An assessment of the Te Arai landslide upstream of the water treatment plant in Waingake has revealed a risk to the city's pipeline. Untreated water from Te Arai Bush Catchment is treated at the plant before travelling along a 30-kilometre pipeline to the city's reticulation network, delivering water to households.
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Although the landslide occurred in September, a severe storm in November reactivated the situation.
“The key point is that it's right next to the pipeline that supplies water to the entire city, so it's now a risk to critical infrastructure to the city,” Gisborne District Council principal scientist Dr Murry Cave said.
The assessment showed the Waingake River, which was originally 90 metres away from the pipeline before the landslide, had been diverted to within 30m of the structure due to the movement of the landform.
“It (the landslide) is only going to keep getting closer so we have to try to manage it.”
Dr Cave said the council had routinely used drones to map the scale of the landslide.
“The drone supplies a grid and takes about 1000 photographs, which are then stitched together and it comes up with a high-resolution aerial photograph.”
Contouring — the lines on maps that show landforms — is also used as a tool for mapping.
Dr Cave said aerial images of landforms such as landslides could be taken a month apart and then compared to identify changes in landform over time.
“We have also done some work using structure from motion survey with the help of Massey University, which is a detailed survey done with drone footage,” Dr Cave said.
“We have been able to establish that using the structure from motion data there is actually subtle movement occurring all the way up the landslide and particularly close to the top of the hill where there has been a lot of movement, suggesting there is risk higher on the hillside as well.”
Even though the landslide was categorised as mid-sized, its proximity to the pipeline and especially its movements were the cause of concern as there was a possibility of a “larger-scale potential failure”, he said.
Such a failure would also hinder access for maintenance of the pipeline, and farming operations at Patemaru Station, which is part of Maraetaha Incorporated.
“If we have a pump failure, for example, that means the pipeline may not operate properly and we would have to rely on the Waipaoa water treatment scheme, which isn't a good option.We are hoping that the landslide does not reach the pipeline itself but if it does look like it is going to do that, then we have to do some armouring along the areas where the pipeline crosses the river.”
Maraetaha Incorporated chair Bella Hawkins said assessing and mitigating the risks was “important for ensuring the safe passage for all as part of our whenua (land)”.
“The connection between Maraetaha Incorporation and the relationship with GDC for the purposes of supplying water via the pipeline is a necessity for Turanganui-a- Kiwa.
“The pipeline provides a vital source to the community of Gisborne and we are doing all we can to support and assist with ensuring the pipeline is protected,” she said.
Ms Hawkins said further slips would not only affect farmland production and access for farm staff and operations but also block access for council staff to “monitor and read the weir and the water flow that is coming into the Waingake River.”
Dr Cave said to mitigate the risks, methods such as “dewatering” could be used.
“We have a big spring further upstream from the landslide and we'll probably dewater (remove) that to divert water from the landslide.”
The council and Maraetaha Inc would also look at planting the whole landslide with manuka and permanent vegetation cover to help mitigate further landslide risk.
“The vegetation will provide a protective cover to this soil via root mass, and it also takes out all the moisture, reducing instability and the risk of landslides.
“Hopefully that will be the option that we can run with for the longer-term risk and do as much as we can in the meantime,” he said.
Dr Cave said there were plans (yet to be approved) to build a 24-metre-long permanent bridge for better access over a small lake that had appeared due to the landslide.
The assessment highlighted how vulnerable the whole region was to landslides, and the need to work around and mitigate those problems, he said.
Filmed and Edited by
Ben Cowper
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