Pakistan floods - the extraordinary duration of the elevated water levels

The floods in Pakistan may have faded from the headlines in Europe, but unfortunately the impact continues, even though the rainfall events that caused them occurred more than a month ago.  The most dramatic illustration of this is a set of satellite images collected by NASA using the MODIS instrument.

This is an image of the area around Sukkur taken on 7th July, before the rainfall event that initiated the disaster:

By way of comparison, this image was taken on 7th September, showing the same area:

Two things to note.  First the extensive flooding along the line of the main part of the Indus remains.  This may well be an indication that water is draining very slowly from behind the broken levees, which are now serving to keep water in despite their failure to keep the water out in the initial event.  Such problems with broken levees are common.  Second, note the parallel course of the Indus that has been created, where the flooding is much more intense.  I assume that this is the river occupying one of its old drainage paths.  The problem here may be that the downstream end of this huge (check out the scale bar on the imagery) new watercourse is Mancchar Lake.  This is an artificial lake, created in the 1930s, with a range of existing environmental problems.  The lake appears to be connected to the main Indus River by just two small canals (see Google Earth image below):

This limited drainage path is may cause the floods to drain very slowly, prolonging the agony for those affected.  The flood waters must now be highly polluted, increasing the potential for health problems for the affected population.

38 years ago today - the Chungar landslide in Peru

Today is the 38th anniversary of a notable landslide - the Chungar rock avalanche in Peru. This landslide occurred on the banks of Lake Yanahuani (sometime spelt Lake Yanahuin), about 120 km north-east of Lima (see image below):

The landslide, which had a volume of about 100,000 cubic metres, is shown on the image below. Unfortunately the Google Earth image resolution is low in this area, but you can see enough to get an idea. I have annotated the image to show the main features (click on the image for a better view):
The rockslide descended a vertical distance of about 400 m before entering the lake, whereupon it created a displacement wave that crossed the body of water at high speed. The wave struck a mining camp located on the other side of the lake, running up a vertical distance of 30 m and erasing all traces of the settlement. Between 400 and 600 people were killed. The landslide occurred on highly-fractured limestone rocks on a slope that had been over-steepened. The slope remains highly dangerous. Unfortunately the landslide is poorly documented - for example, the trigger is really not at all clear.

Norwegian landslide - is this a quick clay slide?

Most people would probably not think of Norway as being hazard prone, but it does have two particular landslide issues. The first is that occasional large-scale failures occur on the walls of fjords. Given the height of these rock walls, these failures can be large and energetic, and sometimes trigger small scale tsunamis. The other is a strange type of failure known as a quick clay slide. Perhaps the best way to explain a quick clay slide is to direct readers towards the original landslide viseo, which shows the Rissa slide. This is described at the site below, which also contains the video in two parts:

http://www.ce.washington.edu/~geotech/courses/cee522/RissaLandslide/rissa.html

Do take a look - the film is amazing. quick clay slides occur in marine clays that have an unstable structure. Disturbance of that structure can cause a massive reduction in strength, which then destabilises the slope allowing failure on even low slope angles. In the case of Rissa, a small excavation for the foundations of a barn was enough to trigger the failure.

Today, reports have emerged of a landslide at Namsos in Norway that has destroyed ten or so houses. Fortunately there have been no fatalities. Various pictures of the landslide are available - these two particularly caught my eye (from here and here - click on the image for a better view in a new window. The people are homeowners I think):




Familiar? It is clear that this is a landslide occurred in clay on a pretty low angle slope on the edge of a lake. What's more, the newspaper reports (e.g. this one) suggest that "It happened near a road construction site, and NRK said it could be related to blasting or excavation." I don't know for certain that this is a quick clay landslide, but I have my suspicions, especially when I look at this image of the site (from here):


There are a some other pretty good images around too, mostly focusing on the aftermath. For example, this one (from here) shows the displaced houses:

Whilst this one (from here) shows just what a low angle slope the failure occurred on. Presumably the road works in the foreground are the proposed cause of the problems:


Quick clay landslides are pretty rare (I have never seen one) if rather dramatic.

Landslide on the Mahakali River on the Nepal / India border

Thanks to both David Hopkins and Ripendra for drawing my attention to a landslide on Friday on the Mahakali River on the border between Nepal and India. Although it has occurred in a remote area, it is interesting because the landslide appears to have partially blocked a fairly large river coming down from the high Himalayas.

The landslide occurred in the Chautuldhar (Chetalkot) area of Rephalikot, close to Tawaghat in Dharchula, right on the border of Nepal (to the east) and India (to the west) (see Google Earth image below - click on the image for a better view).

The reports suggest that cracks were first seen on Thursday, which led to a large scale failure on Friday. The location is shown on the Google Earth perspective view below (the actual location of the landslide appears to be in a small area of low resolution imagery):


Reports suggest that the river is flowing through a gap of just 2.5 metres, which has created a barrier lake that is threatening a number of villages upstream. The main road is blocked and may take as much as a month to reopen, and there is a threat of further landslides.

An interesting dimension to this is the proximity of the Dhualiganga I hydroelectic scheme. I have annotated the diagram above to show the location of the dam and the power house. This is a run of the river scheme in which the Dhauliganga River has been dammed with a 56 m high concrete faced rock fill dam to create a head of water. The water is then passed through a tunnel beneath the mountain to the powerhouse, where turbines are located. The project was completed in October 2005. Some newspapers are speculating that the landslides may have been associated with the construction of the tunnels as the failure occurred in the dry season with no obvious trigger. However, the tunnel is 6 km from the landslide site, so this is not likely. However, if a large lake were to form at the landslide site, and then was to be released rapidly, then the powerhouse would be threatened. It is not clear to me whether this is a possibility, but given that the river is now reportedly flowing again I suspect the chances at present are low.

As an aside, the dam site itself appears to be quite interesting from a slopes perspective:

Two interesting recent landslides

Two interesting landslides to report in the last few days.

1. Landslide in Iztapalapa, Mexico City
On Thursday morning a 50 cubic metre landslide in the Itzapalapa, a poor suburb of Mexico City slid onto a house at the toe of the slope, killing two people. Chinagate has published a rather dramatic picture of the site:

The interesting thing about this image is the very large (apparently 5 m high) retaining wall that had been built between the slope and the houses. Clearly the wall has failed under the weight of the landslide (not surprising given the size of those boulders actually), but I wonder who built the wall (surely not the people living in these houses - it is far too big)?

2. Landslide-induced wave on Lake Roosevelt, Washington State, USA
The second is a report from the Spokane arm of Lake Roosevelt near to Spokane in Washington State. The website Spokesman Review is carrying a report that says:

"Property owners in the Spokane arm of Lake Roosevelt were swamped by a huge wave caused by a landslide last Friday. The 17-acre landslide on Jan. 16 near Mill Canyon – northeast of Davenport and downstream from Little Falls Dam – slid into the water, unleashing a wave that reached 30 feet above the full-pool mark across the lake into Breezy Bay, according to Adam Kelsey, chief ranger for the Lake Roosevelt National Recreation Area. Private docks and vessels were destroyed or damaged for at least 1.5 miles downstream from the site."

They also have an image of the site:


It is reasonable to assume that the displaced mass on the far side of the lake is the landslide as the Google Earth image does not show a slide at this site:

It is interesting to note that the section of slope that has failed shows clear signs of toe erosion and some interesting linear features on the Google Earth imagery:

Interestingly, in the years after Lake Roosevelt was filled there were a series of landslides. The Emergency Management Division of Washington Military Department has a document here that describes a series of events as the lake was filled and emptied:

"1944 to 1953 – Massive landslides generated a number of inland tsunamis in Lake Roosevelt in Eastern Washington:
• April 8, 1944 - A four to five million cubic yard landslide from Reed Terrace generated a 30-foot wave, 5,000 feet away on the opposite shore of the lake about 98 miles above Grand Coulee Dam.
• July 27, 1949 - A two to three million cubic yard landslide near the mouth of Hawk Creek created a 65-foot wave that crossed the lake about 35 miles above Grand Coulee Dam; people 20 miles away observed the wave.
• February 23, 1951 – A 100,000 to 200,000 cubic yard landslide just north of Kettle Falls created a wave that picked up logs at the Harter Lumber Company Mill and flung them through the mill 10 feet above lake level.
• April 10 – 13, 1952 – A 15 million cubic yard landslide three miles below the Kettle Falls Bridge created a 65-foot wave that struck the opposite shore of the lake. People observed some waves six miles up the lake.
• October 13, 1952 – A landslide 98 miles upstream of Grand Coulee Dam created a wave that broke tugboats and barges loose from their moorings at the Lafferty Transportation Company six miles away. It also swept logs and other debris over a large area above lake level.
• February 1953 – A series of landslides about 100 miles upstream from Grand Coulee Dam generated a number of waves that crossed the lake and hit the opposite shore 16 feet above lake level. On average, observed waves crossed
the 5,000-foot wide lake in about 90 seconds.
• April – August 1953 – Landslides originating in Reed Terrace caused waves in the lake at least 11 different times. The largest wave to hit the opposite shore was 65 feet high and observed six miles away. Velocity of one of the series of waves was about 45 miles per hour."

Earthquake triggered landslide at Aratozawa Dam in Kurihara, Miyagi prefecture, Japan

Many thanks to reader Ripendra for alerting me to the following, and indeed for doing nearly all the work for this post. The 14th June 2008 earthquake in Japan is known to have triggered a number of landslides, as per earlier posts, but the most notable appears to have occurred at Aratozawa Dam in Miyagi prefecture.

The Aratozawa Dam (Figure 1) is a rockfill dam used for flood control, irrigation and hydro-electric power generation. It is 74.4 m high and 413.7 m long, impounding up to 14,130,000 cubic metres of water.

Figure 1: Miyagi Prefectural Government image of the Aratozawa Dam before the earthquake and landslide.

In the earthquake, a landslide occurred in the catchment of this reservoir. Professor Sassa of the International Consortium on Landslides has provided a brief description of this failure here:

"A huge landslide akin to a massive hole being punched in the side of a mountain was triggered by the Iwate-Miyagi Inland Earthquake at an upstream section of the Aratozawa Dam in Kurihara, Miyagi Prefecture, causing widespread damage and making roads impassable. Kyoji Sassa, head of the International Consortium on Landslides, said the volume of collapsed earth near the dam is believed to be more than 10 million cubic meters. Sassa believes that this is comparable to the largest landslides that occurred as a result of the 2004 Niigata Prefecture Chuetsu Earthquake, which also struck in a mountainous region, and to the volume that fell in single landslides after last month's cataclysmic earthquake in China's Sichuan Province.

"It may have been a 'deep landslide,' in which a large amount of earth completely collapses from a point deep in the mountain slope," Sassa said of the landslide set off by Saturday's quake. In such deep landslides, tremors that seem to thrust up from below cause the pressure of underground water to rise, making the ground above seem to float before abruptly collapsing.

Hiroshi Fukuoka, an associate professor at Kyoto University's Disaster Prevention Research Institute's Research Center on Landslides, said there was a complicated relationship between the landslide and the dam downstream of it. "Water is stored in the dam, and it seems that the underground water level rose and caused a high-speed landslide. Earth on the outside of the slope fell into the artificial lake behind the dam, bringing the danger of flooding," Fukuoka said. "It is necessary to establish whether such a risk exists when building a dam."

In many ways this description does not really do justice to the landslide that actually occurred, which is rather impressive. Figure 2, from Getty Images, provides a rather more helpful illustration of what occurred here:

Figure 2: Getty images picture of the Aratozawa Dam landslide.

Updated@ Tangjiashan images deepen the mystery

NB - I have now updated this with an image at the end that shows this more clearly.

A quick image search today showed up the following Reuters image, entitled: "Water from the Tangjiashan "quake lake" flows down in quake-hit Mianyang, Sichuan Province June 7, 2008. Chinese troops on Saturday eased pressure on a swelling "quake lake" threatening hundreds of thousands of people, but a smaller lake burst its banks in a show of destructive force."

According to the caption this was taken on 7th June - i.e. yesterday.

Now, lets compare this to the earlier image that we had by putting them side by side, with the older one on the left and the newer one on the right:



A few things to note here (see annotated image below):
1. There does appear to be more flow in the second image, though not vastly so;
2. The activities of the excavators in the second image appear to be to deposit soil and rock, not to be excavating it (highlighted with the red circle below). Are they filling something in here?
3. The small waterfall adjacent to the blue trucks in the first image has eroded back a considerable distance in the second one (highlighted with a green circle below). This shows that erosion is occurring, but through a nick point.

The timing of the image is crucial here. If it was indeed taken yesterday, and the nick point eroded back that far during the day, then the erosion back of this nick point becomes a very possible failure mechanism for the dam.

Updated - here is another Reuters image that shows this more clearly:

Tangjiashan (Beichuan) Lake - update

China Daily is today reporting that real progress is being made with the lake above Beichuan, which is now known as Tangjiashan Lake:

Amid low visibility, a Russian helicopter successfully delivered a large bulldozer and other eight sets of big machineries near the swelling Tangjiashan lake, in order to dig a diverting channel to prevent a flood, said Minister of Water Resources Chen Lei. "I feel relieved this morning as we have reached the area," said Chen, who is in charge of dealing with the emergency situation of dozens of quake lakes in Sichuan. Quake lakes were formed after landslides blocked rivers."All these mean we can officially start to deal with swelling lakes now," said Chen, adding it first time he spoke to the media, as he was not in the mood the previous days to do failures to deal with the possible flooding situation. Other helicopters managed to airdrop materials for the operation by 7:49 am Monday at the top of the quake lake at Tangjiashan in Beichuan County. It was formed by landslides that blocked a local river known as Jianhe after the May 12 earthquake. While military forces are preparing to dynamite the swelling Tangjiashan quake lake, rescue headquarters have urged the expert team to quickly give "risk evaluation" reports on bursting of the barrier. Chen asked the provincial and Mianyang municipal governments to draw "three red lines" on the map until Tuesday noon, with each representing which area will be affected if the barrier bursts by one third, half or to nothing.

This sounds much more promising. The best way to deal with this issue is a combination of a channel constructed using heavy equipment, backed up with an assessment of the areas likely to be inundated by a flood. Blasting the dam should be avoided if at all possible - it is a real last gasp approach. However, great care will be needed in the construction of the channel, in particular in preventing rapid erosion of the channel bed and thus a rapid collapse event.

AFP image entitled: The build-up of water in devastated Beichuan county is a massive concern

This is progress for sure but they are not yet out of the woods. The lake now has a volume of >128 million cubic metres of water.