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The GeoNet landslide team visited the Young River landslide on Friday 28 September. The source area and margins of the lake were inspected from the air for signs of possible further failures, prior to landing on and inspecting the landslide deposit near the lake margin.
The dam height above original river bed is estimated to be 100 m. The lake volume at crest level is of 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.
Prior to being reported the date of formation was unknown, but seismic signatures indicate that the landslide occurred about the 29 August.
The probability of rapid failure at or after overtopping cannot be discounted. The maximum outbreak flow rate for such a failure could be as high as 3500 m3s-1 (estimated from Costa/Schuster empirical relationship). Any flood wave would attenuate as it moves down the Young valley to the Makarora confluence. Given this, 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.
Failures associated with overtopping are more likely to occur during periods of heavy rain and lake discharge flows will be added to high flood flows in the Young and perhaps in the Makarora Rivers. Unusual water surface elevations and sediment deposition may occur in the Makarora River as a result, both upstream and downstream of the confluence. It is likely that the Makarora River will be deflected towards the east at the Young confluence by this flood.