Rockslide on Glacier Exacerbates Flooding in New Zealand

New Zealands Southern Alps Courtesy of Geee Kay, Flickr)
New Zealand’s Southern Alps Courtesy of Geee Kay, Flickr)

On 2 January 2013, large piles of rock tumbled down Mt. Evans in New Zealand. The avalanches, set off by the collapse of the mountain’s west ridge, sent rocks onto the Evans and County Glaciers and eroded snow and ice. As the rocks tumbled down, they triggered flooding in the Wanganui River.

The event was not the first time rock avalanches caused severe damage in the region; glaciers, landslides and rivers are the main cause of erosion in New Zealand’s Southern Alps.  Historically, rockslides such as these occurred once every hundred years, according to a new report by authors J.M. Carey, G.T. Hancox and M.J. McSaveney, but have increased in recent decades. There were 4 per decade between 1976 and 1999 and more than 20 per decade since 1999.

Some, the report found, are caused by the region’s frequent earthquakes, but many of these rock avalanches cannot be attributed to one factor alone. Instead, factors including heavy rainfall, high slopes and fractured rock each contribute to avalanche-prone rock conditions.

Understanding the underlying causes and effects of rock avalanches can help researchers assess the likelihood of future rock avalanches and the potential damage they will cause. Already, researchers expect boulders above the Evans Glacier to collapse at any time onto the ice.

“The increase may relate to accumulating geodetic strain in the region as the change in occurrence rate correlates closely with change in accumulating seismic moment release in the New Zealand region,” wrote the authors. “It also has been linked to global climate change which is likely an additional rather than an alternative influence.”

The consequences of frequent rockslides can be severe. In the case of the most recent event on Mt. Evans, rocks travelling at 35 meters per second, or 78 miles per hour, set in motion cascading events which inundated farmland, cut off a road and severed a fibre optic cable. The floods were initially attributed to heavy rainfall, but a reconnaissance mission five and a half months later revealed that the landslide onto the Evans Glacier was the main trigger. Heavy rains exacerbated the flooding in the Wanganui River.

“The rock avalanche onto Evans Glacier ran out at high speed onto a broad flooded river flat over a kilometre long,” the authors wrote. “The rock avalanche significantly bulked up with snow and flood water and also may have bulked up with alluvium [deposit left by flood water] and possibly old glacial deposits.”