The Waiho river in Westland, New Zealand, is not large by world standards (its catchment area is less than 200 square kilometres), but it has four characteristics that combine to make it of global interest:
- Its whole catchment is increasing in elevation due to tectonic uplift at about 5 mm per year;
- The catchment receives in the order of 10,000 mm of rain per year
- It is in a highly seismic environment; and
- Where it leaves the Southern Alps, it flows adjacent to the rapidly-developing township of Franz Josef Glacier, a key centre of the vital New Zealand tourism industry.
Because of its natural environment, the bed of the Waiho River has been increasing in elevation (“aggrading”) at about 16 cm per year since the mid-20th century. Lateral flood banks (“stopbanks”) have kept it in the channel it occupied in the 1970s and prevented it flooding the township, but it has aggraded within these banks by about 8 metres since the mid-20th century. As a result the river bed is now elevated well above the ground level in the township, so a stopbank failure will be catastrophic.
As a challenge for engineers, scientists and managers, this river rivals any on Earth; not only is the river glacier-fed, it also flows out of the mountains exactly where it crosses the active plate boundary, the Alpine fault– which generates a M8+ earthquake every 300 years or so, the last one being in 1717. The coseismic landsliding in the steep catchment that will accompany the next earthquake will certainly and seriously exacerbate aggradation, while the earthquake shaking will probably damage the stopbanks severely.
The challenge is to develop a management strategy that will reduce the risks to the township caused by extreme storms, earthquakes and landslides to acceptable levels.
Tim Davies is a Professor in the School of Earth and Environment | Te Kura Aronukurangi at University of Canterbury, Christchurch, New Zealand. He has been studying the Waiho River and its behaviour since late last century, and has co-authored a number of publications and reports on it.