

After Morakot

A new blog, called After Morakot, has been started to highlight the impact of Typhoon Morakot in Taiwan. The most recent post describes a journey by the author into the village of Namsia, which was one of the most seriously affected villages in the typhoon. The images and the commentary are remarkable.

I reproduce here two of the images, of the village of Mintsu - there are many more on the blog:

A dreadful weekend of landslides

The last few days have been a dreadful period for landslides. I really cannot cover all that has happened in enough detail, so all I can do is to outline the main events:

1. Rotorua rockfall, New Zealand
Actually the period started with a very lucky escape in New Zealand on Thursday when a rockfall on a slope under maintenance crushed a car with two passengers. Both were trapped but were extracted from the car with minimal injuries. Given the size of the blocks and the state of the car this is pretty remarkable image from here:



2. Shunhe township, Hanyuan county, Sichuan province, China
The large valley-blocking landslide in Hanyuan County also occurred on Thursday. This slide, which is reported here, is believed to have killed 31 people. The partial blockage of the river continues to cause problems:

3. Mount Pinatubo, Philippines
On Friday the Philippines was hit with very intense rainfall that appears to be from the outer edges of Typhoon Marakot, which hit Taiwan and then China. There were two disastrous landslides - in the first a lahar hit a tour group and their guides on the flanks of Mount Pinatubo, killing two locals and three tourists.

4. Kias, Baguio, Philippines
In the second incident, also on Friday, a landslide hit a group of miners at Kias, near to Baguio in the Philippines. A group who went to their rescue were then hit by a second landslide, killing 14 people in total.

5. Pithoragarh district, Uttarakhand
On Saturday a large landslide occurred in northern India (see image below from here). The true impact of this is a little unclear, but the current estimate of loss of life appears to be 43 people. Thanks to David Hopkins for the heads-up on this one, and to Sekhar for the photo).

6. Typhoon Marakot, Taiwan
The true impact of Typhoon Marakot is far from clear, with reports of debris flows that might have killed hundreds. It is important to stress that these are unconfirmed, but with 2.4 m of rainfall in a weekend who knows? A slightly clearer report suggests that Taoyuan Village in the county of Kaohsiung was hit by a debris flow that killed 16. We will wait for the morning to see whether the reports of much higher loss of life are correct.

Two recent landslides at hydroelectric sites in China

In the past week China has suffered two rather extraordinary landslide accidents at hydroelectric plants.

The first occurred an 20th July 2009 near to the Xiaowan hydroelectric power station in Yunnan province. The dam, shown in the artist's impression below, will apparently be the world's tallest arched dam, standing 292 m high when finished. It is still under construction, with the first turbine expected to be operational later this year:


The image above is a bit fanciful as it doesn't show the lake or the extensive slope works that have been needed. The image below, taken in an earlier phase of construction, gives an idea of the state of the slopes:

Now interestingly, late last month there was a rather triumphant article in the Chinese media about the second stage filling of the site. The Google translation of the article says this:

"It is reported that the second phase of Xiaowan storage power station will be carried out in two steps, 20 July, water power stations will reach 1125 meters elevation; the end of August, water storage, power generation water level will reach 1160 meters elevation, the first unit into operation by the end of September this year power generation is just around the corner" (excuse the rather mangled English).

In other words, the lake level was being elevated for the first time to 1125 m elevation on 20th July. The landslide occurred on that day. The landslide itself was described thus:

"Waves up to 30 metres high swept 14 people into the turbulent upper reaches of the Mekong River following a huge landslide into the river in south-western China's Yunnan province, state media said on Tuesday. Rescue workers recovered two bodies from the river, which is known as the Lancang in China, and were still searching for 12 others missing since they were swept away in the early hours of Monday, the semi-official China News Service said. An estimated 13 hectares of land plunged into the river near the Xiaowan hydroelectric power station, creating giant waves that engulfed 14 farmers as they camped nearby, the agency said. "

This slide apparently had a surface area of 130,000 square metres and created displacement waves 30 m high. That is a very large landslide!

Then, on 23rd July 2009, another landslide disaster occurred, this time at the site of the Changheba Hydroelectric power (HEP) works in Kangding County, Sichuan Province. It appears that in this case a debris flow hit a construction camp for the dam, which will be a 2.2 GW plant across the Dadu river. Unfortunately, the very large (500,000 cubic metre) debris flow occurred at about the worst possible time, 3 am. It hit a major road and 136 temporary buildings housing workers. It is believed to have killed 57 people. The site of the landslide appears to be shown in this Xinhua image:

The rescue operations have been hampered by another large failure, this time on the access road to the site. Here it appears that a failure has occurred in a hillside store of boulders that are to be used in the construction of the dam:

The boulders clearly have been generated recently, as this close up image shows:

It appears that they were being retained by gabian walls above the road:


The collapse has blocked the highway, meaning that until today heavy machinery could not access the disaster site. This has hampered the rescue operation, although to be honest there seems to be very little chance that there could be any survivors from an event of this size.

If anything, 57 fatalities from 136 temporary houses would seem to be a surprisingly low number.

Three new landslide / debris flow videos

Thanks to Lynn Highland at the USGS for drawing my attention to this video of a debris flow in Costa Rica:



This is a slightly strange event. It occurred on the Sarapiqui River in Costa Rica after the 16th January 2009 Cinchona earthquake. It is not all clear why the earthquake should have triggered such a debris flow given that there was no rainfall, but a suggestion is that there may have been mobilisation of shallow groundwater. Another possibility is the creation and then collapse of a barrier lake.

The same event also appears to have been captured in this video:



If anyone has any more information then I would be very interested in hearing about this event.

In Hong Kong last month I saw a video taken of a collapsing hillside during the Wenchuan earthquake. This appears to have been shot in the Wolong Panda Reserve area. The quality is poor, but the footage of the collapsing slope is dramatic:

An update on landslides in Beichuan

At the time of the Wenchuan earthquake I posted on numerous occasions about the landslides around the town of Beichuan. One of the concerns has always been the likely behaviour of the landslides during heavy rainfall events.

As part of the forthcoming Sinorock 2009 conference in Hong Kong there will be a tour to the earthquake-affected area to look at and discuss the landslides. The organisers have put together a web page about this tour (see here). Interestingly, thy have posted three images of Beichuan:

First, before the earthquake:

Second after the earthquake:


And third, most importantly, after a very intense rainfall event on 24th September 2008:


Note the extensive movement of sediment, which has caused the river bed to agrade and bury many structures. This is very similar to what happened in parts of Taiwan after the 1999 Chi-Chi earthquake. Such behaviour represents a serious challenge for the authorities.

Landslide disaster in Yunnan, China

Xinhua, the Chinese Official news agency, is reporting a series of landslide disasters in Yunnan Province, China this weekend:

"Twenty people died and 42 were missing after landslides and mud-rock flows hit southwest China's Yunnan Province over the weekend, said local authorities. The disasters hit Chuxiong Yi Autonomous Prefecture, and four counties and a district of the provincial capital Kunming, the provincial civil affairs department said. More than 1,000 houses collapsed or were damaged in the disasters. Tents, quilts and clothes were being transported to the areas. The casualties and damages were also being assessed."

Mud-rock flows are debris flows.

The worst landslide appears to have occurred close to Chuxiong City, where Reuters (and others) are reporting 15 deaths and 34 people missing. It is unclear at the moment whether this is a single location or whether there have been multiple flows.

Interestingly, the TRMM have identified this as an area in danger of landslides today, as this image shows:

The Yigong Rock Avalanche, Tibet

Occasionally I like to take a look back at a significant landslide. Today I thought I'd write about the amazing Yigong rock avalanche in Tibet. The location of the landslide is shown on Figure 1: it's location is 30°14' N, 94°59' E, located high on the Tibetan Plateau.

Figure 1: Google Earth image showing the location of the Yigong landslide. Click on the image for a better view.

The landslide itself occurred at midday (UT) on 9th April 2000, when a wedge failure at about 5,500 m elevation (3,300 m above the valley floor) detached a volume of rock of over 100 million cubic metres.

the landslide occurred in Zhamu Creek. It took only 10 min to travel down a horizontal distance of 8 km through a vertical elevation difference of 3330 m from its source at 5520 m a.s.l. to its sediment fan at 2190 m a.s.l.(Fig. 3). The trigger for the failure is not clear, but no obvious rainfall or seismic event was recorded. The debris travelled down the valley (Fig. 2) at velocities of up to about 14 m/sec (Shang et al. 2003). Over the next 10 minutes the landslide travelled about 10 km, entraining debris, snow and ice as it flowed to generate a deposit with a volume of about 300 million cubic metres. The landslide came to rest on the valley floor at an elevation of 2190 m, blocking the Yigong River to a height of 60 m over a distance of about 1000 m (Fig. 3).

Figure 2: The track of the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Figure 3: The deposit the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

The landslide itself is rather well shown in Google Earth, although unfortunately the source area has a slightly lower resolution than does the track (Figure 4). It is clear that the debris travelled down a comparatively linear tributary valley until the main valley was reached. In the latter stages of the movement the landslide was technically a distal debris flow, essentially meaning that it entrained a large volume of water (from snow and ice). This would have been immensely destructive in an inhabited area, but fortunately the area is sparsely populated.

Figure 4: The track the Yigong landslide as shown on Google Earth. Click on the image for a better view.

Of course the landslide did create a major problem in that it blocked the valley. A lake rapidly started to fill, and over the next two months a volume of about 3 billion cubic metres of water accumulated. The authorities sought to mitigate the problem by relocating the 4,000 living in the path of the flood whilst simultaneously constructing a drainage channel across the landslide deposit. This lowered the peak height of the dam to 44 m above the river level. The water broke through the channel on 10th June and the lake drained over a period of two days. 17 km downstream a peak discharge of 120,000 cumecs (cubic metres per second) was recorded, with the river level rising to 32 m above the deck of the bridge across the river. The trimline created by the flood is clearly visible in Fig. 5. The area that had been inundated by the lake is clear on Figure 6.

Figure 5: The trimline created by the flood resulting from the overtopping of the Yigong landslide dam. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Figure 6: The area inundated by the landslide dammed lake at Yigong shown on Google Earth. Click on the image for a better view.

The emergency measures were effective in preventing loss of life in Tibet, although there was considerable damage to infrastructure downstream. However, the flood did cause an international incident as the Chinese authorities failed to warn their counterparts in India that the flood was coming. As a result, the people of Arunachal Pradesh were not prepared for the event, leading to about 130 fatalities and 50,000 being rendered homeless (Yin and Wang 2005).

Finally, as an aside, the team that successfully built the drainage channel in this case was kept intact after the event and were thus ready to respond to the landslide dams created by the Wenchuan (Sichuan) earthquake this year. Their experience at Yingong was undoubtedly critical in their success in mitigating the major landslide dammed lakes that were created. It appears that their skills continue to be needed.

References:
Shang, Y. et al. 2003. A super-large landslide in Tibet in 2000: background, occurrence, disaster, and origin. Geomorphology. 54 (3-4), 225-243.
Yin, Y. and Wang, S. 2005. Landslide hazard and reduction strategy in China. In: Hungr et al. Landslide Risk Management. Tayor and Francis, 423-6.
Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Debris flows and fatalities

One question that I am quite often asked is why it is that fatalities in landslides occur more at night than in the day. In some ways this is counter-intuitive as one might expect that people would be safer in their houses than they are when they are out and about.

At least a part of the answer lies I think in the destructive potential of comparatively slow landslides. Most people have probably seen the video of the recent Hong Kong debris flow, which I blogged about earlier in the year. Clearly when a landslide is as rapid and destructive as this one then the chances of escape are quite limited, although running sideways out of the path, rather than away from the slide, would give the best chance.

However, many fatal landslides are much slower than this. The following Youtube video shows a debris flow from Clear Creek County in Colorado.



If you cannot see this above, click here.

This is a fairly typical debris flow. They very often occur in a series of pulses, each of which start with the movement of large boulders that are moving quickly but not exceptionally fast. This is clear in the video above. These boulders continually roll-over each other, bulldozing everything in their path. Behind the boulders comes of a flow of much finer and more fluid material, which often moves quite quickly, as in this case. The flow slowly declines before the next pulse arrives.

In many cases people who are awake and alert are able to move out of the way of one of these flows, not least because they are pretty noisy. However, people who are asleep in a house often do not know that the debris flow is coming until it hits the building. Unfortunately very few buildings can withstand the huge forces that the flow can generate, as the USGS image below shows. Even where people do survive the initial event, they are quite often then hit in the dark by the next pulse.

USGS image of the aftermath of the Vargas debris flows in Venenzuela

Clearly in this case the required mitigation is first to try to prevent these flows from occurring at all - for example by stopping deforestation and by placing catch structures in the tributary valley areas in which the flows start. Second, it is important to identify where flows might occur and then to try to ensure that people are not in the way of them. Finally, in some cases a warning system can be helpful, but this should be seen as a last resort once the above two measures have been implemented as effectively as is reasonably practicable.

Hurricane landslides in Haiti

A perennial landslide story at this time of year is the triggering by a tropical cyclone of landslides in Haiti. This year the hurricane season has been particularly cruel, with three large events in a month. Hanna, the most recent, appears to have stalled close to Haiti for a day or so, causing torrential rainfall. At the moment reports suggest that 136 people have been killed in flash floods and landslides as a result of Hanna, but that figure may well rise as the picture becomes clearer. Unfortunately, there is another, very intense "Cape Verde" type hurricane (called Ike) lurking to the east. Current forecasts suggest that the track will swing north of Haiti (in fact Florida could get a direct hit), but at this stage such forecasts are at best tentative. A direct hit from a hurricane as large and intense as this, on the back of Hanna, could be disastrous for Haiti.

So why is Haiti so vulnerable to hurricanes? Basically, Haiti is the most extreme illustration of the impact of deforestation on landslides and flash floods. Haiti is the poorest country in the Caribbean - over half the population live on less that $1 per day ($1 is the recognised mark of extreme poverty) and over 75% live on less that $2. More that 60% of the working population do not have formal employment. The consequence of this has been extreme deforestation, primarily for firewood to create charcoal.

This is rather well illustrated by this Google Earth image:

The border between Haiti and the Dominican Republic is the river that runs across the centre of the image. On the west (leftish) side is Haiti, to the east (right) is the Dominican Republic. The contrast between the two is striking - in the Dominican Republic deforestation has been limited, in Haiti the loss of forest is almost total.

The result is that during hurricanes the landscape has little capacity to intercept and store water, and once flows across the surface begin the landscape rapidly erodes. This is perhaps best illustrated by the following Google Earth image of the city of Gonaives in the east of the country. Gonaives has been very seriously affected by erosion and landslides again in the most recent floods. The image shows the hills on the outskirts of the edge, with the suburbs clearly visible. The hills are clearly suffering from extreme deforestation. In September 2004, Hurricane Jeanne triggered mudslides and debris flows from these hills that killed over 3000 people in the city. Bearing in mind the fact that Haiti should be densely vegetated with tropical forest, the disaster that is the landscape in this country is all to clear to see. Unfortunately, posts about landslide disasters in Haiti will be a feature of this blog in the late summer for years to come.

Debris flow video from Hong Kong

Thanks to a former student for bringing the following video to my attention. Hopefully it will play directly when you click on the box below - if not then take a look at:
http://www.youtube.com/watch?v=R2uTKyK1c9k




This channelised debris flow occurred on Lantau Island, near to Hong Kong airport, on 7th June when the area was affected by an exceptional ("black") rainfall event. In a single hour Hong Kong observatory recorded 140 mm of rain, the highest intensity on record there. During the day, which saw over 300 mm of rain across most of the area, about 400 landslides were triggered on Lantau alone, including one that killed two people.

Helpfully, the Hong Kong Observatory make rainfall isohyet charts available online. I have annotated below the map for 7th June and the approximate location of the landslide. Note that the landslide occurred in an area in which rainfall totals were at their highest (>400 mm).

Hong Kong Observatory isohyet map of the rainfall distribution for 7th June 2008. The approximate location of the landslide in the Youtube video is indicated.

A couple of words about the landslide. This is technically a channelised debris flow. It appears to have started as a comparatively small shallow slip in weathered materials high on the slope. The mobile material has then picked up (technically this is termed "entrained") debris and water in the channel. Once such flows start, and assuming that they have a steepish slope to travel down, they build momentum and volume to create a highly turbulent and destructive pulse of material that moves very rapidly. Slides of this type typically occur in pulses, as can be seen in the video, which is of course particularly nasty for people in the way. Needless to say these types of flow are very dangerous, particularly if they overflow the channel banks. Fortunately, in this case the flow appears to have remained mostly in the channel itself.

Hong Kong has a long history of these types of landslides, and is adept at managing them. To enhance this the Hong Kong government has just started a large project to identify natural terrain areas prone to these failures and to mitigate areas of high risk.

Updated: Tangjiashan (Beichuan) dam - a summary of what we know

In the next 24-48 hours the water level at Tangjiashan should reach the spillway and flow should start. I thought therefore that it would be useful to summarise what we know about the site:

• The landslide dam is 124 metres high;
• At the time that the over topping begins the lake will have a volume of about 205 million cubic metres of water (UPDATED - the volume is now estimates as 211.6 million cubic metres as at 2 pm (loca) Wednesday. The lake level had 1.37 m to go before reaching the spillway)
  
• The channel that has been cut appears to be about 5 (now known to be 7 m) wide at the bed (though note that the film with the journalist in the bed suggested less than this) and about 15 m at the top. It is 12 m deep The bed is unlined and unprotected. It is formed in silty materials with a few clasts (stones);
• Failure can occur through any of three mechanisms:
• The water can reach the spillway and start to flow over the dam;
• The two known seepage points (total reported flow = 10 cubic metres per second) in the dam could cause collapse;
• A landslide into the lake could trigger a wave that would overtop the dam. A dangerous slope has been identified at Xuanping township.
  
• Downstream there are three further (smaller) landslide dams in the Beichuan area;
• The Chinese government has relocated about 250,000 people. In total, about 1.3 million would be threatened by total collapse.

Reuters image of the top of the dam showing the lake and the spillway under construction

These are the best and worst case scenarios for the dam as I see them:
Best case: water flows over the spillway and erodes it slowly. The lake level reduces gradually over a few weeks;
Worst case: a large landslide into the lake triggers a wave that overtops the dam, causing complete collapse. The entire volume of the lake is released very rapidly. The resultant flood pools behind and then breaks through the downstream dams, creating a devastating debris flow.

The most likely scenario at this stage is that water will flow over the spillway. A ball park flow rate appears to be about 100 cubic metres per second when there is no rainfall. This channel will not be able to cope with this as far as I can see, leading to rapid erosion and therefore collapse of the dam. I suspect that most of the lake volume will be released in a few days at most. The water is likely to pool briefly behind the landslide dams downstream, which will probably increase the size of the flood wave once released.

Where will the water go? I have tried to produce a quick Google Earth summary image below of the anticipated flow path (click on the image for a better view):-


The flood is likely to be large, fast and very damaging in the mountains, but should spread at least a little and slow on the plains. We can only hope for the best.

I am travelling to Nepal this afternoon, but will do what I can to keep this updated from there.