Volcanic landslide in the Democratic Republic of Congo

Whilst the Attabad landslide continues to be my major blogging preoccupation at the moment (I'm guessing you might have noticed...), of course other landslides continue to occur around the world.  On16th May a slip happened on Nyiragongo volcano in the Democratic Republic of Congo.  The reports suggest that 43 people are likely to have been killed, and a further 1,500 directly affected by this slide.


Nyiragongo volcano is active, so this is likely to be a lahar - i.e. a debris flow in volcanic deposits.  This Google Earth image, although of low resolution, suggests that this is not the first failure on the east side of this volvano:

The El Salvador landslides of last year provide a good indication of the nature of a lahar - see previous posts here, here and here.

Recent landslide updates

A few updates on landslide events over the last few days:
1. Highway 97: various newspapers now report that the teams trying to stop the movement of the slope are making headway. The rate of movement has now slowed to about 8 mm per day (half of the rates measured early in the crisis). The drilling and blasting crews have now moved 5,000 cubic metres per day from the crown to the toe of the slope. There are hopes that the road might be reopened within a week.
2. Pitrap, Poot District, Kenya: Two landslides struck this small village on 7th November, killing a total of eleven people, most of whom were women and girls who had gathered to celebrate the birth of a child. Heavy rain was the trigger.
3. Landslides in Yunnan Province, China. Xinhua has now published some images of the landslides in China last week that left at least 83 killed or missing:

Xinhua image of Dasongshu Village of Xishelu Township in Chuxiong Yi Autonomous Prefecture

Xinhua image of landslides in Yunnan Province. Note that a number of the landslides, which are all shallow, have started on or by the road

Xinhua image landslides in Yunnan Province.

4. Landslide in Hunan Province, China: Xinhua reports that six people were killed by a landslide in a landslide on 7th November in Huanxin Village, Pingjiang County. The landslide was triggered by heavy rainfall.
5. Landslide in Burma (Myanmar): This is a location in which I suspect that I do not manage to record all of the landslide events that occur, given the news black-out imposed by the Burmese authorities for much of the time. However, reports have emerged that a landslide on 28th October in Mongton Township, Southern Shan state killed 13 workers on a tea farm. The cause was heavy rainfall.

A strange report from Nigeria...

The Nigerian newspaper "The Punch" has been carrying a quite strange and intriguing story over the last few days regarding an apparently huge landslide in the Awgu area (i.e. at about 6.0046N, 7.4024E). The key points of the story (from the Punch) are:

"Residents of Ugwueme, Umuhu, Ugulesi Awgu and Ezinese Mgbidi communities in Awgu Local Government Area, Enugu State could not believe their eyes when they visited their farmland in the morning. Expansive farmlands measuring more than 20 square kilometres had been destroyed by a tremor. On account of the incident which was later explained as a landslide, there were huge cracks and depressions on the land as if a straying ballistic missile had exploded there. Some trees sank in the depression while others were completely uprooted."

The traditional Prime Minister of Awgu, Chief Stephen Onuorah, who visited the scene on the second day of the incident, said he felt a massive shaking and reverberations under his feet. He said he heard a noise that sounded like the roaring of a bulldozer. Surmising that it was an earthquake, he contacted the Chairman of Awgu Local Government Area, Chief Uche Anioke, who later led some officials to the site. Amazed at the level of devastation, the council boss contacted experts from the Federal Ministry of Mines and Steel Development, but the confusion over the incident was dispelled by the team of experts from the National Geo-hazards Monitoring Centre led by the director of the agency, Mr. Alex Nwegbu, and Prof Cornelius Nwajide, a lecturer at the University of Nigeria, Nsukka and consultant to Shell Petroleum Development Corporation.

The team which also included geologists, geophysicists and seismists, visited the farmland on Tuesday. They described the incident as a landslide which occurred on account of massive rainfall which “peculated into the land to an impermeable layer and caused land cracks”. Nwegbu in his preliminary report traced the remote cause of the incident to the geographical positioning of Awgu which was sitting on a ridge belt called clay star. The belt runs from Idah in Kogi State through Awgu to Arochukwu community in Cross River State, all of which are prone to geo-hazards.

The Chairman of the House Committee on Petroleum Resources and Environmental Management, Chris Ugwu, in a report sent to the state assembly, described the incident as an unusual disaster...Ugwu said that his committee, which visited the area, observed that large portions of land were compressed into crooked ridges and valleys."

This is intriguing because the area affected appears to be very large (>20 square kilometres) and the level of destruction appears to be high, even though the landslide has not been catastrophic in terms of movement. The terrain is question is not particularly steep or rugged, and this is not an area with notable seismicity. I would be very interested to hear more about this slide. I wonder whether this earlier report, although possibly covering a different area, might provide an indication of the root cause of the issue?

Although Nanka has bared the brunt of the erosion-based devastations, it has however not been relegated only to Nanka. Other towns have been faced with their unfortunate share of these hellish gullies. Enugu Ukwu is another town ravished by gully erosion. According to a 1993 State report on the state of gully erosions in Anambra, “the number of gullies presently pervading and ravaging the lands of Enugu Ukwu defies solution”. The story of the gullies in Enugu Ukwu begins with the Etti-Umuatulu-Osili Enugu Ukwu gully networks which starts from Etti Awobu village and gallops down a slope stretching down into Umuatulu Awobu where it splits into two branches; to run into Obuagu Osili where it converges to a point where the gully depth stands at about 80meters. The gully network within the Urukpaleke and Akama Osili areas originate which is reported to have been aggravated by a flood from a faulty drainage line constructed along Enugu-Onitsha road. The same gully stretches to another Akama Osili village and joins Urukpaleke gully. According to Inter Press Service New Agency who was one of the many New Agencies that reported on the incident, “the inhabitants of Umuchiana, one of the villages that make up Ekwulobia community in Anambra state, were woken up at night by a noise only to find some houses at the edge of the village giving way to landslide. They deserted their homes, taking refuge in nearby forest and villages. By the time they returned to their village the following morning, several houses, a church and some roads were washed away. Their farmlands, palm and cashew trees were not spared either. Though nobody died in the incident, more than 250 families (made up of more than 1,500 persons) were displaced.

Retaining wall collapse, Uganda

On Tuesday morning a collapse occurred in a retaining wall being constructed for the foundations of a new tower block in the capital of Uganda, Kampala. Two images have been published online of the failure, which buried nine workers, killing eight of them:

Monitor online image of the Pensions Towers collapse site

New Vision image of the Pensions Towers collapse site

Collapses like this are now quite rare in more developed countries, though not unknown (see here, here and here for example). However, a properly designed retaining wall, based on good site investigation and laboratory data, and a properly executed design, should not collapse in this way. So it will be interesting to find out what has gone wrong in this case. There will be a formal investigation, so we will wait and see, but the pictures and reports do point to a couple of issues.

First, the pictures above show a very red soil. This is of course characteristic of deep weathering - the red is iron oxide. Deep, tropical residual soils like this are often problematic materials because they can be weak and also highly variable. Experienced engineers know to treat them with respect.

Second, the pictures show that the excavation is deep (one of the newspaper articles suggests it was 25 m high!). The wall was being supported, but it appears that this was with nothing more than "wood, iron bars and wire mesh" see here). The images suggest that the wall was just being faced, not supported properly. This is surprising. However, the reasons behind this are perhaps explained by this:

"At the heart of the queries will be NSSF’s decision to change building plans which KCC insists it had advised the developer against. In building Pension Towers, NSSF had initially planned an eight-storey tower consisting of two-basement parking levels. The Fund subsequently changed the plans for the complex, into a 26 storey complex and tripled its costs from Shs36b to Shs120b."

The suggestion being that as a result of the extra storeys on the tower the excavation was deepened from two to four storeys without taking into consideration the need to change the design of the wall. It has also turned out that:

"The developers of the Pension Towers, whose retaining wall collapsed on Tuesday killing eight people, had earlier ignored a warning from Kampala City Council advising against using unapproved building plans."

The state of play at the site might be indicated by this report into the aftermath of the collapse:

"When the New Vision visited the site around 11:00am, the horrified workers of [the] construction company were desperately trying to dig out their buried colleagues, using spades and hoes. Enraged onlookers demanded that the two bulldozers on the site be used to remove the soil. As fate would have it, the wheel chain of one of the bulldozers broke off as soon as the engine was started by a volunteer as its operator had vanished, while the second bulldozer developed a mechanical fault and caught fire. A third bulldozer had to be rushed in from another...construction site. "

Finally, the interplay between western and traditional views of disaster causation on sites like this is nicely captured here:

"Foreman Hajji Kigongo said the accident could have been prevented if they had sacrificed three bulls before construction began. He scolded this boss for failing to perform the ritual, a common practice in Buganda. Susan Kataike, the works ministry spokesperson, said prior to the works, they had tested the soil to establish if it could hold a building of that size."

Submarine landslide in southern Africa?

The award for the strangest landslide story of the week goes to this one, from southern Africa. Late last week reports started to emerge of strange tidal patterns around the southern Cape of Africa (Fig. 1).

Fig. 1: Google Earth image of the southern Cape of Africa, showing the location of St Helena Bay and Plattenburg Bay.

In particular, on 21st August at about 8:45 am (local time) a series of tidal oscillations were recorded in St Helena Bay (Fig. 2). These tidal oscillations were reportedly large enough to damage some factories, pull a car into the sea and to cause problems for boats navigating into the harbours. A couple of days later, a report emerged from the National Sea Rescue Institute (NRSI) suggesting that the "mini-tsunami" was caused by a seismically-induced submarine landslide. Fig. 2 shows that some anomalous tidal activity certainly did occur.

Fig. 2: Tide gauge data for Port Nolloth showing the anomalous behaviour on the morning of 21st August 2008 (graph from the South African Weather Service).

Now, of course submarine landslides can trigger localised tsunamis (as occurred in Papua New Guinea in 1998), and the most likely cause of such an event is an earthquake. However, aspects of this cause do not seem to ring true. First, southern Africa is not a strongly seismic area. Second, no earthquake event has been recorded at the time that the tidal oscillations were observed. And third, the oscillations appear to have occurred over a long time period, and at markedly different times along the coast. One would expect that a landslide induced tsunami would be quite short-lived and to arrive at the coast over quite a short time period. Thus, the evidence does not really seem to support this mechanism.

In the last couple of days a more considered analysis from NSRI has been reported, recognising that the cause is more likely to be meteorological than the effects of a landslide. The South African Weather Service have proposed that atmospheric gravity waves might have been the cause, although they are sitting on the fence a little.

Whilst these tidal oscillations are clearly strange, a landslide cause does not really seem to be very likely.