The Young River landslide

Chris Massey, Mauri McSaveney, Neville Palmer, Vern Manville & Graham Hancox

Introduction Haunted Spur (1680m amsl) The North Branch of the Young River, near (m) Main scarp Source area 1500 Lake , , was blocked Tension ROCK AVALANCHE TRAVEL PATH cracks by a large landslide at 4:40 am on 29 Schist

August 2007. The debris avalanche approx. profile of present slope North branch (failure surface) occurred in closely jointed schist, 1000 of Young River approx. profile of pre-failure slope Landslide dam Overtop forming a dam 70 m high across channel (580m) the valley. Water immediately Young began impounding behind River 500 Former level of river the dam and has now channel (480m amsl) overtopped, scouring an note: The pre-failure topography is derived from 1:50,000 Topographic map WILKIN F38. Post failure topography is based on data from Terrestrial laser scan surveys. outflow channel on the 0 downstream face of the 0 500 1000 1500 2000 (m) dam. Cross section of Young River debris avalanche and landslide dam Solar panels provide power to lake level monitoring instrumentation and communication equipment.

800 Rainfall at 700 Makarora Telemetry

600 Albert Burn Rainfall at Dragon Fly 500 400

300 Landslide dam 200 overtopped

500 m 100

Cummulative Rainfall (mm) Landslide dam formed 0 1-Aug-07 31-Aug-07 30-Sep-07 30-Oct-07 Date Data from Regional Council

Young River area rainfall data.

The source of the avalanche was located on what is locally referred to as Haunted Spur, above the river's true right bank. The main scarp is at an elevation of 1600 m, with an elevation difference (between the top of the source area and toe of the debris) of 1100 meters. Several View of landslide debris forming dam, looking north west. terrestrial laser scans (TLS) of the landslide dam (deposit) and source area (using a RIEGL LMS-Z420i scanner) have been carried out. View of landslide and dam looking north west. These data have been used to generate a three-dimensional model of the landslide deposit. This model has been compared with the pre- failure topography, to calculate the landslide and lake volumes. This information has been used by both GNS scientists and emergency Summary of Key Facts management officers at the Otago Regional Council and has provided essential information for assessing its future stability as a dam. Location Young River Landslide, North Branch Young River (Mount Aspiring National Park), New Zealand, which 16:40 25”, is a tributary of the Makarora River. The dam is about 70 metres high at the outflow and the impounded Initiation Date/time 04:40 on 29 August 2007 lake is approximately two kilometres in length and 500 metres at its 28 August, Trigger No obvious trigger (no rainfall three weeks prior to widest point. Water overtopped the dam on Friday 5 October, initiation and no earthquakes recorded) 2007 (UT) Landslide Volume 11 Million m3 approximately 5 weeks after its formation. Water continues to flow Velocity 100 km/hour (minimum) over the dam crest and has scoured material from along the outflow Runout distance 1.8 km 3 channel after periods of high rainfall and associated outflows. Impounded Lake Volume 23 Million m Equipment to remotely monitor the lake level has been installed . Dam height (at outflow channel) 70 m

The Young River landslide is in a remote headwater tributary of the Seismic trace generated by the landslide. Makarora River and was first recognised by a helicopter pilot three weeks after initiation. Seismic shaking generated by the landslide was the equivalent of a magnitude 2.9 earthquake, and was recorded by the GeoNet seismic network. Debris from the landslide travelled approximately 1.8 kilometres in 60 seconds, suggesting a minimum debris speed of 100km/hour.

Using a tripod-mounted laser scanner, geologist Chris These images show the 3D model before, and after the Massey and surveyor Neville Palmer, both of GNS Science, landslide. have measured the volume of the landslide at 11 million cubic meters.

Discussion Rapid dam failure during high outflows remains possible. The maximum outbreak flow rate for such a failure could be as high as Assumed 3500 m31 s- (estimated from Costa/Schuster empirical relationship). lowest point of 582.0 dam crest Any flood wave would attenuate as it moves down the Young valley to November 2007 579.7 m the Makarora confluence. Given this the Department of Conservation have closed access to the lower Young River to ensure public safety, 580.8 and the Otago Regional Council is monitoring the situation to manage the safety of people in the Makarora Valley. GNS Science is providing information gathered from our monitoring to both Department of 579.7 Conservation and the Otago Regional Council.

Lake Level (m above mean sea level) Assumed lowest point of dam crest April 2008 579.4 m Failures associated with overtopping are more likely to occur during 578.5 3-Nov-07 18-Nov-07 3-Dec-07 18-Dec-07 2-Jan-08 17-Jan-08 1-Feb-08 16-Feb-08 2-Mar-08 17-Mar-08 1-Apr-08 periods of heavy rain when lake discharge flows will be added to high Date flood flows in the Young and perhaps in the Makarora Rivers. Unusual water surface elevations and sediment deposition may occur in the Young River lake level monitoring data. Makarora River as a result, both upstream and downstream of the confluence. It is likely that the Makarora River will be deflected east at the Young confluence if a flood were to occur. However, it is also Engineering geological map of the landslide. possible the dam and lake could be part of the landscape for a long time.

Acknowledgements: Gavin Palmer (Otago Regional Council), Paul Hellebrekers (Department of Conservation), Tim Davies (Canterbury University), Oliver Korup (Swiss Federal Institute for Snow andAvalanche Research) GNS Science