The dunes west of Gisborne city would block a lot of a tsunami but there were city areas with a moderate to
high level of risk, such as along the riverside walkways, the logging terminals, the lower ends of Customhouse and Grey Streets, and the Waikanae campground.
Wainui properties would also be protected by the dunes, but some areas up and around stream outlets had a moderate to high level of risk.
The council is seeking legal advice on the implications of the information, as it could affect people’s land information memorandums (LIMs) and property values, said GDC strategic policy manager David Wilson.
As well as calculating the flow depth, which is the difference between ground level and top of the water surface
when the tsunami comes on to land, the report also maps the annual individual fatality risk (AIFR).
Data from national studyThat is the risk of someone being killed in the event of a tsunami caused by an earthquake in a certain location. The research incorporated data from a national study that simulated 100,000 years of tsunamis caused by earthquakes at various faults, such as the Hikurangi subduction zone, and it takes into account sea level rise.
The AIFR was used by Christchurch City Council for land use planning in the Port Hills following the Canterbury earthquakes, where there were unstable rock slopes, Mr Horspool said.
Mayor Meng Foon asked what the AIFR information meant for Gisborne and how many people could be affected.
“This report only looks at the individual risk for land-use planning,” Mr Horspool said.
“Further work could look at the number of people injured or killed in a given scenario.”
Councillor Craig Bauld asked if the AIFR affected property values in the Port Hills.
“In the hills where there were lots of damage, prices dropped initially but have now gone back up,” Mr Horspool said.
“Painting evacuation zone lines on the ground in Wellington did not appear to have affected property prices.”
Councillor Amber Dunn asked if GNS would look at other sources of tsunami, not just earthquakes.
“Earthquakes are the dominant source,” Dr Power said. “Gisborne is less exposed to volcanic sources to the north. There is a big question about landslide sources in this area.
“There was a lot of debate about whether it was a landslide or earthquake that caused the 1947 tsunami that affected this coast. We think it was an earthquake.”
No modelling of Hikurangi ShelfMr Foon also asked if they had modelled the Hikurangi Shelf, as people in Ruatoria wanted to know what the effects would be if it collapsed.
“That is outside my expertise, but we do know a massive landslide off Ruatoria probably caused a big tsunami several hundred thousand years ago,” Dr Power said.
“There is a big question mark about those types of events.”
GNS had not been asked to research that area, GDC environmental and regulatory services group manager Kevin Strongman said.
Mayor Foon also asked if the council should not recommend any further intensification of coastal communities. Mr Wilson said this type of planning would be part of the council’s natural hazards program that staff were working on.
“In the future we would like to workshop with the committee, not just on tsunami but also results from liquefaction studies, coastal inundation work, all the different natural hazards, to gauge your level of risk and the community’s so we can start throwing questions around.
“For example should we be encouraging development in these areas or should we be steering away from that as we develop long term plans and infrastructure plans.”
Property values would be the biggest anxiety, Mr Foon said.
The council was taking legal advice on what the implications of the information are, Mr Wilson said.
“Some of these things have to go on to peoples LIM’s which can affect property values. All councils are grappling with this.”
The report, titled Probabilistic Mapping of Tsunami Hazard and Risk for Gisborne City and Wainui Beach, can be found on the council’s website.
