Zhouqu landslide - NASA satellite image

NASA have somehow managed to find a gap in the clouds to collect a satellite image of the Zhouqu landslide site.  Readers at the time of the landslide last month will remember that I was wondering about the source of the slides.  Although this image does not give a full answer, it gets us a part of the way there:

A zoom into the affected area yields this:

The slides (note the smaller one to the right of the main event) clearly originated as a series of smaller slips on the walls and headwaters of the upstream catchments.  These appear to have coalesced to form the main events.  This is consistent with processes observed elsewhere during periods of exceptional rainfall.

Meanwhile, I am now in Christchurch in New Zealand, site of the earthquake of 12 days ago.  Every 30 minutes or so my chair shakes as an aftershock rattles the area, which is a very surreal experience.

Pakistan flood update - 15th August 2010

The second flood wave is now starting to affect the most devastated areas of Sindh.  The PakMet FFD hydrograph for Guddu has started to show an increase in discharge once again:

Note that the flood level has been above the exceptionally high (danger) level for ten days now.  Downstream at Sukkur the water level continues to fall very slowly, again remaining well above the danger level:

Expect this to start to rise once again in the next few days. Meanwhile, the final downstream station at Kotri the flood level remains very surprisingly low:

Assuming that these values are correct this must mean that a vast volume of water is being stored in the landscape between Sukkur and Kotri.  The flood is certainly spreading across the plains, as this report on new flooding in Jaffarabad demonstrates.  This NASA MODIS satellite image shows the area around Sukkur.  Notice how the area flooded changes at Sukkur, presumably because of the restriction in the flow by bridges and barrages:

There is a first class UNOSAT flood extent map here.   It demonstrates the areas in which the flood is still spreading - a close look is quite horrifying actually - the scale of the disaster is so huge that the media are understandably struggling to provide an adequate picture of the extent.

To multiply the problems the new flood wave travelling down the river.  This must be a very worrying situation once the second flood reaches Sukkur.  Although smaller than the first wave, the cumulative effects and the existing damage to levees spells a potentially very difficult situation.

Meanwhile, the focus of the media coverage is now on the downstream areas.  With 20 million people now directly affected by the floods, this is unsurprising.  However, the situation upstream also remains critical.  The Pamir Times has a good report on the situation in Gilgit-Baltistan, which is effectively cut off downstream by the loss of the Karokoram Highway and upstream by the Attabad landslide dam:

"The devastating landslides and flash floods that resulted in death of over 120 people in different parts of Gilgit – Baltistan have also blocked supply routes increasing misery and sufferings of hundreds of thousands more.

Blockage of the Karakuram Highway since last week of July has resulted in depletion of POL products, including petrol, diesel and Kerosene oil in the entire regions.  Hospitals have been closed and hotels are shutting down due to absence of electricity.  Banks have also closed down temporarily because of lack of electricity.Supply of electricity to Gilgit has been cut off for more than one week due to destruction of power houses and transmission lines. Transportation system has also collapsed due to closure of fuel stations that relied entirely on supplies from down country through the Karakuram Highway.  In Gojal valley the boats have stopped ferrying passengers and goods because of depletion of fuel reserves. The local people are cut off from other parts of the region without proper medical facilities and supplies.  Ghizar, Astore, Diamir and the two districts of Baltistan are also facing similar situations. Hundreds of displaced families in different parts of the region have still not been reached by the government and relief agencies. One such example is of the displaced people of Gaise village in Diamir where the food reserves have reportedly finished and starvation is setting in. Local people have demanded of the government to intensify its efforts in Gilgit – Baltistan to end miseries of the suffering people.  Deaths due to starvation and malnutrition seem to be a sad possibility in some far flung parts of remote affected districts of Gilgit – Baltistan."

Jeff Master's remarkable blog has an article on the causes of the Pakistan floods.  It remains my plan to follow this up in the next few days, if I can find the time given everything that is going on.

More El Salvador lahar satellite imagery

In an earlier post today I highlighted the availability of satellite imagery covering the El Salvador lahar disaster, triggered by Hurricane Ida earlier this month. Whilst sitting in a conference session this morning it occurred to me that the Disaster Charter, which provides satellite images for disaster relief and recovery operation, was also triggered by this event. So I had a quick look at the web page for this triggering of the charter, and sure enough there are some good materials there.

Perhaps the best image is this interpretation of the lahar tracks, produce using the Taiwanese Formosat2 satellite (famous for its images of Beichuan and Tangjiashan after the Wenchuan Earthquake) - do click on the image to see this properly, it is worth it!:

There is also a more detailed look at the town of Guadeloupe using the same satellite, showing before and after images of the town:

Viewed in conjunction with the NASA images in my earlier post, this is a very useful resource.

Satellite imagery of the El Salvador lahars

NASA has released before and after satellite images of the lahars (volcanic debris flows) triggered by Hurricane Ida in El Salvador earlier this month. The final toll of the landslides remains unclear, over 60 is the widely reported statistic.

The images are false colour composites collected by the ASTER instrument. These images differentiate between vegetation, which appears as red colours, and bare ground, which appear as a blue-grey colour. Such images are ideal for delineating new landslides, which usually strip away vegetation to leave bare soil.

This is the before image of the area affected by the landslides:

As usual, click on the image for a better view in a new window. Here is the image after the hurricanes passed through:


The lahar tracks are pretty clear. Note that they start high on the volcano, mostly with small, translational landslides. These slides then enter the channel, entraining (eroding and incorporating) material along the channel. The towns are quite clear on the images - the debris flows have hit the settlements in several locations, leaving this type of damage (Image from here):


When the landslides reach the plains below the volcano they spread out and stop, leaving a large area covered in debris. This is also clear on the image.

Are satellite-based landslide hazard algorithms useful?

In some parts of the world, such as the Seattle area of the USA, wide area landslide warning systems are operated on the basis of rainfall thresholds. These are comparatively simple in essence - basically the combination of short term and long term rainfall that is needed to trigger landslides is determined, often using historical records of landslide events. A critical threshold is determined for the combination of these two rainfall amounts - so for example, it might require 100 mm of rainfall in hours after a dry spell, but 50 mm after a wet period. These threshold rainfall levels have been determined for many areas; indeed, there is even a website dedicated to the thresholds!

In 1997 NASA and JAXA launched a satellite known as TRMM (Tropical Rainfall Monitoring Mission), which uses a suite of sensors to measure rainfall in the tropical regions. Given that it orbits the Earth 16 times per day most tropical areas get pretty good coverage. A few years ago Bob Adler, Yang Hong and their colleagues started to work on the use of TRMM for landslide warnings using a modified version of rainfall thresholds. Most recently, this work has been developed by Dalia Bach Kirschbaum - and we have all watched the development of this project with great interest. The results have now been published in a paper (Kirschbaum et al. 2009) in the EGU journal Natural Hazards and Earth Systems Science - which is great because NHESS is an open access journal, meaning that you can download it for free from here.

Of course a rainfall threshold on its own doesn't tell you enough about the likelihood of a landslide. For example, it doesn't matter how hard it rains, if the area affected is in a flat, lowland plain then a landslide is not going to occur. To overcome this, the team generated a simple susceptibility index based upon weighted, normalised values of slope, soil type, soil texture, elevation, land cover and drainage density. The resulting susceptibility map is shown below, with landslides that occurred in 2003 and 2007 indicated on the map:


A simple rainfall threshold was then applied as shown below:

Thus, if an area is considered to have high landslide susceptibility and to lie above the threshold line shown above based upon an analysis using 3-hour data from TRMM, then a warning can be issued.

Kirschbaum et al. (2009) have analysed the results of their study using the landslide inventory datasets shown in the map above. Great care is needed in the interpretation of these datasets as they are derived primarily from media reports, which of course are heavily biased in many ways. Examination of the map above does show this - look for example at the number of landslide reports for the UK compared with New Zealand. The apparent number is much higher than in NZ, even though the latter is far more landslide prone. However, in New Zealand the population is small, the news media is lower profile, and landslides are an accepted part of life. However, so long as one is aware of these limitations then this is a reasonable starting point for analysing the effectiveness of the technique.

So, how did the technique do? Well, at a first look not so well:



In many cases the technique failed to forecast many of the landslides that actually occurred, whilst it also over-forecasted (i.e. forecasted landslides in areas in which there were none recorded) dramatically. However, one must bear in mind the limitations of the dataset. It is very possible that landslides occurred but were not recorded, so at least to a degree the real results are probably better than the paper indicates. Otherwise, the authors admit that the susceptibility tool is probably far too crude and the rainfall data to imprecise to get the level of precision that is required. However, against this one should note that the algorithm does very well (as indicated by the green pixels on the map above) in some of the key landslide-prone areas - e.g. along the Himalayan Arc, in Java, in SW India, the Philippines, the Rio de Janeiro area, parts of the Caribbean, and the mountains around the Chengdu basin. In places there is marked under-estimation - e.g. in Pakistan, Parts of Europe and N. America. In other places there was dramatic over-estimation, especially in the Amazon Basin, most of India, Central Africa and China.

All of this suggests that the algorithm is not ready for use as an operational landslide warning system. Against that though the approach does show some real promise. I suspect that an improved algorithm for susceptibility would help a great deal (maybe using the World bank Hotspots approach), perhaps together with a threshold that varies according to area (i.e. it is clear that the threshold rainfall for Taiwan is very different to that of the UK). Kirschbaum et al. (2009) have have produced a really interesting piece of work that represents a substantial step along the way. One can only hope that this is developed further and that, in due course, an improved version of TRMM is launched (preferably using a constellation of satellites to give better temporal and spatial coverage). That would of course be a far better use of resource than spending $4,500 million on the James Webb Space Telescope.

Reference
Kirschbaum, D. B., Adler, R., Hong, Y., and Lerner-Lam, A. 2009. Evaluation of a preliminary satellite-based landslide hazard algorithm using global landslide inventories. Natural Hazards and Earth System Science, 9, 673-686.

Guatemala landslide - new satellite image

The Advanced Land Imager instrument of the NASA Earth Observation satellite EO-1 has recently collected an image of the Guatemalan landslide, which they have made available online. The image is here. I have cropped it to show just the slide - it is quite useful:

Satellite imagery of the landslides in the Sichuan earthquake

Now that there is a brief weather window, satellite imagery is starting to emerge of the Sichuan Earthquake zone. At present most of these images are too low resolution to allow detection of the real damage to the buildings (which is notoriously difficult to achieve), but they are excellent for looking at landslides.

The best to date that I have seen has come from the Taiwanese Formosat-2 instrument, which is a moderately high resolution imaging system that passes over Taiwan (and thus China) on a twice daily basis (10 am and 10 pm). They have helpfully released before and after images of the Beichuan area:

(Annoted from the BBC - click for a better resolution version)

The town in the centre of the image located around the river is Beichuan. The landslides are the brown areas on the left hand image - it is clear from this that about 20% of the land mass in this area has slipped. The southern edge of the town has been hit by a big landslide (see here) that appears from this image to have spread over a large area. Worryingly, some of the rivers also appear to have been blocked in two locations by landslides. I hope that urgent works are underway to resolve this as there is a real danger of a secondary disaster when the blockages are breached. This is an absolute crisis that needs very urgent action.

The Formosat-2 image has also been enlarged for Beichuan, again in before and after format. This is available here. This the before image:

This is the after image annotated to show the big landslide:

Note that the river appears to be blocked in this image too. There is no evidence of flow. Note also that the location of the very damaging landslide does not appear exceptional in the "before" image - this is a forested slope that looks the same as the surrounding slopes.

Finally, there are now ALOS images (both optical and radar) available. Radar adds little as the data are very difficult to interpret without processing. The optical image is better, and indeed has been annotated to indicate the location of landslides. To my eye there are many landslides that have not been annotated. Unfortunately the resolution is too low to be really helpful in this form. This is the image (click for a bigger view - warning this is a big file):