Activity of the landslide Te Horo and Te Koroka fan, Dart River, New Zealand during January 2014

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Cox, S.C.; Rattenbury, M.S.; McSaveney, M.J.; Hamling, I.J. 2014 Activity of the landslide Te Horo and Te Koroka fan, Dart River, New Zealand during January 2014. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/07 45 p.

Abstract: The landslide Te Horo lies on the eastern side of Cosmos Peaks, Dart Valley, Otago. It as an active compound rock and debris slide with ancillary debris topples and debris flows commencing from its toe. Te Horo has an area of 0.9 km2 with a steep toe area that is rapidly eroding on its southern lateral margin to feed 105 – 106 m3 of sediment annually onto a debris-flow fan on the valley floor. Debris flows in 2013 and January 2014 crossed the fan and entered Dart River, impeding flow, and impounding a lake which grew from 0.47 km2 to 1.48 km2 between December 2013 and January 2014. Between 5 and 15 January 2014, a continually surging debris flow added about 1–2 x 105 m3 of sediment to the toe of the fan, but the main surge appears to have occurred on 4 January after heavy rain in the area. The lake is expected to persist for decades as the landslide continues to supply debris. The lake level and extent will fluctuate. There is no downstream danger of a catastrophic lake outburst flood. The landslide-derived sediment is redistributed downstream by Dart River, to Lake Wakatipu. Known as Te Koroka to Iwi/Mâori, the locality is significant as a source of Pounamu (nephrite/greenstone) and is recognised and managed as a special Topuni area in the Mount Aspiring National Park with entry by permit only. (auth)