The January 2013 Wanganui River debris flood resulting from a large rock avalanche from Mt Evans, Westland, New Zealand

At 9:16 a.m. on 2 January 2013, a flood in Wanganui River, South Westland, severed a fibre-optic communications cable and the road link to south Westland (State Highway 6) at the northern approach to the bridge. This flood occurred during heavy rain which triggered both a rapid rise in stream flows and caused several large landslides in the headwaters of Wanganui River. The flood also inundated farmland in the lower valley. Flows in other South Westland rivers at the time reached high but not flood levels. The flood was caused by a rock avalanche from a 4.5–5.5 Mm³ collapse of the west ridge of Mt Evans (2620 m a.s.l) which fell onto both Evans Glacier and County Glacier in the headwaters of different catchments. One extremely rapid landslide (>35 m/s) swept down Evans Glacier, eroding snow and ice, and then slowed as it descended a gentle river flat eroding flood water, river alluvium and underlying glacial sediments. After traversing the river flat, the water-laden front of the rapid (~35 m/s) rock avalanche evolved into a debris flow in a steeper reach of the stream channel. The debris flow bulked up further with water and boulders from the stream bed and became a debris-laden flood, in places up to about 400 m wide and 80 m deep. The flow deposited large boulders, caused considerable erosion and triggered further landslides that extended up to 120 m above the river channel up to 12 km downstream from the source on Mt Evans. A much smaller rock avalanche on County Glacier did not reach County Stream and had no visible effect on its downstream catchment, Waitaha River.