article: Oct 5 2007 - Young River update

Updated information for the Young River landslide dam.

Summary of activities

The initial observation of the landslide was on 26 September when a Makarora based helicopter pilot flew over the area. The GeoNet landslide team (comprising Dr Mauri McSaveney, Dr Tim Davies and Dr Oliver Korup) inspected the Young River landslide on Friday 28 September. Examination of seismic traces recorded on the GeoNet seismograph network indicate that the landslide occurred at 4:40 am on 29 August 2007.

On 3 October as part of the ongoing GeoNet landslide response, Dr Vern Manville (GNS Science) installed an automatic remote camera that takes pictures every minute during daylight, and an Acoustic Flow Meter (AFM) that can measure vibrations associated with large volumes of mobile water and debris (possibly as a result of overtopping or breach of the dam). Both the camera and the AFM require manual download of data which will be carried out at set intervals (weather permitting). The same equipment was previously deployed on Mt Ruapehu. Although the camera was effective on Ruapehu, Dr Vern Manville expressed caution, saying: "Given that the dam is likely to reach capacity during heavy rain, snow, or the hours of darkness, and the number of keas in the area, there is a small probability of getting good images. It's a case of waiting and seeing when we are able to download the camera".

Further investigation and monitoring activities are planned by GNS Scientists early next week (8 - 10 October weather permitting) when a pressure transducer to monitor lake level will be installed and a terrestrial laser scanner will be used to produce a highly accurate map of the landslide and dam.

Summary of facts

A landslide blocking the North Branch of the Young River near Haunted Spur occurred at 4:40 am on 29 August 2007. The landslide occurred in shattered and sheared schist rock adjacent to the Moonlight Fault Zone. The landslide formed a dam 100 m high blocking the valley. Immediately after the landslide occurred water began impounding behind the dam. It has taken more than five weeks for the lake to fill completely and at the time of this report now needs only a further rise of less than one metre to overtop the dam. The lake volume at crest level will be in the order of 2 x 10⁷ m³ (about 40 Wellington Stadiums), with a surface area of ~ 10⁶ m². After heavy recent rainfall the lake is now close to overtopping.

Previous experience with dams of this type indicates that if they do not fail during initial overtopping then it is probable that they will fail soon afterwards. The Department of Conservation have closed access to the Young River to ensure public safety for this likely event, 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 Conservation and the Otago Regional Council.