Monsoon rains triggered a large earth flow near the town of Koslanda yesterday. The landslide struck early in the morning, and according to the Disaster Management Center destroyed more than 120 homes, with current estimates suggesting there could be more than 100 casualties. New video from the site gives the first really good impression of the location and scale of the earth flow, as well as some insight into the contributing factors.

Aerial image of the landslide release area in what appears to be a deep, weathered soil, and
evidence for strong surface or near-surface water flows from the headscarp.

 

Using a still image from the video to map the landslide in Google Earth, it appears the disturbed area is approximately 700 m long, and a little over 100 m wide. While this is not unusual for earth flows in such material, the tragedy here is that there was a large housing complex in the path of the landslide. This is presumably where many of the casualties occurred.

Map of the extent of the earth flow. The housing complex is located on the right-hand margin
of the slide, approximately half-way downslope.
The grey outline indicates the margin of what appears to be a pre-existing landslide complex.

 

The local landscape indicates this may not have been the first such failure at the location. With a river actively eroding at the valley floor, and very thick weak sediments overlying what appear to be harder layers that wrap around the hillslope to the upper right of the image above, the grey line in this image bounds a somewhat linear, concave depression that is certainly a candidate for intermittent slope instability. Failure from the head of the slope, and deposition near the toe typically produces a relatively flat region on the lower half of slopes affected by this kind of instability. As population pressures increase, these flat though often very wet regions can be popular targets for development. Tragedies such as this provide a reminder of the growing need for good geotechnical investigation in developing regions.

Interactive map indicating the estimated extent of the landslide (orange), and evidence for a pre-existing landslide complex (grey), as well the aerial photograph of the release area (double click the photo to view). A .kmz file is available as a Google Earth download here
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