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.

A surprising spell of landslides

In most years in which I have been collecting landslide fatality statistics (since 2002) November has been a month with quite low landslide occurrence. Not this year though - in fact the last few days have been more like the summer monsoon season, with fatal landslides occurring in a wide range of places. Examples include:

• El Salvador on 6th November, in which the volcanic lahars triggered by Hurricane Ida appear to have killed over 100 people, with a few (very speculative) reports of rather more;
• South Sulawesi in Indonesia, when 13 people were killed in a slide Palopo;
• Tamil Nadu on 9th and 10th November, when a number of landslides killed at least 30 people;
• Casamicciola on Ischia in Italy on 9th November, when a 15 year old girl was killed by a slide that carried her out to sea in a bus;
• Bukittinggi in West Sumatra on 10th November, when two people were killed;
• Wassa-Akropong in Ghana on 10th November, when up to 30 people were killed in a landslide in an illegal mine;
• Goha village in Tanzania on 10th November, when 20 people were killed in a landslide.

In addition there have been reports of non-fatal landslides in the USA, Serbia, Vietnam, etc. I cannot tell at the moment whether this is just coincidence or something more significant, perhaps due to the El Nino conditions. With Cyclone Phyan currently bringing heavy rainfall to central western India, an area that is landslide-prone, there could be further events in the next few days.

El Salvador landslide disaster

The nature of the landslide disaster in El Salvador caused by Hurricane Ida is now becoming a little clearer. The largest event appears to have happened in the town of Verapaz, where it appears that a debris flow hit and destroyed part of the town. The reported death toll is 16 people, with a further 47 thought to be missing.

There are two intriguing aspects to this landslide. The first is that the maps seem to show that Verapaz is some distance from the San Vicente volcano (also known as Chichontepec volcano) from which the landslide (probably actually technically a lahar) came, as the Google Earth image below shows:

So, I wonder whether the landslide actually affected this town - in which case it is very large indeed- or whether it was one of the smaller communities nearer to the volcano.

Second, this area was affected by an earthquake on 13 February 2001, which killed over 300 people. www.elsalvador.com has an interesting comparison image:

This is clearly not the same street.

Acciording to this abstract, In that earthquake two landslides were triggered on San Vicente volcano (the volcano shown above). Part of the abstract states (note my emphasis):

"Large landslides dammed two major rivers, the Río El Desagüe and Río Jiboa, and two large slides occurred on Volcán San Vicente. The ±1.5-million-m³ Río El Desagüe landslide temporarily dammed the river and formed a shallow, 1.5-km-long lake, but the dam has been overtopped and is stable. The ±12-million-m³ Río Jiboa landslide blocked about 700 m of the valley with debris composed of poorly consolidated tephra, and the upstream lake could potentially have been as deep as 60 m and about 2 km long. A 20-m-deep spillway was excavated to decrease the maximum lake volume and reduce the possible catastrophic failure of the unstable landslide dam. On the northern flank of Volcán San Vicente, ±250,000 m³ of loose landslide debris filled the upper part of Quebrada El Blanco; remobilization of this material in debris flows could inundate part of a downstream village. On the volcano's northwest flank, ±200,000 m³ of lithified andesite blocks slid in the upper part of Quebrada Del Muerto, but this material will not likely remobilize and threaten downstream settlements.."

Verapaz is to the north of the volcano.

The presence of potentially dangerous landslides on the northern flank of the volcano is fairly clear:

Reports of a very large landslide tragedy at San Vicente volcano in El Salvador caused by Hurricane Ida?

It is becoming clear that Hurricane Ida has wreaked havoc in El Salvador over the last 48 hours, with many landslides. Over 100 people are known to have been killed, with the toll expected to rise over the next few days as more remote areas resume contacts. Intriguingly, the El Salvador news site ElSalvador.com has a report on the disaster here. It reports:

"Según los últimos reportes, en Verapaz, en las faldas del volcán de San Vicente, un alud ha cubierto un área de ocho kilómetros aproximadamente. El panorama es sombrío: Grandes rocas y árboles se observan por todos lados, por lo que la movilización es difícil. La tierra ha cubierto colonias enteras. Familiares y socorristas buscan desesperadamente a las víctimas. Testigos afirman que la tragedia es similar a la que ocurrió en Las Colinas, Santa Tecla, en 2001 cuando el país fue afectado por un terremoto."

This roughly translates as:

"According to recent reports, in Verapaz, in the foothills of the San Vicente volcano, a landslide has covered an area of eight kilometres. The outlook is grim: Large rocks and trees are found everywhere, so that mobilization is difficult. The earth has covered entire colonies. Relatives and rescuers desperately looking for victims. Witnesses say the tragedy is similar to what occurred in Las Colinas, Santa Tecla, in 2001 when the country was affected by an earthquake."

The Santa Tecla landslide killed about 600 people.

I have no way to validate or refute this report at this time, so let's hope that this report is erroneous. This is a Google Earth perspective view of the volcano in question, San Vicente:

I will post again when information becomes available.