KEY POINTS:
Eight areas around Lake Taupo are at high risk of erosion, including Taupo's scenic shoreline, raising the possibility of restrictions on development to manage the problem.
A new study has found eight of 44 areas around the lake's foreshore are unstable and actively moving.
Ongoing movement is also anticipated.
The study, commissioned by Taupo District Council and Environment Waikato, said it was important development did not take place in the high-risk zones.
"That's a fairly significant statement," said Chris Todd, project manager for the council's foreshore risk management strategy.
"We would want to talk with the community about that as an option."
The high-risk areas are mostly on the lake's eastern and southern shores and include the area from the Waikato River to Wharekawa Point (which includes Taupo), Waitahanui, Te Rangiita and Kuratau.
The other four areas are Hatepe, Motutere, Wakaipo Bay and Whangamata Bay (including Kinloch).
A further 15 areas were identified as facing a moderate risk of erosion. These included Acacia Bay and Whareroa.
The study said further monitoring of all areas was needed to determine whether the erosion was due to long-term changes in the shoreline or natural fluctuations.
Mr Todd said most of the eight high-risk areas were buffered by lakeshore reserve, so the numbers of private property owners affected were limited.
But at Waitahanui, an area of older baches, and Te Rangiita, erosion of private property has been reported.
So far, the worst erosion seen is at Kuratau, where about 20m of a 40m wide reserve has disappeared.
No houses have yet been threatened.
Mr Todd said the study raised the possibility of preventing development within a certain distance of the lake and building structures to counter erosion, but no such measures would be taken without the public's consent.
"The implications are for the community to decide through the consultative process - what value they place on the foreshore and whether they wish to have continued public access to some areas, and how that foreshore might look in the future."
Leaving erosion to continue naturally was also an option, he said.
The study identified the likely causes of erosion, finding that a combination of natural and manmade factors contributed to the shoreline subsidence.
Natural factors included: wind and waves; sediment inputs from rivers; tectonics (land rise and fall); climate change; and fluctuations in sand and gravel.
High winds were also found to be a factor.
The eastern and southern shorelines were found to have a low resistance to erosion because of their geology (soft sediment with a pumice base), while the western and northern shorelines had a moderate to high resistance because of a strong geology of ignibrite rock cliffs.
Human influences included development close to the shoreline, boat ramps, rock walls, vegetation removal, and a reduction in sediment from rivers as lake levels were controlled by dams.
"All of these things can have an impact on the little processes that occur on beaches," Mr Todd said.
The study was the third stage of a project examining erosion around the lake.
The next stage is identifying whether assets including properties, pipelines, reserves and ramps are at risk.
The final plan will set out options to counter the problem.
Environment Waikato project manager Adam Munro said the study had filled a void and provided valuable information for dealing with erosion at Lake Taupo.
"We're being proactive and looking at addressing it now," he said.
