5 October 2021
A significant landslide near to the water supply pipeline for Gisborne in New Zealand
Loyal reader Dr Murry Cave of Gisborne District Council has kindly highlighted an interesting landslide, close to the water supply pipeline for the city of Gisborne in New Zealand. The image below, provided by Murry, shows the landslide, which occurred on 27-28 September 2021:
The landslide covers an area of 5,440 square metres. The vertical image below shows its extent and the tension cracking around the rear scarp:-
The location of this landslide is -38.802, 177.792 if you want to take a look. Note that the slip is on the outside of the bend, suggesting that toe erosion may have played a role in its initiation. The landscape of this area of New Zealand is dominated by natural landslides, so a failure is not unusual or surprising, but clearly in this case it is potentially problematic given the water supply pipe.
Perhaps the most interesting aspect of this landslide is that it has occurred at the toe of a very much larger landslide complex. Murry has kindly provided this annotated image of the site site, with the 27-28 September landslide on the west side:-
Note that the main landslide complex extends right up to the ridgeline. The risk is that the failure of the toe of this much larger complex will lead to movement on a greater scale, and indeed as the image above highlights, there are signs that parts of the complex have reactivated.
This will now require careful monitoring as a failure on this scale could threaten the pipeline, which can be seen on the west side of the image above. This will require careful monitoring. New Zealand has very extensive experience of dealing with these types of hazards.
New Zealand is now leaving the wetter winter period, so for a few months the risk of a major failure will reduce. However, early in the new year the cyclone season will bring a risk of further heavy rainfall events.