19 July 2009

Intriguing landslide at Nachterstedt in Germany

Posted by Dave Petley

NB: I have posted an update on this landslide here.

A somewhat intriguing landslide occurred on Saturday at Nachterstedt in Germany (the location is 51.808 N, 11.343 E for those who are interested). The site is the edge of Lake Concordia, a flooded open cast coal mine:

The mine apparently closed in 1991 whereupon it was converted into a recreation area. The lake was apparently created in 1994. Reports suggest that on Saturday the area received some rain, but that this was not exceptional in any way. The slide appears to have been rapid, with a volume of about 1 million cubic metres. Reports suggest that it created a mini-tsunami on the lake (not surprising given the images below), and that two houses were carried with the slide into the lake. It is thought that three adults were killed in one of the houses. The images of the site are quite remarkable:

Image published in the LA Times providing an over-view of the site

Image published in Deutsche Welle providing a more detailed of the landslide

Close up published in the LA Times of one of the houses affected by the landslide

This is quite an intriguing slide, both in terms of the mechanisms of the slide and of the trigger. In particular it is not at all clear to me why this slope has failed when the weather was not wet. Central Europe had heavy rain a fortnight ago, so this might be an interesting starting point to ponder. Earthtimes suggests that the area might have been underlain by old mine adits, though this is not obvious from the images above. The middle image does show what appears to be shallow coal seams on the right side, but I can’t see any evidence of coal seams or abandoned workings elsewhere.

The Google Earth image of the site is quite high resolution. I have zoomed in below to show the area of the slide:

A few of things to note. First, the area that has slid appears to have been suffering some erosion of its toe. Second, this area also has extensive herringbone drains around the large gullies that cross it. I wonder why these gullies are there (they seem to be better developed than those elsewhere on the walls of the lake), and why these herringbone drains have been built. Is this an indication of a drainage problem. Third, there is another small slide on one of the images – see the right side of the Reuters image below:

This smaller slide also looks quite fresh (i.e. recent). It could of course be that this was triggered by the displacement wave, but it could also be an indication that there is a general stability problem. Finally, the presence of the ponds on the slide behind obviously displaced blocks suggests that there was some water in the slope.

The Earthtimes article suggests that the mine was still being progressively flooded, so it could be that the key issue here is instability under raised groundwater conditions.

NB: I have posted an update on this landslide here.