The village not only straddles the Alpine Fault but faces a potential triple whammy — a huge earthquake, flooding from the Waiho River and landslips.
The natural hazard assessment by GNS, prepared for the West Coast Regional Council, says unpublished data suggests the chance of a major quake on the Alpine Fault may now be as high as 50% in the next 50 years — not the 27% previously thought.
The last big earthquake on the fault-line was in 1717.
The report says a strategy is needed for a managed retreat from the main area around the Alpine Fault, recommending that the town centre should be moved northeast along the extension of Cron St, following the recent move there by Ngai Tahu Tourism and the Department of Conservation.
However, to greatly reduce the danger of shaking in a major quake, the town should move 5km to 10km northwest of the fault.
"We recommend that the council undertakes a cost-benefit analysis in consideration of relocating the town of Franz Josef," GNS says.
The Alpine Fault could move 1m to 2m vertically and up to 9m horizontally at the southern end of the township.
Houses would be shifted off foundations, even reinforced building frames could partially collapse, and there would be widespread landslips and rockfalls.
The report also raised the possibility of liquefaction.
Earthquakes in turn would cause landslips, and it is "highly likely" a landslip would fall above the town and the Tatare River subdivision.
The Waiho River, which destroyed the former Scenic Hotel earlier this year, is still aggrading (gathering sediment) and at the current rate the riverbed could be 4m higher in just 20 years.
If a one in 100-year flood then hit, it could send water 1m deep through the township.
There is also the risk of catastrophic rock avalanches, especially in the range immediately east of the town.
This could lead to a "considerable portion, if not the entire town, being overrun".
The regional council has referred the 70-page report to the Franz Josef natural hazards working party, but has yet to discuss the findings.
- Laura Mills, Greymouth Star