The Dart River Landslide

Dart River aerial view 400

In early January 2013, heavy rain in the Southern Alps reactivated a massive landslide in remote Dart River valley. To Maori, the area is known as Te Koroka and the slip as Te Horo; the area is culturally significant as a source of pounamu and has the status of tōpuni.

Maori shared stories about Te Koroka and Te Horo with Captain James Cook. In the last decade the 50,000,000 cubic metre landslide has become increasingly active. Debris-flow activity from the landslide toe increased during 2013, covering a fan below with fresh debris and sometimes blocking Dart River. Debris flows consisting of wet concrete-like slurries (boulder-sized to fine-silt) traveled downslope from Te Horo and across the gently sloping fan, burying all in its path. Debris flows over the last year have delivered more sediment to Dart River than it has been able to carry away, resulting in the fan partly blocking the river.

In January 2013, a small lake (0.13 square km) formed upstream of Te Koroka /Slip Stream. More debris flows over the year caused the lake to reach 1.48 square km by January 2014. A large debris flow on 4 January 2014 delivered nearly a million cubic metres of sediment to the fan and was sufficient to dam the flooded Dart River. For the rest of the month the debris flow pulsed continuously, up to several times each hour, switching to new channels across the fan, and intermittently raising the dam crest and lake. The growing fan pushed the Dart River channel against a terrace edge on the opposite side of the river, eroding the steep, forested bank. A section of the popular Rees-Dart walking track fell into the river and another part is submerged in the lake.

Dart River Mark & Simon 400

The continuing event presents a rare and unprecedented opportunity to study the dynamics of debris-flow initiation and deposition, and the impacts of a very large sediment pulse on a river. Using aerial photography and satellite imagery, GNS Scientists Simon Cox, Mauri McSaveney and Mark Rattenbury have updated the erosion and sedimentation history for 2011 to 2014. Debris-flow source areas, fan deposits and the lake extent were mapped for successive years. The maps show the different parts of the landslide that were active at different times, and the debris-flow source area on the landslide that is progressively enlarging upslope over time. The maps also show how the fan has grown in size, restricting and temporarily blocking flow in the Dart River at its toe.

The upper part of the landslide is intensely fractured, and the slope area at 1200–1470 m provides a source for future debris flows. Until such a time as sediment delivery from the landslide reduces radically, the periodic debris flows and associated events are likely to continue.

At this writing, landslide-related debris flows do not appear to add any additional danger to users of the lower Dart River in areas below the dam, though the Te Koroka /Slip Stream area itself is hazardous. A catastrophic lake-outburst flood is unlikely. However, Dart River visitors downstream of Te Koroka /Slip Stream need to check with the local information centre at Glenorchy for updates on the situation. Simon Cox describes what's happening at Dart River (VIDEO).

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Last updated 7 Nov 2014