It was regrettable, but completely inevitable. The floods that damaged the West Coast town of Franz Josef were the work of a combination of factors at work. As the clean up continues and the West Coast Regional Council, Westland District Council, the locals, the businesses and the tourists there count the cost of the flooding on Thursday, the debate about the future of the town has reignited.
But to understand the debate and the lessons that will emerge from the thick layer of sediment laid down, we need to look at the underlying geomorphology of the area. It is important to understand how the climatology, the hydrology and the geomorphology work together to create one of New Zealand’s most dynamic river systems. And the planning decisions that lead to properties being sited in high risk areas.
Franz Josef is town nestled against the Southern Alps. This mountain range has very high rates of uplift at nearly 30mm/year and equal rapid erosion, which means a steady supply of sediment is trapped in the upper reaches of the river catchments heading on the range. Because of its location in the roaring forties belt of westerly winds, the Southern Alps is a magnet for substantial orographic rain coming off the Tasman Sea, with some parts receiving up to 14,000mm per annum. Whilst much falls as snow in the winter, during the warmer months during heavy rainfall events 200mm or more in a day is not uncommon.
Franz Josef is sited on the north side of the Waiho River, a short but steep river that has its source underneath the Franz Josef Glacier. The Waiho River also has a significant tributary called the Callery River which has its confluence immediately upstream from the State Highway 6 road bridge. The Waiho River and the Callery River have quite different hydrological systems. The former is largely hidden under the Franz Josef Glacier and the associated ice pack, which slows the progress of rainfall into the river itself. The Callery River however is nearly all hard rock, with little absorption capacity meaning rainfall ends straight up in the river.
In the past two decades, the Waiho River has threatened the stop banks and the town several times, washing out the northern approaches at least once. Another time under prolonged heavy rain the roof of the glacial tunnel carrying the Waiho River caved in, forming an internal dam of ice that burst violently with chunks of glacier ice found several kilometres downstream on top of the stop banks.
Prior to the flooding on Thursday, several commercial properties including a hotel were nestled against the southern stop bank. This stop bank was built across a part of the river bed that has been historically quite active. This area was marked in 2002 by Civil Defence as being at very high risk of a landslide dam burst flood event in either the Waiho or the Callery River. The stop bank on active riverbed has had the effect of making the river pile up large amounts of sediment. In order to protect the premises from a river whose bed is steadily rising and State Highway 6 have necessitated the construction of ever bigger stop banks, costing millions and is ultimately something that cannot be sustained.
The flooding on Thursday however was widespread, right across the riverbed and also broke out on the north side, inundating properties there as well.
But there is a bigger problem in Franz Josef Glacier which may in time supercede all of the other factors. It sits on the Alpine Fault, which is coming to the end of its average repose period and is likely to generate a magnitude 8.0+ earthquake in the next 50-100 years. In 2013, recognizing that the fault poses a substantial threat to the town, Westland District Council released Plan Change 7 to their District Plan, which called for restrictions on the businesses that could sit on the probable fault rupture zone. Due to no clear scarp being visible in the town there is a buffer zone in the plan change. As the rupture is likely to involve up to 8 metres horizontal and perhaps 2 metres of vertical movement, there are suggestions that Franz Josef township should be moved from its present location altogether. The plan change itself does not necessarily suggest moving the town, though discussion has been had about the idea.
Not surprisingly there has been resistance to the proposals. Some people feel that it will kill their businesses and others think the District Council has its priorities wrong. However long time residents know that the river bed is very active and that the Waiho often flows across all parts of it. Whether they admit it or not, time is not on their side as the stop banks cannot get much bigger and the river continues to pile up sediment. Eventually the portion housing the commercial premises and State Highway 6 on the south side will become river bed again as well.
In conclusion, several lessons can be drawn:
- The south side premises are not safe from flooding
- The stop bank will fail again and does not justify the expenditure on it
- State Highway 6 where it passes the river may have to be relocated
- Given the high rate of sediment movement down the river and the high annual rainfall, another such event is a certainty