Franz Josef is a picturesque town in Westland District. It is nestled against the lower flank of the Southern Alps, with the Waiho River immediately to the south of the township. The town and the nearby Franz Josef Glacier are named after the Austrian Emperor Franz Joseph, by explorer and geologist Julius von Haast.
But for all of its mighty charm nestled in temperate rainforest, Franz Josef is caught between a rock and a hard place. In terms of geological and geomorphological hazards it is in a location that in the long term, and increasingly likely in the short term, untenable. This article takes a look at the danger facing Franz Josef.
New Zealand straddles the boundary of two tectonic plates. To the west is the Indo-Australian Plate and to the east is the Pacific Plate. The onshore boundary is denoted by the Alpine Fault, a large fault line with a repose period of 300-350 years and a tendency to only move in magnitude 8.0+ earthquakes. The last one was about 1717AD.This onshore boundary is where about 25-30mm of tectonic uplift occur per annum as well as a similar amount of erosion, which means there is a continuous supply of sediment waiting to enter the catchments of the West Coast and east coast rivers.
This relentless uplift creates a lovely mountain range with steep hydrology – from the summit of Mt Cook to the Tasman Sea is about 45 kilometres. Being in the prevailing westerly belt of winds that sailors call the “Roaring Forties” because of the latitude, moist westerly air comes off the Tasman Sea and empties its moisture content – often over 200 millimetres and up to 500 millimetres in a day – on the West Coast side. Unsurprisingly flooding becomes a major problem.
How does this affect Franz Josef?
Immediately after one crosses the Waiho River heading south, the road takes a hard right turn. In the corner is a hotel that sits behind a substantial stop bank. On the other side of that stop bank is a riverbed that is rising at a rate of about 300mm each year. The rise is because a large volume of sediment is continually entering the Waiho catchment. This poses an increasing flood risk on a river where water levels start responding to heavy rain in less than an hour.
Franz Josef straddles the Alpine Fault, which runs right through the middle of the township. It crosses the Waiho River in the immediate vicinity of the Milton Hotel, which was flooded and suffered severe damage in a 2016 outbreak. Westland District Council published Plan Change 7 (P.C.7), which was meant to identify a zone through central Franz Josef, where there is high confidence of the Alpine Fault’s exact location, with a view to moving essential services and businesses out of the zone. However after considerable public opposition, P.C.7 was scrapped.
Time is running out. The stop bank is about as tall as it can realistically get without massive supporting earth works. When the river tops it, it will start eroding away the stop bank and try to reclaim the riverbed that the stop bank was originally built over. This may claim several farms when it happens.
But there is a bigger problem. The Alpine Fault is now due for another earthquake. Should it rupture whilst Franz Josef is in its current location, the town will be subject to immediate and unmistakably violent shaking lasting up to 3 minutes. There will be between 8-10 metres lateral displacement to the right and up to 3 metres vertical displacement. Only the newest structures would probably be still standing.
Before then though, there may be another rain storm of similar magnitude to the one that occurred between 25-27 March. Should that happen, similar damage to what happened as a result of that storm should be expected. This has a high probability of including the bridge over the Waiho River, which was destroyed on Tuesday 26 March. The next rain fall event might not even need to be that big.
What is the solution?
In the absence of P.C. 7 existing, one option is to give up on the stop bank and let the Waiho River reclaim the riverbed. The problem here is that several farms and the air field would have to move. It also does not address the long term problem of the Alpine Fault. Perhaps the most feasible option is progressively relocate Franz Josef township’s population and amenities to neighbouring towns. Westland District Council and West Coast Regional Council have a duty of care to the residents and the tourists and other visitors to their District/Region to make sure that they are in no undue danger.
Where could the people go?
There are several nearby townships where the people of Franz Josef could be moved to. Ross and Whataroa are two, though these are quite near the Alpine Fault. Harihari is a third. All are on the same road, State Highway 6, as Franz Josef. It would be likely that West Coast Regional Council and Westland District Council would need to prepare a joint request for Government assistance purchasing land and working out appropriate resource management issues.
For their part the Government would most likely need to provide assistance. The West Coast is economically one of the poorer parts of New Zealand. It has a small rate payer base and this has a good chance, even if well planned and executed, of blowing whatever budget is set. And if the plan went ahead, it might have to be applied to Fox Glacier as well, as it too is very near the Alpine Fault.