Things you might not know about the Nelson slips
About 350 homes in Nelson have been affected by landslides in August’s floods, and about 130 of those households were evacuated as a result.
The magnitude of the slips has left many wondering: Why here? How have developers gained resource consent for unstable areas? And why does Nelson seem to be getting hit so badly with slopes turned to the consistency of yoghurt?
Civil Defence geotechnical response lead Grant Maxwell said Nelson was ‘‘quite complex’’ geotechnically and geologically.
‘‘We’ve got some great hills, which everyone loves to live on, because we’ve got such fantastic views, so it’s a fantastic place to live. [But] that does mean it comes with some hazards and some risks.’’
In terms of land stability, we have ‘‘some quite good geology underneath’’, he said.
Depth
Maxwell said: ‘‘Most of our failures tend to be quite superficial. They do tend to be down to about 2 metres deep maximum, but when they liquidise and when they mobilise they do get up some speed sometimes.’’
Noise
Many of the people who have witnessed slips describe the noise as ‘‘cracking’’.
Maxwell said that comes from trees that are cracking and the properties and the homes that crack when things may hit them, as the earth itself doesn’t tend to make a lot of noise.
Different kinds of landslides
Landslides have different forms, Maxwell said.
‘‘We have landslides that operate as more material, soil and rock flows, and have others that liquidise into what we call a debris flow – that some people see as cake batter chocolate mix running down the hill, and unfortunately they can move very fast.’’
Should we build on the hills?
Although this is a council issue, Maxwell said we’re running short on development land.
‘‘We don’t have a lot of spare space to be developing. The lowest risk is in the best geological places for this sort of thing.
‘‘Building on hills is globally an environment where you have to manage the risks . . . and there is a raft of factors that are assessed and understood before building takes place – such as liquefaction, fault hazards, fault rupture.’’
In terms of new suburbs, Maxwell said there were ‘‘lots of great developments in Richmond’’ on the sand gravels that had different and lower risks.
What about the Slump?
The Tāhunanui Slump area,
Maxwell said, is ‘‘really complex’’ and the site of ‘‘large deep-seated instability’’.
About 50 properties evacuated in the area – from Rocks Rd right the way back up the hill, much of which was ‘‘precautionary’’ so as to not put people at risk.
The slip at Grenville Tce was a ‘‘reactivation’’ of a 2011 event. Maxwell said they have seen slightly more movement than previously in 2011, though data was still being captured.
The neighbourhood was the ‘‘largest singular area’’ where there have been evacuations.
I was evacuated. What now?
‘‘It’s a significant event, and it’s heartbreaking to see people taken out of their homes,’’ Maxwell said. But at the same time, he said, ‘‘They’re not the sort of things you can fix in an instant.
‘‘We don’t know the effects of a new storm event on a specific piece of land. It changes lots.
‘‘There are risks, but with the science, we can create some tools.
‘‘We can try and add resilience, but the best thing we can do is we can be a community and when things happen, help each other.’’