15 March 2021
Knappensee: a large landslide at an old open cast mining site in eastern Germany
On Thursday 11 March 2021 a large landslide occurred on the banks of an abandoned and flooded open case coal mine site at Knappensee in eastern Germany. The images after the event are impressive:-
The event was captured on a remarkable video that has been posted to Youtube. This video starts with an orientation problem that is soon resolved:-
Judging by news reports, the banks of the Knappensee have been undergoing remediation to allow the site to become a recreation attraction. This bank had been undergoing work recently – I note the various vessels in the video – and was a site with known geotechnical problems. The unslipped areas in the image above have little or no vegetation, suggesting engineering works since the last growing season.
The landslide at Knappensee was probably triggered by renovation work in the bank area. Experts from the Saxon Mining Authority have come to this conclusion. In the past few weeks, the first careful security measures have been carried out in this area. Among other things, dump material has been removed and work has been prepared in the bank area, said the Mining Authority. This was also the case on the day of the landslide, which is why this work is suspected to be the trigger.
The landslide appears to have been retrogressive, starting with a small failure at the toe. That in turn may have been triggered by a slip under the water, but that is speculation at this point. The final scar is reported to be 500 m by 200 m.
The landslide generated a displacement wave – reported to be about 1.5 metres high. Whilst this would be small in a marine environment, in a non-tidal lake this is significant event. This tsunami damaged properties on the other side of the lake:-
The level of the lake has been raised by 32 cm as a result of the landslide. This is now being lowered, but slowly because of the fear that rapid drawdown will trigger further instability.
Large failures in open cast coal mines, or in coal mine waste piles, are not rare, Recent examples include:-
- The 2014 Mina Pecket landslide in Chile
- The 2013 Hatfield Colliery landslide in the UK
- The 2017 Kakanj landslide in Bosnia