Shotover rockfall in New Zealand

Reports today suggest that an interesting landslide problem in New Zealand has finally come to a close. The location of the problem was the Shotover River in Otago, which is a little to the north of Queenstown on South Island. Shotover is famous as an extreme sports location - in particular jet-boating on the Shotover River, white water rafting and a canyon swing (a sort of bungee jump I think) in the gorge, plus walking, pony trekking, etc.

The problem that arose was the occurrence on 12th July of a rockfall on the canyon wall above the river in an area upstream of that used by the jetboats. The Google Earth image of this area is quite good (Fig. 1), illustrating well the steep topography, narrow valley and the location of the jetboat station.

Fig. 1: Google Earth image of the Shotover River site.

The rockfall was pretty intriguing - a large pillar of schist had detached from the rock mass and was creeping downwards (Fig 2). The mass was pretty large - about 30,000 tonnes and as the image shows it was quite unstable. Fortunately, the authorities in New Zealand are pretty well set up to deal with such things, so quickly swung into action. Perhaps unsurprisingly, the gorge had to be closed to rafters and other users, although the jetboats were essentially unaffected. A monitoring programme was put into place whilst a strategy was formulated.

Fig 2: The detached rock block early soon after the problem first arose. Picture from the Otago Daily Times

Clearly the block needed to be brought down so that rafting etc could be restarted, so a fire hose was used to try to induce failure (this is sometimes called water jacking). Unfortunately, the weather over the last month has been very cold, which has meant that this approach has been slower than had been anticipated. Nonetheless the block continued to move. The main block finally detached at 6:30 am yesterday after temperatures rose and there was a few hours of heavy rainfall (Fig. 3). The block has broken up and has not blocked the river, meaning that the crisis is finally over. The local council hopes to reopen the river next week.

Fig 3: The site of the detached rock block early yesterday after the block finally collapse. Picture from the Otago Daily Times