14 January 2012
I have spent the last week collecting samples from the rock slopes affected by the Christchurch earthquake sequence, and am now travelling back up to Wellington, were I will spend the coming week. En route I have stopped off for the weekend in Nelson so as to be able to take a look at the landslides at Golden Bay. These landslides were triggered in an exceptional rainstorm in mid December – I covered it briefly at the time – and have been quite devastating for the local people.
In Golden Bay the most seriously affected area that I saw was the margin of the Abel-Tasman National Park, as shown in the Google Earth image below, especially between Pohara and Tata Beach, although there is almost certainly further problems to the east that I could not access. The road shown on the map across the area was closed at Tata Beach.
A good overview of the main impacts of this storm is shown below – this is the small community of Ligar Bay, which has been an area of beachside houses, with new subdivisions on the slopes behind. If you look carefully you will see:
- Slips below the road affecting the carriageway;
- Slips on the slopes above the road, which deposited debris that blocked the highway;
- Multiple slips in the slopes behind the village; and
- Debris released from those landslides that has then damaged the houses.
The landslides themselves are quite interesting – and are not just simple shallow, translational failures. This is one of the areas of slips in the hills behind the village:
There are multiple anastomosing slips in regolith materials, but some are several metres or more deep. These then appear to have entrained material downslope to form debris flows that have damaged the communities below. Note also the amount of wood in the debris. As an aside, the slips above are in an area that was for sale for residential development:
“Location, location, location” indeed!
In some places the landslides have directly affected properties; in others it came perilously close, as in this case:
As described above, the debris from the landslides travelled downhill, following but often overtopping the channels. The houses lower on the slope are quite vulnerable:
In some places across the affected area the houses were too close to the drainage lines, and were thus in the path of the flows as they travelled downslope. It must have been truly terrifying to have been in one of the houses affected in this way:
Note the depth to which the garage has been buried, and the way that the debris has forced its way in through the front door. Note also the amount of timber, and that the wood is stripped and shattered, which indicates fast, turbulent flow. This house clearly had debris flows straight through the living room:
This is shown after the flood in the video here.
There is one intriguing, very large landslide. This is at Bird Road in Clifton. I managed to get some of the way up the landslide, but not to the source area. The landslide appears to have travelled down a very steep section of channel, entraining debris en route:
Further down the slope it had become a very energetic debris flow. Note on the right side the way that the debris has travelled up the side slope as it came around the bend. This is known as super-elevation, and is an indication of high speed flow.
He also has a video of the flow in a small ravine during this rainstorm: