42 years evolution of Slip Stream landslide and fan, Dart River, New Zealand

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Thomas, J.S.; Cox, S.C. 2009 42 years evolution of Slip Stream landslide and fan, Dart River, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/43 32 p.

Abstract: Slip Stream landslide is a 0.9 km2 slow-moving, landslide on the eastern side of Cosmos Peaks, Dart Valley, Otago. A steep 30o-46o toe and eroding gullies at the margins of the landslide regularly feed 104-105 m3 and smaller volumes of rock debris from the landslide (volume 107-108 M3) onto a talus and 1.4 km3 fan below, thereby creating one the more geologically active sites in the region. Aerial photographs, satellite imagery, oblique photos and field visits provide a history of rainfall-induced debris flows and fan aggradation from 1966 to 2008. Local channel avulsion and debris movement are recorded in numerous photographs at the site of a 20 tonne pounamu (known otherwise as jade, nephrite or greenstone) boulder which has been progressively buried. In 1966, active debris channels were in the centre of the fan, with mature forest and minor grassland either side. The channel remained active until 1979, when only 15% of the fan was little-vegetated, fresh debris. During the following thirty years, an additional 0.7 km2 of active channel area formed. Some 65% (0.9 km2) of the fan has been inundated by between 0.5 to 1 million m3 fresh debris which destroyed mature forest and buried a pounamu resource of cultural and archaeological significance. Slip Stream exemplifies the duration over which an active debris source can destroy mature forest downslope, which has generally not been considered in studies using trees or other forest disturbance to infer the timing of earthquakes. The Slip Stream fan and landslide toe area are subject to life-threatening hazards in times of heavy rain. (auth)